Citrus Fruits:Diagnostics

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‘‘‘Albinism: ‘‘‘ Albinism is a sporadic problem of germinating citrus seedlings. The entire seedling may be albino, or only parts of the hypocotyl or cotyledons may be affected. Totally albino seedlings die, but partially affected ones recover and produce normal plants. Albinism can be prevented by proper storage of seed and treatment with thiram or 8-hydroxyquinoline sulfate.


‘‘‘Algal Disease (Cephaleuros virescens) ‘‘‘ Algal disease usually occurs in the warmer regions of the world and is normally troublesome only on lemon and Tahiti lime. Colonies of the alga appear as a raised, yellowish green or gray felty growth on the surface of leaves, fruit, or twigs. When fruiting bodies are present, the colonies become reddish brown. On lemons and limes, algal disease causes cracking of the bark, which may result in girdling and dieback. The best form of control is a routine copper spray for fungal diseases.


‘‘‘Algerian Navel Orange Virus: ‘‘‘ This virus is retransmitted mechanically from herbaceous hosts to healthy citrus, but it has not been associated with any symptoms in citrus. Flexuous rod-shaped particles, approximately 780 nm long, have been observed in extracts from infected citrus and herbaceous hosts.


‘‘‘Alternaria brown spot of mandarins: ‘‘‘ This disease is characterized by brown lesions on the fruit that enlarge slowly and circular, firm lesions develop on some tangerines and their hybrids.


‘‘‘Alternaria-induced diseases: ‘‘‘ Brown lesions with darker brown concentric rings develop on rough lemon, Rangpur lime, and some mandarins and mandarin, hybrids.


‘‘‘Alternaria Rot (Alternaria citri) ‘‘‘ Diseased fruit in the orchard color prematurely and may develop a light brown to blackish discoloration of the rind at or near the stylar end. Some fruit, however, show no external evidence of infection and must be cut to reveal internal decay, called black rot or center rot. Typically, affected fruit develop stem-end browning as well as center rot. Stylar-end infection is usually only important in cultivars with poorly formed navels. Stem-end rot is usually minimal in fruit harvested at optimum maturity. Control has been obtained by the use of imazalil or 2,4-D (or both) on harvested fruit. Use of the growth regulator 2,4-D delays the onset of senescence of the fruit button, thereby delaying or restricting the movement of the pathogen into the fruit.


‘‘‘Anthracnose (Colletotrichum gloeosporioides) ‘‘‘ This fungus is a common symptomless invader of citrus leaves, stems, and fruit rind, which usually causes no injury to the tissue even when it ages. The few disorders known to be initiated by C. gloeosporioides include postbloom fruit drop disease and the post harvest anthracnose that develops on the rind of susceptible cultivars. C. gloeosporioides has also been reported to cause a tearstain on fruit, particularly on grapefruit, but such symptoms do not normally appear until after the fruit has been exposed to cold, wet weather or frost. The cause of this disease on these cultivars is a host-specific strain of Alternaria citri.


‘‘‘Anthracnose Tearstain (Colletotrichum gloeosporioides): ‘‘‘ Anthracnose tearstain may blemish the rind tissue of mature Valencia and navel oranges, grapefruit, and occasionally lemon. The disorder affects mainly fruit on stressed trees with old, dead wood. The Anthracnose fungus usually first infects weakened twigs. During wet or foggy weather, Anthracnose spores drip onto fruit, where they infect the rind and leave dull, reddish green streaks.


‘‘‘Ants and Termites: ‘‘‘ In some countries, ants or termites feed on living bark, and they may cause serious girdling of the crown roots or trunks of young trees.


‘‘‘Ants or earwigs: ‘‘‘ Ants or earwigs often appear in ripening fruit. Apply sticky material around trunk of tree, prune away twigs and branches providing other routes for insects to reach ripening fruit are the best forms of control.


‘‘‘Wind injury, Aphid injury, Thrips injury, Psyllid injury, Broad mite injury: ‘‘‘ Distortion of emerging shoots or leaves, but little or no drying.

==APHIDS  ==

‘‘‘Aphids: ‘‘‘ Heavy infestations of aphids on new shoots cause the leaves to be distorted and tightly curled. Honeydew and sooty mold also appear on the leaves.


‘‘‘Areolate Leaf Spot (Pellicularia filamentosa) ‘‘‘ Areolate leaf spot occurs in some of the humid, tropical areas of South America and is most serious on grapefruit. Light-colored dry areas with dark, concentric rings form on leaves. These areas cease to expand under unfavorable conditions and thus vary considerably in size, from 0.2 to 3 cm in diameter. Yellow halos form around the lesions, and severely affected leaves abscise. Preventive sprays with copper fungicides or benomyl are the only known means of control.


‘‘‘Armillaria Root Rot (Armillaria mellea): ‘‘‘ The Armillaria root rot fungus is native to many oak trees and affects many tree crops planted in former riverbeds, flood plains, or other land subject to overflow. The first symptoms of Armillaria root rot are poor growth or dieback of shoots, small yellowing leaves, and premature leaf drop. The pathogen invades the roots and crown, eventually girdling the crown region and destroying the entire root system. From the infection site, the fungus invades lateral roots and the crown region, where it spreads as white mycelial plaques between the bark and wood. The best forms of control include avoid planting in infected areas and remove and burn infected trees and the neighboring healthy trees that are possibly infested.


‘‘‘Aschersonia growing on whitefly nymphs: ‘‘‘ Red or yellow fungal structures develop on leaves that are removable by rubbing.


‘‘‘Aspergillus Rot (Aspergillus spp.) ‘‘‘ Aspergillus rot has been reported in many citrus-producing regions and on all types of citrus fruit. Initially, the decay is light-colored, very soft, and easily punctured, somewhat like sour rot. On oranges, the lesions are pale orange, yellow, or primrose yellow, and they eventually become sunken and wrinkled. Mycelium is produced on the lesion surface, and later the rotted surface is covered with a black, powdery layer of spores. Aspergillus rot is controlled effectively by storing fruit at or below 15°C.


‘‘‘Biuret Toxicity: ‘‘‘ Biuret is formed by excessive heating during the manufacture of urea and toxicity can result from soil applications or foliar spray applications of urea that contains biuret. The leaf symptoms appear as an irregular, butter yellow area starting at the tip of the leaf. Young leaves are more vulnerable than old ones, and affected leaves do not turn green again. Succeeding growth flushes appear normal provided applications of biuret-containing urea are not repeated.


‘‘‘Black pit: ‘‘‘ Brown lesions develop on fruit of varying size on lemons.


‘‘‘Black Root Rot (Thielaviopsis basicola) ‘‘‘ Black root rot can cause stunting and yellowing of green-house-grown seedlings. Black, localized, dry lesions up to 1.5 mm in diameter are produced on fibrous roots which may blacken the entire rootlet. Affected roots remain firm and do not disintegrate. Most of the commonly used rootstock varieties are susceptible to black root rot, but certain trifoliate orange selections are tolerant to it.


‘‘‘Black Spot (Phyllostictina citricarpa) ‘‘‘ Leaf symptoms, when present, are small dry spots with a gray center surrounded by a dark brown ring and a yellow halo. Fruit symptoms can be classified as hard spot, freckle spot, virulent or spreading spot, and false melanose. The most reliable diagnostic criterion is the presence of pycnidia, but they do not always appear. Hard spot lesions are the most typical black spot symptoms and sometimes appear before the color changes from green to orange. When this occurs, the area around the lesion turns yellow. On colored fruit, green tissue may remain outside the lesion, which forms as a dark brown circle enclosing a depressed, round, light brown to gray-white center. Freckle spot usually appears after the fruit has colored. Individual freckle spots sometimes coalesce to form one big lesion or even a tearstain, similar to symptoms of melanose. Virulent spot develops late in the season, particularly when the fruit is fully mature and temperatures rise. The best form of control is to remove infected out-of-season fruit and to apply protective materials such as copper fungicides.


‘‘‘Blast and Black Pit (Pseudomonas syringae) ‘‘‘ Blast and black pit are bacterial diseases that occur under cool, wet conditions. Blast lesions most commonly appear on the wings or petioles of leaves, as water-soaked or black areas, which quickly move in both directions, up the mid vein of the leaves and down to the twigs. Drying may occur on the leaves and usually progresses for only a short distance into the twigs. It then becomes surrounded by a reddish brown to chestnut-colored scab or crustlike callus. Occasionally, entire twigs are girdled and killed. Black pit lesions on fruit may be small pits or specks or larger sunken spots from 5 to 20 mm in diameter. They originate as light brown spots in the rind, later become reddish brown, and finally turn black. In locations where the disease is severe enough for control measures, copper fungicides are applied during the winter or spring.

==BLIGHT  ==

‘‘‘Blight: ‘‘‘ The first indication of blight is usually a wilt or a zinc deficiency pattern in the leaves. A general decline of the canopy follows, with leaf loss, twig dieback, and weak growth flushes. The symptoms may initially be restricted to only one sector of the tree canopy. The root systems of trees beginning to decline appear healthy, but drying of the major roots is common in advanced decline. Water is transported poorly in blight-affected trees. Zinc usually accumulates in the bark and outer xylem of the trunk. The cause of blight is unknown. The only known means of control of blight is to replace declining trees with replants on a more tolerant rootstock.


‘‘‘Blight, Greening, Stubborn, Some nutritional deficiencies: ‘‘‘ Interveinal yellowing generally is distributed over the whole leaf blade. Leaves are small and upright and fruit abnormalities may occur in orchards.


‘‘‘Blue and Green Mold Disease (Penicillium spp.) ‘‘‘ Although early infections are almost impossible to detect, the disease is easily recognized once the whitish mycelium and the blue or green spores appear. Blue and green mold usually occur together, although the blue mold often over grows the green mold. To reduce infection with blue and green molds, do not pick wet fruit and handle fruit carefully during picking and transport.


‘‘‘Blue Mold (Penicillium italicum) ‘‘‘ Diseased tissue becomes soft, watery, and slightly discolored and is easily punctured. The lesions do not enlarge as rapidly as those of green mold. A white, powdery growth of mycelium develops on the lesion surface, and soon a blue spore mass forms, leaving only a narrow white fringe of mycelium surrounding the lesion. A pronounced halo of water-soaked, faded tissue surrounds the lesion, between the fringe of mycelium and the sound tissue. The blue spores covering the fruit may become brownish olive with age. The control of blue mold is the same as that of green mold.

==BORON  ==

‘‘‘Boron: ‘‘‘ The most conspicuous and characteristic symptoms of the deficiency are expressed in the fruit which are small, hard, and misshapen and have a thickened peel. Pockets of brownish gum are found in the peel and sometimes also in the core. The deficiency is readily corrected by the application of borax in fertilizer or as soluble compounds in foliar sprays. Boron toxicity can easily occur if too much of the mineral is applied in an attempt to correct a deficiency. The foliar symptom of the toxicity is a yellowing of the leaves, beginning at the leaf tips and gradually extending back along the margins. Gum is secreted on the lower surface of affected leaves. Excess boron may be leached out of the root zone by heavy irrigation or rainfall.


‘‘‘Botrytis Blight (Botrytis cinerea) ‘‘‘ The disease may develop in blossoms or on a twig or limb, appearing first as a yellowish buff spot. Only twigs or branches less than 2 cm in diameter are normally attacked. The fungus is most frequently found attacking flowers and young fruit of lemons. It can reduce fruit-set, injure young fruit, and cause the formation of ridges on the rind, thus lowering the grade of fruit for the fresh market. On lemons growing in the cool coastal areas, control has been achieved by spraying the trees with zineb or copper fungicides before rain or fog is expected.


‘‘‘Botrytis Rot (Botrytis cinerea): ‘‘‘ During prolonged wet periods, the Botrytis fungus, also called gray mold, infects blossoms and also twigs. Petals carrying the fungus often spread the disease when they contact small fruit. At the point of contact, the fungus stimulates cell growth, so that ridges develop. General preventive measures, such as avoiding mechanical or chemical injury, protecting against frost and brown rot, and pruning regularly, help reduce the incidence of Botrytis rot.


‘‘‘Brittle Twig Yellows: ‘‘‘ The causal agent is uncharacterized, but some similarities to stubborn were noted. No stem pitting was observed.


‘‘‘Fertilizer burn, Herbicide injury: ‘‘‘ Brown, sunken lesions develop on low-hanging fruit.


‘‘‘Brown Rot (Phytophthora citrophthora) ‘‘‘ The decay is first observed as a light brown discoloration of the rind. The affected area is firm and leathery, and it retains the same degree of firmness and elevation as the adjacent healthy rind. Delicate white mycelium forms on the rind surface under humid conditions. Infected fruit have a characteristic pungent, rancid odor, which distinguishes this disease from stem-end rots. Cultural practices that help minimize the disease include proper irrigation, pruning low-hanging branches, maintaining adequate soil moisture, and preventing ground vegetation from growing too tall. Copper fungicide sprays applied to the soil and the lower part of the tree canopy prior to anticipated wet weather help to prevent brown rot. Brown rot can be controlled by submerging fruit in hot water (50°C) for 2-4 min. Storage of fruit at about 5°C significantly delays the development of brown rot.


‘‘‘Brown rot fungus: ‘‘‘ Fungus spores on ground are splashed onto fruit on lower branches. Ripe fruit turning light brown and soft. Remove diseased fruit, do not store infected fruit with clean fruit, and a preventive Bordeaux treatment applied before first fall rains can be applied to tree skirts up to 4 feet and ground beneath trees as control measures.


‘‘‘Brown rot gummosis: ‘‘‘ Brown rot gummosis is caused by a fungus that infects the trunk and may spread to crown and woody roots. Leaves turn yellow and drop and beads of sap may be found on trunk which is cracking. Keep the trunk dry and scrape away all diseased bark and a buffer strip of healthy light brown to greenish bark around margins.


‘‘‘Alternaria brown spot: ‘‘‘ Excessive fruit drops on susceptible mandarins and their hybrids.


‘‘‘Bud-Union Creases and Abnormalities: ‘‘‘ Many graft or bud-union creases and abnormalities between scion and rootstock cultivars have been observed. In some cases, the only symptom is a crease or stain at the union, which is apparent only after the removal of a bark patch. In others, there is an obvious overgrowth of the scion or rootstock. Pitting or pegging of the wood and gum impregnation of the bark or wood near the union are common symptoms associated with these disorders. Some affected trees are severely stunted, but in other cases the crease at the union seems not to affect growth. There is no known cause of the problem.


‘‘‘Bud Union Disorders: ‘‘‘ Certain scion-rootstock combinations show an incompatibility reaction, which may appear shortly after grafting or may take 10 to 20 years to develop. A crease or fold forms at the bud union, and with increasing overgrowth, the food-conducting tissue is compressed, resulting in a slow girdling of the tree. When planting or replanting, avoid susceptible combinations.


‘‘‘Bud-Union Incompatibility: ‘‘‘ Some bud-union disorders are known to be caused by virus diseases. Others do not seem to be caused by infectious agents but seem to result from genetic incompatibility between the scion and the rootstock.


‘‘‘Cachexia: ‘‘‘ Symptoms of cachexia in reactive citrus hosts are discoloration and gum impregnation of the bark, which is revealed by scraping away the outer bark or by making tangential cuts to remove the outer bark tissues. The inner bark surface is bumpy, with numerous rounded bumps or pegs, which fit into corresponding depressions in the wood. Warm conditions favor symptom expression. Severely affected trees are stunted and yellow and may decline and die. Tristeza also causes stunting, yellowing, and stem pitting of the rootstock in trees budded on lemons, but the pits are sharper and less gum is produced. Infection in new propagations can be avoided by the use of disease-free bud wood. Discoloration and gum impregnation of bark, in distinct pockets, with rounded pegs grow from the bark into, the wood, on sweet lime, lemons, and some tangelos.


‘‘‘Calcium: ‘‘‘ Calcium deficiency rarely occurs in field-grown citrus. It does not produce distinctive foliar symptoms, but it can significantly limit overall tree growth and reduce fruit yields. In some areas, soluble calcium compounds in irrigation water contribute significantly to the supply required by citrus trees.


‘‘‘California red scale: ‘‘‘ Round, red brown scales develop on the fruit, leaves and twigs. Leaves may turn yellow and drop and twig dieback may occur.


‘‘‘Cancroid Spot: ‘‘‘ On leaves, the symptoms are insignificant and appear as numerous small, white spots. On fruit, much larger spots are found, with a striking resemblance to the pustules formed by canker.


‘‘‘Canker (Xanthomonas campestris pv citri) ‘‘‘ Canker lesions start as pinpoint spots and attain a diameter of 2-10 mm. The lesions are initially circular but later may develop irregularly. They are often aggregated at the leaf margin or the leaf tips or in a restricted area of the leaf and all lesions on a leaf tend to be about the same size. On fruit, the lesions can vary in size, because the rind is susceptible for a longer time and further infection cycles can occur. Young lesions are raised or pustular, particularly on the lower leaf surface. The pustules later become corky and crateriform, with a raised margin and a sunken center. A characteristic symptom of the disease on leaves is the yellow halo that surrounds lesions. In countries where the disease is severe, only the more resistant types of citrus may be profitable. Leaf injury from wind-blown sand can be reduced by allowing vegetation to grow between rows of trees. Where canker is a major problem, control requires appropriate cultural practices plus timely use of copper sprays. More sprays are generally required to provide acceptable control on grapefruit, which are very susceptible to canker.


‘‘‘Carpenterworm: ‘‘‘ Damage is caused by boring larvae of a large moth and the limbs become girdled. Conspicuous sap and sawdust can be seen coming out of the branches and trunk. Apply commercial preparations of the entomopbagus nematode, Neoplectana carpocapsae, to burrows according to product label.


‘‘‘Cercospora Leaf and Fruit Spot (Cercospora angolensis) ‘‘‘ On leaves, the fungus produces circular, mostly solitary spots, up to 4 mm in diameter, with a grayish brown, shriveled center and brown border, surrounded by a prominent yellow halo. The lesions are flat or shrunken. When the disease is severe, a major part of the leaf blade may be yellow and premature leaf abscission is common. Sometimes the dry center falls out, creating a shot-hole effect. On fruit, the spots are discrete or grouped together and mostly small. They have a slightly sunken and brown center and, on green fruit, a yellow halo. Copper fungicide sprays have provided satisfactory control at some locations.


‘‘‘Chemical injury: ‘‘‘ Brown or black discoloration appears on the side of the fruit exposed to the sun and gray, brown, or black, half-circular or irregular markings are seen on the fruit.


‘‘‘Chilling Injury: ‘‘‘ Chilling injury occurs most frequently on grapefruit, lemons, and limes and appears as sunken pits on the peel surface. Chilling injury can be avoided by storing and transporting fruit at higher temperatures, but this increases the risk of decay. Partial control can be achieved by applying wax or certain film wraps to decrease moisture loss from the rind or by treating the fruit with benzimidazole fungicides. The risk of injury is reduced by lowering the storage temperature slowly and by raising it intermittently during cool storage.


‘‘‘Chimeras: ‘‘‘ At times, a mutation in part of a tree results in the growth of one or several twigs with characteristics different from the rest of the tree. The leaves on these twigs may have a different shape, size, or color variegation. Or the mutation results in the development of multiple buds, creating a bunch growth or witch's broom. Certain mutations also affect the shape or color of fruit. Pruning abnormalities is the best form of control.


‘‘‘Chimeras: ‘‘‘ If a mutation occurs in a branch or twig and survives, it may develop into a new twig with characteristics different from the rest of the tree, called a chimera or sport. New characteristics may affect the color of the rind or pulp or the shape of the fruit, often resulting in bizarre forms, for example, the so-called cock's comb. The mutation may affect the shape, size, or color variegation of leaves.


‘‘‘Citrus Blast (Pseudomonas syringae): ‘‘‘ Citrus blast is restricted mainly to citrus-growing areas, where wet, cool, and windy conditions during winter and spring favor development and spread of the blast bacterium. The bacterium infects small injuries caused by thorn punctures, wind abrasions, or insect feeding. Infections usually start as black lesions in the leaf petiole and progress into the leaf axil. Once the petiole is girdled, leaves wither, curl, and eventually drop and entire twigs may die back and diseased areas are covered with a reddish brown scab. Infections result in small, black spots on the fruit. Preventive treatment against citrus blast alone is generally not economical, but sprays against brown rot or Septoria may provide some protection against citrus blast. Certain cultural practices can reduce the incidence of citrus blast which include planting windbreaks and using bushy cultivars with relatively few thorns to help prevent wind injury, pruning out dead or diseased twigs in spring after the rainy period to reduce the spread of the disease, and scheduling fertilization and pruning during the spring or early summer to prevent excessive new fall growth, which is particularly susceptible to blast infection.


‘‘‘Citrus bud mite injury: ‘‘‘ Citrus bud mite is a very small, barely visible yellow mite. It infects plants by creating oddly misshapened flowers and fruit. The mites can be controlled with a petroleum spray during May and June or September through November.


‘‘‘Citrus cutworm: ‘‘‘ The citrus cutworm is a brown to green, smooth-skinned caterpillar with a prominent white stripe on each side. It curls up when disturbed and feeds on the surface of holes in blossoms of very young fruit. The damage usually occurs in the spring and natural enemies are often effective.


‘‘‘Citrus red mite: ‘‘‘ Citrus red mite is tiny, barely visible without a hand lens, found mostly on young leaves. The leaves and green fruit have a pale yellow stippling but no webbing occurs. Natural control should usually be sufficient in unsprayed backyard trees. An oil spray made between August and September will control problems in most areas.


‘‘‘Citrus rust mite injury, Melanose, Anthracnose: ‘‘‘ Citrus rust mite creates tearstain markings on the rind.


‘‘‘Citrus thrips: ‘‘‘ Citrus thrips are a tiny yellow insect about 1 mm long and is very active. A ring or partial ring of scarred tissue appears around the stem on fruit skin. Young leaves may be deformed and scarred. Botanical pesticides ryania or sabadilla can be used in commercial orchards if the thrips are a serious problem.


‘‘‘Citrus Yellow Mottle: ‘‘‘ A graft-transmissible agent associated with veinclearing and symptoms like those of ringspot have been reported in scattered satsuma mandarin trees, causing a disease described as citrus yellow mottle. It is graft-transmitted to a wide range of citrus cultivars and infection in citrus is associated with a rod-shaped particle, with dimensions of 12-14 X 690-740 nm.


‘‘‘Concave Gum: ‘‘‘ Bark scaling or external gumming is uncommon, but gum is deposited in concentric rings in the wood of affected trees and may be present at the base of the concavity or pocket when the bark is removed. In the spring, young leaves of trees affected with concave gum or blind pocket may develop yellow flecking or oak leaf patterns. Some healthy mandarins and mandarin hybrids may exhibit trunk deformities or depressions similar to those caused by concave gum or blind pocket, but not leaf symptoms. Concave gum disease does not cause the bark to become abnormally thickened over depressions in the wood. The principal control measure is the use of disease-free propagating materials.


‘‘‘Cool weather, insufficient irrigation, weak trees: ‘‘‘ Choose varieties suited for your area that do not require cross pollination and follow guidelines for irrigating and fertilizing to deal with cool weather problems.

==COPPER  ==

‘‘‘Copper: ‘‘‘ Typical symptoms of copper deficiency are excessive growth of young shoots, resulting in enlarged leaves on long, willowy branches. Pockets of brownish gum may form on the stem, and affected twigs commonly die back from the tips. The symptoms on fruit are brownish, corky lesions on and in the peel. Copper toxicity can occur where excessive amounts of the mineral accumulate in the soil. It can greatly reduce the growth and vigor of citrus without causing visible symptoms in the foliage. Copper toxicity can be prevented by maintaining the soil pH at 6.0-6.5 or higher.


‘‘‘Cottony cushion scale: ‘‘‘ Cottony cushion scale is characterized by newly hatched nymphs that are red and found on leaves and twigs, older scales are on twigs and covered with a cottony secretion. Eggs are in a fluted white egg sac about 1/2 inch long. Fruit and leaves are covered with honeydew and sooty mold. The scale becomes a pest when its natural enemies, including vedalia beetle and a parasitic fly, are destroyed by insecticides. Reestablish natural enemies and avoid use of insecticides.


‘‘‘Cottony Rot (Sclerotinia sclerotiorum) ‘‘‘ Infected fruit first develops a discoloration and a slight softening of the peel. The peel changes from lemon yellow to straw yellow and later to yellowish brown. In a humid environment, a fluffy, white, cottony mycelial growth covers the fruit, and black sclerotia are produced. In the cool, moist environment of the lemon storage room, the fungus spreads rapidly by contact, eventually attacking most of the fruit in the box. For orchards with cover crops, the obvious preventive measure is to switch to clean cultivation.


‘‘‘Creasing: ‘‘‘ Creasing consists of grooves or furrows in an irregular pattern in the rind. Thin-skinned fruit are especially susceptible to creasing as well as subsequent splitting. Creasing is thought to be related to a complex of preharvest factors, including potassium nutrition, but the causes are not fully understood. Preharvest sprays of gibberellic acid have been reported to reduce creasing.


‘‘‘Crinkle Scurf: ‘‘‘ The term crinkle refers to the twisting of leaves, and scurf refers to the corky banding that develops on the trunk and branches of affected trees.


‘‘‘Cristacortis: ‘‘‘ The disease causes conspicuous wood pitting, but its effects on yield are unknown. The cachexia viroid also produces wood symptoms in some hosts affected by cristacortis, but not in sour orange. Cristacortis also causes leaf patterns similar to those occurring on trees affected by concave gum. The disease is transmitted by propagation and by grafting. The use of disease-free budwood is the only recommended control.


‘‘‘Damping-Off (Rhizoctonia solani) ‘‘‘ The typical damping-off symptom results from fungal penetrations just above the soil line. However, damping-off fungi can also cause a seed rot or a preemergence rot, resulting in sparse stands of seedlings. Adequate control of damping-off depends on avoiding the use of infested soils or eliminating the causal pathogens from seed beds. Care must be taken to avoid reinfesting treated soil. Seed should be subjected to hot-water treatment.


‘‘‘Robinson dieback: ‘‘‘ Dieback of some twigs and limbs on otherwise vigorous Robinson tangerine trees, often with gumming at the border between dead and living bark


‘‘‘Diplodia-Associated Diseases: ‘‘‘ Even though it plays a primary role in the development of Diplodia stem-end rot of harvested fruit, it is doubtful whether this fungus can initiate disease symptoms in citrus trees. D. natalensis is commonly present in dead bark and wood, but it is usually considered a secondary invader, following the weakening of twigs or limbs by unfavorable environmental conditions, such as extreme cold, flooding, drought, or mechanical damage. Sometimes the dieback proceeds into the main trunk, and the whole tree dies. Spraying the trees with benomyl reduced the incidence of dieback, which further suggested the involvement of this fungus.


‘‘‘Diplodia Stem-End Rot (Physalospora rhodina) ‘‘‘ Diplodia stem-end rot is rarely observed on fruit attached to the tree, even when they are mature. The fungus becomes active at the stem end of the fruit and rapidly penetrates the rind and core. Occasionally, the decay develops at injuries on the side or stylar end of the fruit. The fungus progresses rapidly through the spongy central axis of the fruit, usually reaching the stylar end much sooner by this route than through the rind. Decayed tissue is initially firm and later becomes wet and mushy. Surface mycelium appears only in advanced stages of infection in very moist environments. Good cultural practices can aid in the control of Diplodia stem-end rot by producing thrifty trees with minimal amounts of dead wood. Harvesting by pulling rather than clipping can reduce the incidence of decay. The growth regulator 2,4-D, applied in water and wax formulations, has been used to retard the senescence of the button and, therefore, the entry of the pathogen. Significantly more Diplodia stem-end rot develops if the ethylene concentration used in degreening is too high. Post harvest application of benzimidazole fungicides provide good control of Diplodia stem-end rot.


‘‘‘Disease Diagnosis: ‘‘‘ Some diseases can be identified relatively easily by the symptoms present and others require a search for microscopic causal agents or appropriate laboratory analyses and testing before reliable diagnosis can be made. The causal agents of many diseases do not reside where the visual symptoms are apparent. Growers need to be aware of this possibility when collecting and submitting samples to a diagnostic laboratory for disease identification. Often, it is impossible for a plant pathologist to draw any conclusions about a problem without observing the disease in the field and obtaining information on the local topography, soil conditions, cultural practices, types of rootstock, or other possible contributing or associated factors. Fungal organisms with sporing structures are often observed in dry citrus tissue, but these may not belong to the causal agent. Secondary invasion of dry tissue by other organisms is common, either from external sources or from symptomless fungal infections that are already present. Diseases that produce distinctive and specific symptoms, such as most diseases caused by bacteria and fungi and certain diseases caused by viruses and viruslike agents, can be reliably identified in the field once the observer becomes sufficiently familiar with them. It is important to eliminate poor management practices, edaphic factors, genetic defects, or chemical injuries as potential causes of the symptoms.


‘‘‘Disease Occurrence and Spread: ‘‘‘ The occurrence and severity of biotic diseases are determined by the virulence of the pathogen, the susceptibility of the host plant, and local environmental conditions. In addition, with some diseases caused by viruses or viruslike agents, a vector may be required for transmission of the disease agent from one tree to another. The occurrence and severity of most fungal and bacterial diseases that attack fruit and leaves are greatly influenced by climate. In most arid and semiarid climates, these diseases are generally unimportant, and their causal agents may be unable to cause infection or survive. Some fungal diseases occur only where temperatures are low during periods of tree or fruit wetting. Other fungal diseases present problems only in regions with frequent rainfall or prolonged high humidity accompanied by relatively high temperatures. Overall, fungal diseases of foliage and fruit are more prevalent in regions with summer rainfall than in those with winter rainfall.


‘‘‘Dothiorella Gummosis (Dothiorella gregaria) ‘‘‘ Cankers consisting of small to large areas of dead bark form on limbs or trunks. The inner bark tissue becomes chocolate brown and often disintegrates, leaving elongated cavities and grooves in the inner bark next to the cambium, which may ooze gum. Actively growing citrus wood exhibits considerable resistance to this disease. Sunburn, frost injury, wounds, unfavorable environmental conditions, or other debilitating factors appear to be necessary for the development of the disease. Infected branches should be pruned back well below the canker during dry periods.


‘‘‘Dothiorella Rot (Dothiorella gregaria) ‘‘‘ This disease causes a leathery, pliable stem-end rot of citrus fruits. Dothiorella rot forms broad bands of brown discoloration in the rind, similar to the discoloration due to Diplodia stem-end rot. It is generally of minor importance on citrus. The pathogen also causes a minor disease of citrus trees.


‘‘‘Drought: ‘‘‘ Inadequate soil moisture can result in rolled leaves, defoliation, dieback, and dried up leaves. The symptoms appear as yellow areas on the leaf blade between the midrib and the leaf margins and these yellow areas later turn gray to light brown. The disorder seems to occur mostly on young leaves when a sudden change in the soil-moisture status occurs at low atmospheric humidity. The destruction of surface roots by disking or ploughing when moisture is deficient may also contribute to mesophyll collapse.


‘‘‘Dry Root Rot (Fusarium solani) ‘‘‘ The general symptoms are similar to those caused by other agents that damage the roots or girdle the trunk. These include reduced vigor, dull green leaf color, poor new growth, and twig dieback. If extensive root damage occurs, the leaves suddenly wilt and dry up on the tree. Damage usually starts in larger roots and spreads into the crown creating moist, dark decay that adheres to the wood. The wood below the dead bark is hard, dry, and stained grayish brown to purple. Once the crown region is girdled, the tree collapses. Good orchard management, especially careful irrigation, is essential for preventing dry root rot. If the soil around the tree crowns and roots is saturated for long periods of time, the chances for injury and subsequent fungal infections increase. Injury to roots and trunk, especially during the cool and wet season, provide entry wounds for infection. Carefully follow label instructions for applying fertilizer, herbicides, and other pesticides to prevent injury.


‘‘‘Brown rot: ‘‘‘ Excessive fruit drop occurs following unusually long periods of rainfall and fruit wetting.


‘‘‘Exocortis: ‘‘‘ Exocortis is of minor importance in many areas today because strict regulations on budwood sources have kept new plantings largely free of this viroid disease. The characteristic symptom of exocortis is the shelling of susceptible rootstocks. The viroid kills the bark, which dries, cracks, and may lift in thin strips. Some droplets of gum-often appear under the loose bark. Sometimes lesions develop slowly and shelling is restricted to a small area for several years and at other times, the disease develops rapidly over the entire rootstock. Infected trees rarely die, but growth is retarded and productivity slowly declines. For planting or replanting, it is best to use viroid-free budwood available to nurseries or growers through the Citrus Clonal Protection Program. It is best to remove infected trees from the orchard because pruning clippers and saws can transmit exocortis unless thoroughly disinfected with hypochlorite (bleach) and heat does not kill the viroid.


‘‘‘Fatal Yellows Disease: ‘‘‘ Fatal yellows disease causes veinclearing, vein corking, leaf epinasty, shoot distortions, and stunting in alemow seedlings and similar but generally milder symptoms in some other citrus cultivars.


‘‘‘Felt (Septobasicium pseudopedicellatum) ‘‘‘ In humid climates, twigs may be covered with a soft, leathery, felt-like fungal growth. The fungus commonly appears in association with certain scale insects, which may provide secretions for the fungal growth. The disease generally seems harmless, since there is no fungal penetration of the bark. Control measures consist of pruning out affected branches and spraying the trees with a copper fungicide to prevent further occurrences.


‘‘‘Fertilizer Burn: ‘‘‘ Heavy deposits of fertilizer on leaves and fruit can cause a burn, particularly if their surfaces are wet at the time of contact.


‘‘‘Fig scale: ‘‘‘ Fruit has warty appearance and shaped scales appear on leaves, twigs or fruit. The best forms of control include dormant season sprays with superior type oil and parasites are often effective.


‘‘‘Flowering Parasites: ‘‘‘ Flowering parasites commonly known as mistletoes develop from seeds that germinate on the bark and send rootlike suckers into the twigs of citrus trees. Mistletoes have leaves and stems containing chlorophyll and are therefore dependent on the host only for water and some of their food requirements. These parasites are more prevalent in the tropics and subtropics and are found mostly in trees that have thin canopies because of neglect or unfavorable growing conditions. Branches with mistletoe should be pruned out to rejuvenate the tree and to eliminate seed, which could threaten other trees in the vicinity. Another type of flowering parasite starts in the ground as a seedling and later grows like a vine into the canopy of the tree. Once in the canopy, the vine sends suckers into the bark of twigs, and the connection with the ground is then severed. These flowering parasites are leafless and have little or no chlorophyll, and they form stringlike, yellowish brown branching growths over the tree canopy. Gall formation is sometimes induced where suckers of the parasite penetrate twigs. Cultivation under the trees helps to destroy seedlings that arise from previous infestations.

==FOVEA  ==

‘‘‘Fovea: ‘‘‘ Fovea was originally described as a disease causing a decline and death of Murcott tangor trees. The affected trees also had inverse stem pitting. Later, it was shown that inverse stem pitting in Murcott could also be induced by inoculation with tissue from cachexia-affected plants. The causal relationship between inverse pitting and tree decline in Murcott has not been clarified, partly because tree decline from periodic overcropping is also common in this cultivar.


‘‘‘Freeze Injury: ‘‘‘ Cold damage is caused by frosts and freezes. During a radiative frost, conditions are calm and clear, and heat is lost rapidly by radiation. During advective freezes, or windy freezes, winds blow in a cold air mass. Advective freezes are often more damaging to orchards than radiative frosts, because of the rapid heat loss from the trees. Though often there is no external evidence, freeze injury causes excessive water loss, drying of the juice sacs, and separation of the segments. Mild frosts cause water-soaking of leaves or temporary leaf curling. Sometimes, when a freeze is not severe enough to kill the whole leaf, small dry spots develop on the leaf blade. With lower temperatures or increasingly long periods of freezing, progressively more injury occurs, consisting of leaf drop, death of twigs and branches of increasingly larger diameters, and even death of the trunk, if the wood has been killed, leaves often dry and remain on the branches for several weeks before eventually falling. Cold damage can be reduced by planting on elevated ground, so that cold air can drain away, planting freeze-tolerant scions and rootstocks, planting near lakes or warm bodies of water, reducing weed cover in winter, to allow for soil heating and cold-air drainage, providing adequate soil moisture, and maintaining a dense canopy with proper cultural practices.


‘‘‘Frost Damage: ‘‘‘ Fruit is most susceptible to frost damage, although twigs and leaves may be affected. On fruit, the oil from ruptured oil cells corrodes the rind surface, causing watery, brownish specks or pits called ice marks. Frost damage mainly appears on the outside of fruit exposed to radiation frost, and the pulp underneath the ice marks ultimately dries. Ice marks also provide entry sites for decay organisms. Certain cultural practices can help reduce the impact of frost.


‘‘‘Anthracnose (Colletotrichum gloeosporioides) ‘‘‘ Anthracnose symptoms associated with bruised or injured rind are brown to black spots, 1.5 cm or more in diameter. The decay may appear firm and dry and under humid conditions, the masses of spores on the lesion surface appear pink or salmon-colored, under drier conditions, they appear brown to black. Good cultural practices aid in anthracnose control by reducing the amount of dead wood available for inoculum production. Fruit should be handled carefully to avoid injury and should not be held too long in storage. Washing the fruit after harvest removes at least some of the quiescent appressoria present on the rind surface, and this reduces the risks of subsequent penetration into the rind. Preharvest sprays of ethephon are useful to enhance the orange color of the fruit without rendering the rind more prone to anthracnose. Postharvest applications of thiabendazole or benomyl and storage of the fruit at temperatures below 10°C also help to control the disease.


‘‘‘Fumigation-Induced Stunting: ‘‘‘ Treatment of nursery soils with fumigants such as methyl bromide, chloropicrin, metam-sodium, methyl isothiocyanate and dichloropropene reduces or eliminates beneficial vesicular arbuscular mycorrhizal fungi that are associated with roots and required for normal growth of citrus seedlings. Symptoms of fumigation-induced stunting are poor growth of seedlings and small, yellow leaves, which become dry at the edges and abscise prematurely. The roots of affected seedlings appear normal but are not colonized by vesicular-arbuscular mycorrhizal fungi. Fumigation-induced stunting may be avoided by reducing fumigant rates to levels that eliminate soilborne pathogens in the rooting zone but preserve some mycorrhizal fungi deeper in the soil, for recolonization. Rapid buildup of mycorrhizal fungal populations can be achieved by fumigating well ahead of planting time and establishing a cover crop, which is disked in prior to planting with citrus.


‘‘‘Fungal Diseases in Nurseries and Orchards: ‘‘‘


‘‘‘Fusarium Rot (Fusarium spp.) ‘‘‘ Fusarium rot develops slowly and is most important on fruit in lengthy storage. Infected tissue is leathery, beige to light or dark brown, and sunken. Under humid conditions, a white surface mycelium appears. The core of infected fruit is white, beige, or pink, depending on the species of Fusarium involved. Control has been obtained by applying 2,4-D and imazalil in wax to the fruit before packing and storage. Fusarium stem-end rot has also been effectively reduced by providing adequate ventilation in storage.


‘‘‘Fusarium Wilt (Fusarium oxysporum) ‘‘‘ The first symptoms of Fusarium wilt are a mild, reticulate yellowing and epinasty of young leaves, followed by wilt, leaf abscission, and dieback of young twigs. Gum exudation and vascular discoloration frequently occur on the stems of dying plants. Highly susceptible species are usually killed by the disease and less susceptible species exhibit mild reticulate yellowing and wilt and are stunted, but they seldom die from the disease. Frequent soil drenches with benomyl have provided some control. The most effective control measure is to exclude the pathogen from greenhouses by not introducing infected plant material or infested soil.


‘‘‘Granulation: ‘‘‘ Granulation, sometimes known as crystallization, is a condition in which the juice sacs shrivel because of gel formation rather than because of desiccation. In Valencia orange, tangerines, and tangerine hybrids, granulation occurs mostly at the stem end of the fruit, whereas in navel oranges, it generally extends through the center of the fruit. Gelled vesicles have higher magnesium and calcium contents than unaffected vesicles. It occurs more frequently on fruit grown in the sun than on fruit grown in the shade and seems to be correlated with the amount of heat to which the fruit is exposed during the growing season.


‘‘‘Rind Stipple Of Grapefruit: ‘‘‘ During prolonged periods of cold, wet weather, the rind of grapefruit develops small, brownish pits. The pits often have concentric rings around them or they coalesce into larger, irregular lesions. If the fruit is injured while green, a green halo appears around the pits and persists until the fruit has turned yellow. The symptoms occur mostly on exposed fruit on the north side of the tree. No treatment is available.


‘‘‘Grasshoppers and Katydids: ‘‘‘ Grasshoppers and katydids feed on the rind of very young fruit and on leaves. The chewing damage on fruit is very shallow and becomes covered with scar tissue.


‘‘‘Gray Mold (Botrytis cinerea) ‘‘‘ Rot associated with gray mold on lemon fruit appears as a brown, leathery decay, similar to cottony rot, Trichoderma rot, and brown rot. The odor of Botrytis-infected fruit is not as distinctive as that of fruit with brown rot or Trichoderma rot. At high humidity, distinctive patches of gray-brown to olive spore masses appear on the fruit surface. The pathogen spreads readily by contact with adjacent fruit, giving rise to large nests of diseased fruit in packed containers. Gray mold on lemon trees is difficult to control with fungicide applications, because the trees bloom over a long period. Excessive humidity in storage rooms encourages the growth of Botrytis on decayed fruit and promotes contact spread.


‘‘‘Greasy Spot (Mycosphaerella citri) ‘‘‘ Leaf symptoms first appear as a yellow mottle on the upper side of the leaf, with a matching, slightly raised, pale orange to yellowish brown blister on the lower surface. Later, affected areas of the leaf become darker brown, or even black, and greasy in appearance, and much of the yellowing disappears. Leaves often drop long before the lesions develop the dark, greasy symptom that gives the disease its name. The symptoms on fruit rind are dry specks in the parts of the epidermis between oil glands. These specks do not generally begin to appear until 3-6 months after infection. They consist of dead guard cells and varying numbers of cells in the vicinity of the substomatal chamber. The lesions are pink at first and later become brown or black. Spray oil and certain fungicides are used to control greasy spot. Thorough coverage of the underside of the leaf is essential with all materials. Copper fungicides have proven to be the most reliable materials for controlling the disease in Florida, generally providing better control than oil alone.


‘‘‘Greasy-Spot-Like Diseases: ‘‘‘ The diseases referred to here as greasy-spot-like diseases all produce leaf symptoms similar to those of greasy spot. In addition, they all induce cell enlargement in the spongy mesophyll, thus causing the lower epidermis to become raised and blister-like. Eventually, the affected areas become dark brown or black, because of gum formation within the leaf. The etiology of these diseases is still uncertain. A disease known as greasy spot, is apparently different from pseudo-greasy spot. Although generally of minor importance, it has caused premature defoliation, particularly of grapefruit, in years of above-average rainfall. The symptoms first appear as black specks, up to 0.5 mm in diameter, on the lower leaf surface. They are surrounded at this stage with a distinct yellow halo. Later they become blister-like and enlarge to a diameter of up to 3 mm, and the yellow halos become less distinct. A disease resembling greasy spot but of uncertain etiology also occurs in Argentina. The first symptoms of the disease appear in early winter as small blisters and dry stomata on the underside of leaves. Severe leaf drop can then follow through the winter and into the spring. A single application of copper fungicide in the fall has shown promise as a control measure.


‘‘‘Greasy spot rind blotch, Chemical injury, Citrus rust mite injury: ‘‘‘ These diseases are characterized by superficial brown or black discoloration of the rind, consisting of discrete or coalescent groups of dead, cells between oil glands. These discolorations are associated with the death of guard cells and nearby epidermal or substomatal cells.


‘‘‘Green fruit beetle: ‘‘‘ Green fruit beetle is characterized by the surface of the fruit is scarred and large green beetles with a yellow band around the margins of wings are present.


‘‘‘Green Mold (Penicillium digitatum) ‘‘‘ Initial symptoms of green mold are similar to those of sour rot and blue mold. In the early, pinhole stage, the decay appears as a soft, watery, slightly discolored spot, 6-12 mm in diameter. White mycelium appears on the rind surface, and after it reaches a diameter of approximately 2.5 cm, olive green spores are produced. The sporulating area is surrounded by a broad zone of white mycelium and an outer zone of softened rind. The entire fruit is soon encompassed by a mass of olive green spores, which are easily dispersed if the fruit is handled. shaken, or exposed to air currents. If the relative humidity is low, the whole fruit shrinks to a wrinkled, dry mummy. If the relative humidity is high, other molds and bacteria become involved, and the fruit collapses into a soft, decomposing mass. Careful picking and handling of fruit minimizes injuries to the rind and the risk of green mold. Sanitary practices should be applied to prevent sporulation on diseased fruit and the accumulation of spores on equipment surfaces and in the atmosphere of packing and storage facilities. Benomyl can be applied as an orchard spray up to 3 weeks before harvest or as a post harvest treatment in the packinghouse. Immediate cooling after packing significantly delays the development of the disease, particularly if combined with effective fungicide treatments.


‘‘‘Greening: ‘‘‘ When trees are first infected, leaves may develop vein yellowing or yellow mottling of all or part of the leaf blade. On chronically infected trees, leaves are small and frequently have zinc deficiency symptoms, with green veins and yellow interveinal areas. Greening differs from tristeza in that it does not produce the characteristic stem-pitting symptoms of tristeza on indicator hosts. Fruit on infected trees are frequently small, underdeveloped, and poorly colored. The juice of affected fruit is low in soluble solids, high in acids, and abnormally bitter. Greening can be tentatively identified in the field by foliage and fruit symptoms. Regulation of propagating materials is needed to prevent the introduction of greening to new areas, especially where the vectors exist or could become established. In countries where greening is endemic, control strategies include control of vector populations, reduction of inoculum sources, and, where possible, planting in environmental zones unfavorable for natural spread.


‘‘‘SYMPTOMS AFFECTING GROWTH HABIT AND YIELD: ‘‘‘ Three quite serious citrus diseases affect overall growth habit and yield, often without producing distinctive symptoms on the trunk, roots, or leaves. Stubborn disease is caused by a mycoplasma, tristeza is caused by a virus, and lemon sieve tube necrosis is an inherited disorder of lemon trees.


‘‘‘Cachexia: ‘‘‘ Numerous small, rounded bumps or pegs appear in the bark and fit into the wood, in susceptible scion or rootstock cultivars.


‘‘‘Gum Pocket and Gummy Pitting: ‘‘‘ The symptoms are pockets of gum in the bark and wood tissues of the trifoliate orange rootstock followed by drying of the bark in severely affected areas. Trees are frequently stunted. The causal agent is graft-transmitted but uncharacterized. The disease is avoided by the use of disease-free budwood.


‘‘‘Gummy Bark: ‘‘‘ The disease causes cachexia-like symptoms in sweet orange, but not in Orlando tangelo, which is reactive to cachexia. The causal agent remains uncharacterized, and its relationship to the cachexia viroid is unknown. The use of disease-free budwood has been recommended as a preventive measure.


‘‘‘Hail Injury: ‘‘‘ Young leaves and shoots can be severely tattered by hail, and if the hailstones are large enough, they can rupture fruit rind and even shred bark. Smaller hailstones cause only shallow injury to the rind, and the affected areas soon heal over. If the fruit is still young at the time of injury, the hail-induced scabby areas expand as the fruit enlarges.


‘‘‘Hassaku Dwarf: ‘‘‘ Infected trees are stunted and nonproductive and develop extensive stem pitting. Only certain isolates of citrus tristeza virus cause severe symptoms in Hassaku.


‘‘‘Herbicide Injury: ‘‘‘ Herbicides are very useful for controlling weeds in nurseries and orchards, but great care is needed in their use. Paraquat: Injury from paraquat is caused by direct spray contact with foliage, fruit, and stem tissue that is still green. The injury appears as yellow and dry spots of variable size and shape on leaves and dry areas on green stems. Glyphosate: Defoliation and twig dieback can occur, and the leaves on new growth flushes are small, narrow, and strap-shaped. Later growth flushes may also be affected, because of the long persistence of this chemical in the tree. 2,4-D: Symptoms of 2,4-D injury may occur as a result of direct spray application, drift from adjacent sprayed areas, or soil application. Symptoms on new shoot growth are a curling and rolling of the leaves, with much distortion. Diuron: Injury from root uptake of diuron first appears on leaves as a clearing of the major veins. Leaves injured by direct contact with diuron turn entirely yellow and abscise. Bromacil: Bromacil can cause injury by root up take as well as by foliar contact. Mild injury to leaves appears as a yellowing confined to the veins. More severe injury, such as that caused by direct uptake through sprayed leaves, appears as a more general yellowing or bronzing of the leaves. Simazine: Uptake of simazine through roots can cause leaves to develop an interveinal yellowing, similar to that caused by some nutritional deficiencies. Norflurazon: Injury due to root uptake of norflurazon causes a pale yellowing of the midrib, which later extends through the major veins. Injury caused by direct contact with leaves appears as white spots.


‘‘‘Impietratura: ‘‘‘ The disease reduces yield and fruit quality but does not cause tree decline. Fruit on affected trees are often smaller than normal, and gum pockets frequently appear in the albedo under either sunken or raised areas of the rind, which may also become hardened. These areas do not degreen normally as fruit ripen. Severely affected fruit often drop before those of boron deficiency and certain insect injuries. The recommended control is the use of disease-free bud-wood, which can be obtained by thermotherapy or shoot tip grafting.


‘‘‘Infectious Variegation (CVV) ‘‘‘ Lemons, sour orange, Etrog citron, and grapefruit usually develop yellow leaf symptoms and distortion, which persists in mature foliage. Mild isolates may produce little or no distortion and only a mild mottle or pinpoint yellow flecking. Infected trees may be stunted, and some fruit may be distorted or have yellow patterns. Symptoms are less apparent in warm weather. The use of virus-free bud wood is the only control currently recommended.


‘‘‘Inherited Abnormalities and Weaknesses: ‘‘‘ Various disorders of citrus are not infectious and are classified as genetically induced abnormalities or weaknesses. Some of these disorders appear to express themselves regardless of environmental conditions. Others, such as Robinson dieback and tangerine collapse, seem to result from a combination of genetic weaknesses and environmental stress.


‘‘‘Chemical injury: ‘‘‘ Chemical injury causes superficial gray, brown, or black blotches on fruit.


‘‘‘Water injury: ‘‘‘ Dead bark is evident above the ground at the base of the trunk or in crown roots. Sloughing of bark or roots is also characteristic that has a disagreeable smell.

==IRON  ==

‘‘‘Iron: ‘‘‘ In many citrus regions, iron deficiency is caused by the unavailability of the mineral in the soil, and not by an insufficient supply of total soil iron. Iron is more available in acid soils than in neutral or alkaline soils. The most prominent symptom is a characteristic yellow leaf pattern, which first appears on young, expanding leaves. The entire leaf appears light green or yellow, except for the fine network of midrib and veins, which remain a distinct dark green. The deficiency can usually be corrected with soil applications of iron chelates.


‘‘‘Leaf Curl: ‘‘‘ Infected trees are affected with severe leaf curling, dieback, and decline and must be destroyed.


‘‘‘Leaf Miners: ‘‘‘ Larvae of leaf miners burrow just under the epidermis and produce irregular tracks on the leaf blade and fruit.


‘‘‘Leaf Rugose (CLRV) ‘‘‘ Leaf rugose develops leaf puckering, pinpoint yellow spotting, and stunting. The virus can also be transmitted mechanically to an extensive range of non-citrus plants, most of which are local lesion hosts or support a latent systemic infection. The use of disease-free budwood and decontamination of pruning tools are recommended to avoid further spread of leaf rugose.


‘‘‘Leafrollers: ‘‘‘ New leaves have holes and are webbed and rolled together.


‘‘‘Leathery Leaf: ‘‘‘ Its experimental host range includes a wide range of citrus species and a number of herbaceous hosts. The symptoms are similar to those of satsuma dwarf in many hosts and psorosis-like vein flecking is also observed.


‘‘‘Alternaria Leaf Spot of Rough Lemon (Alternaria citri) ‘‘‘ Lesions form on leaves, varying from large, dry, blighted areas to small, circular spots. The dry areas are usually surrounded by an extensive yellow halo. Lesions tend to extend out along veins. Stem infection and the defoliation that commonly follows infection of the leaf blade cause dieback of the shoot apices. Severe attacks lead to the development of plants with short internodes and excessive branching, which are difficult to bud. Leaves that do not abscise often exhibit shot-hole symptoms. In some countries, spraying with copper fungicides is needed to protect rough lemon seedlings from attack.


‘‘‘Lemon Sieve-Tube Necrosis (LSN) ‘‘‘ Lemon sieve-tube necrosis is restricted to certain lemon clones and rootstocks. Beginning when trees are 3-4 years of age, a drying appears in some of the sieve tubes. There is usually a bud-union overgrowth, with ultimate decline and death of the tree. Only certain budlines are affected by this inherited disorder.


‘‘‘Leprosis and Zonate Chlorosis: ‘‘‘ Leprosis lesions are yellow at first, with or without a dry center. Later they develop into flat or somewhat raised dry areas on leaves and twigs and flat or depressed areas on fruit. The lesions may contain concentric patterns and be gum-impregnated, and they are usually surrounded by a yellow zone. Leaves and fruit abscise when lesions are abundant. Zonate chlorosis causes yellow spots on leaves, twigs, and fruit, but the spots do not dry out. Abscission of fruit and leaves is less common than with leprosis. Trees frequently have psorosis-like bark lesions on old limbs. Control of leprosis is based on the elimination of sources of inocula by pruning affected trees and on the use of acaricides to reduce mite populations.


‘‘‘Lesion Nematodes (Pratylenchus spp.) ‘‘‘ Affected trees are stunted and have thin foliage with small leaves and fruit. Trees on well-drained soil can be damaged as severely by P. coffeae as by the burrowing nematode. Management practices used to control the burrowing nematode have been adopted for sites infested with coffeae and nursery stock needs to be free of lesion nematodes.


‘‘‘Brown rot: ‘‘‘ Light brown, firm, leathery rot appears with a characteristic pungent, rancid odor, covered with white mycelium developing at high humidity.


‘‘‘Lightning Injury: ‘‘‘ A lightning strike in a citrus orchard usually kills only one or two trees. Nearby trees, however, may be partly damaged, particularly at the tops of their canopies. Lightning does not cause citrus bark to split from the wood of the trunk, as it does on some other tree species. Even a citrus tree suffering a direct hit usually has only a narrow strip of dead bark down the trunk. However, dead bark may encircle the trunk at the ground line and kill the tree. Diagnosis of lightning-induced tree death or dieback is usually based on lack of evidence of other factors that could affect a tree so suddenly, coupled with knowledge of lightning activity in the area.


‘‘‘Lime Anthracnose (Gloeosporium limetticola) ‘‘‘ Only young tissues are susceptible to attack. Severe infection of young shoots can cause the tips to shrivel for several centimeters. When infection is less severe or when the leaves are further developed by the time of infection, only parts of the leaves or stems become dry. Leaves may be distorted or develop a localized drying. The dry areas may later fall out, to produce a shot-hole effect. Fruit that are attacked while very young are usually shed. Only limited success has been reported from the use of copper fungicides to control lime anthracnose, and little information is available on the potential usefulness of other fungicides.


‘‘‘Lime Blotch (Wood Pocket): ‘‘‘ On affected trees, the leaves appear variegated, fruit are sectored, and longitudinal splits develop in twigs. The incidence of this disease has been considerably reduced by selecting material for propagation from healthy trees only.


‘‘‘Tristeza: ‘‘‘ Localized veinclearing appears on Key and Tahiti lime leaves.


‘‘‘Magnesium: ‘‘‘ Magnesium deficiency is common in orchards on sandy soils, because of inherently low magnesium contents or poor retention of magnesium applied in fertilizer. It is also common in calcareous soils, because of the antagonistic effect of excessive soil calcium. The most prominent symptom of magnesium deficiency is a yellowing that gradually extends along the leaf margins, until the pattern covers most of the leaf, except for a small, delta- shaped, dark green area at the base. In acid soils, the deficiency is readily controlled with applications of magnesium- containing fertilizers or by liming with dolomitic limestone. It is more difficult to correct in calcareous soils, which require higher rates of fertilizer magnesium and, in extreme cases, supplemental treatments with foliar sprays.


‘‘‘Mal Secco (Phoma tracheiphila) ‘‘‘ The disease attacks trees of any age but is more severe on young ones. When infection occurs in the canopy, the most typical symptom is veinal yellowing. Eventually, the leaves wilt, dry up, and shed. and dieback occurs. Sometimes the fallen leaves have red coloration of the mid rib and some secondary veins. Infected bark, particularly on 1- to 2-year-old shoots, may become silver-gray. A reliable diagnostic symptom of mal secco is the red or orange coloration of recently invaded xylem, revealed by peeling off the bark or cutting through the wood. Propagation using only disease-free material prevents mal secco from reaching new areas. Diseased shoots and branches should be pruned out and burned immediately to prevent infection of nearby healthy trees. Intermittent spraying of the canopy with copper fungicides from autumn until early spring protects trees from infection. Resistant cultivars should be used where available.


‘‘‘Alternaria Brown Spot of Mandarins (Alternaria citri) ‘‘‘ The symptoms on leaves vary from large, dry, blighted areas to small, circular spots, similar to those produced on rough lemon foliage by the host-specific Alternaria that attacks that cultivar. Symptoms develop on fruit within a few days after infection and first appear as small, slightly depressed black spots. Fruit infected soon after petal fall usually drop immediately. One approach to the control of Alternaria brown spot of mandarins is to avoid excessive use of vigor-inducing practices. Overhead irrigation aggravates the situation by increasing the chances for infection. Several applications of copper fungicide during the 3- to 4-month period of fruit susceptibility are usually needed to provide acceptable control on fruit.


‘‘‘Anthracnose: ‘‘‘ Scattered silvery gray, firm, nonsunken lesions cover much of the fruit of some mandarin cultivars.


‘‘‘Manganese: ‘‘‘ Manganese deficiency symptoms appear on young leaves and may persist throughout the life of the leaf. Typically, a light green mottle develops in interveinal areas, while bands along the midrib and major veins remain a normal green. The fine network of veins is not as sharply defined as it is in iron deficiency. Soil applications of manganese sulfate provide effective control in acid soils, but they are less effective than foliar sprays for correcting the deficiency in trees on neutral and alkaline soils.


‘‘‘Mealybugs: ‘‘‘ Mealybugs are characterized by soft oval, distinctly segmented insects covered with a mealy white wax and adults are about 1/8-1/4 inch long. The fruit and leaves become covered with honeydew and sooty mold. A predator, the mealybug destroyer, is available commercially for release.


‘‘‘Measles: ‘‘‘ Sometimes, a few trees in a citrus orchard have leaves that are covered with numerous pale yellow spots. This disorder, known as measles, may appear over the whole canopy or be confined to certain limbs. When viewed from the upper side of the leaf, the spots appear mostly yellow, whereas from the lower surface they appear dirty white to brown and slightly raised. The cause of measles is uncertain, but because the symptoms reappear on the same limbs year after year, this disorder is probably a genetic variant.


‘‘‘Phytophthora-induced root rot, Freeze injury, Mechanical injury, Chemical injury, Ant and termite injury:’‘‘ Leaves become yellowish green and the midrib and major veins are pale yellow, as a result of girdling.


‘‘‘Melanose (Diaporthe citri) ‘‘‘ Melanose symptoms appear about 1 week after infection, as small, brown, discrete or confluent, sunken spots. Pustules on leaves are at first surrounded by a yellow halo. Later the yellow areas regreen, and corky pustules are the only symptoms. Severe infection of shoot apices while leaves are unfolding causes leaf distortion and even dieback. When infection occurs at a later stage of shoot development, the effects are less severe; the pustules are smaller and mostly discrete, and little or no distortion of leaves or reduction in leaf size occurs. Pustules on fruit vary considerably in size, depending on the age of the fruit at the time of infection. When infection occurs soon after petal fall the pustules become relatively large and, if numerous enough, coalesce to form extensive diseased areas. Copper fungicides are still the mainstay for control of the disease on fruit in most countries, but they are effective only if deposited on the fruit surface. Other spray materials used to control melanose in some countries include dithianon, which is similar to copper fungicides in effectiveness, and dithiocarbamate fungicides, which have to be applied more frequently to provide equivalent control.


‘‘‘Mesophyll Collapse: ‘‘‘ Soft interior leaf tissue between the veins (mesophyll) collapses and becomes translucent sometimes causing the leaves to dry completely and turn brown. These symptoms appear when the trees are unable to supply enough water to the leaves either because of low soil moisture or hot, dry winds. A poor root system, saline soil, and heavy mite feeding will compound the stress on the trees.


‘‘‘Milam Lemon Stem Pitting: ‘‘‘ The pitting is confined to the trunk and major limbs, while the trees remain vigorous. No significant economic losses from this disease have been noted.


‘‘‘Mineral Deficiencies and Toxicities: ‘‘‘ Mineral deficiencies occur because of low amounts in the soil or conditions that reduce the plant's ability to absorb them, such as excess moisture, low temperature, and root damage from biotic diseases. An excess of minerals in the soil or irrigation water may be toxic to the plant. The lack or excess of certain minerals usually appears first in leaf symptoms but eventually affects fruit size, quality, or yield. Nitrogen and zinc are most commonly deficient, manganese and magnesium are less frequently lacking, and iron is only occasionally deficient. Toxicities from minerals are less common than deficiencies, although occasionally boron causes toxic symptoms where soils or irrigation water contains high levels of this mineral. All deficiency symptoms except those of magnesium, which usually show up in late summer or fall, are most apparent during the winter and early spring. Foliar sprays can correct zinc and manganese deficiencies. Iron deficiency is more difficult to correct. Where water logging is the main cause of the deficiency, a change in irrigation practices can alleviate the problem. Where you suspect boron toxicity, have your irrigation water checked. If the level of boron is high (more than 0.5 ppm), the best remedy is to switch to another source of water.


‘‘‘Miscellaneous Fungi and Associated Diseases: ‘‘‘ Various fungi grow on living and dead insects and mites. Hendersonula toruloidea Nattrass has been observed to infect freeze-damaged limbs, inducing a disease named Hendersonula branch wilt. A Diaporthe fungus similar to D. citri (see Melanose) causes a rind blemish named microspeck, on satsuma mandarin fruit. Two pathogens of pea, Ascochyta pisi Lib. and A. pinodella L. K. Jones, have been reported to cause a blemish on satsuma mandarins. The resulting disease, named freckle, is important only when peas infected by these pathogens are grown as an intercrop between citrus trees. Tar spot causes leaf and fruit spots, which are initially tan to orange and later become greasy in appearance. Various Ascochyta and Phyllosticta fungi have been listed as pathogens of citrus leaves. However, none of them appears to be important economically, and their taxonomy requires further study.

==MITES  ==

‘‘‘Mites: ‘‘‘ Some spider mites produce a pale, stippled effect on the upper leaf surface. Stippling also occurs when these mites feed on green fruit, but this symptom usually disappears when the fruit change color. If the trees are under severe moisture stress, the feeding of spider mites can cause leaf drying and feeding on younger leaves under moisture stress may contribute to mesophyll collapse. The citrus rust mite causes distinctive injury by feeding on leaves and fruit. The injury to leaves occurs mostly on the upper surface. The more serious consequence of citrus rust mite feeding is the blemish produced on fruit injury to fruit soon after petal fall causes a silvering of the rind, distorted blossoms, and misshapen fruit and a condition described as sharkskin, which resembles wind scar and thrips injury. The broad mite injures young expanding shoots that are stunted and the leaves curl.


‘‘‘Molybdenum: ‘‘‘ The availability of molybdenum in soils is unlike that of most micronutrients in that it decreases with increasing soil acidity. Sometimes, the deficiency can be corrected simply by liming. Foliar symptoms of molybdenum deficiency first appear in midsummer as large, interveinal yellow spots on both surfaces of older leaves. Gum is deposited in the yellow areas on the underside of affected leaves. The deficiency is most easily corrected by spraying the foliage with sodium molybdate.


‘‘‘Mosaic Disease of Citrus: ‘‘‘ Stunting, yellowing, and mosaic symptoms are observed in a range of citrus cultivars.


‘‘‘Mushroom root rot, Rosellinia root rot: ‘‘‘ Rotting of bark, with fan-shaped mycelial sheets occur between the bark and the wood.


‘‘‘Mushroom Root Rot (Armillaria spp.) ‘‘‘ Unlike some other declines, mushroom root rot tends to affect localized groups of trees, and it usually does not affect all of them at the same time. Symptoms of decline do not appear until a major part of the root system has been killed or unless major girdling of the trunk or crown roots has occurred. The only aboveground symptom helpful in the diagnosis of the disease is a rotting of the bark, which sometimes occurs at the base of the trunk. The trunk lesions are similar to those caused by foot rot; however, they always arise from infected roots, whereas foot rot lesions seldom extend much below ground level. To reduce the risks of attack, land with woody vegetation should be cleared of all stumps and large roots well ahead of planting time. Where there is a high incidence of the disease, a complete replanting program may be the best long-term plan.


‘‘‘Chimera or mutation, Infectious variegation: ‘‘‘ Leaves are abnormally shaped or variegated.


‘‘‘Nagami Kumquat Disease: ‘‘‘ A graft-transmissible agent causing a unique, tristeza-like veinclearing has been reported in a selection of Nagami kumquat in Spain. This symptom is induced in several cultivars including sweet oranges, but not in Mexican lime. The causal agent is not readily mechanically transmissible to citrus or herbaceous plants.


‘‘‘Nematode Diseases: ‘‘‘ More than 40 nematode species have been associated with citrus worldwide, but not all reduce tree productivity. The most widely occurring species is the citrus nematode, Tylenchulus semipenetrans, which is especially prominent as a replant problem. The burrowing nematode, Radopholus citrophilus. causes a serious disease called spreading decline. The effects of the feeding of other nematodes on citrus roots are less well known.


‘‘‘Nitrogen: ‘‘‘ The most common nutrient deficiency of citrus is lack of nitrogen. Symptoms of nitrogen deficiency are uniformly light green to yellowish green leaves. Under normal conditions, the deficiency is easily and quickly corrected by the application of soluble nitrogen fertilizers. In some regions, deficiency symptoms occur in early spring even though sufficient nitrogen is present in the root zone. This is caused by the rapid growth and increased activity of the tree canopy at a time when the soil is still too cold for normal root function and nutrient uptake.


‘‘‘Oleocellosis: ‘‘‘ Oleocellosis is caused by the phytotoxic action of peel oil released onto the surface of the rind as a result of abrasion, rough handling, thorn punctures, or other injuries. Foggy, wet conditions exacerbate the disorder by making the rind more turgid and subject to the rupturing of oil glands, and contact between wet fruit and sand during harvest is especially hazardous. Peel oil can also be released when turgid fruit is exposed suddenly to low temperature. The disorder is sometimes called green spot, because areas of the rind affected by peel oil do not degreen. The spots darken and become sunken with time, and these changes make them more conspicuous.


‘‘‘Green mold: ‘‘‘ Soft rots form conspicuous olive green spore masses without the presence of high humidity.


‘‘‘Exocortis: ‘‘‘ Exocortis is characterized by splitting and scaling of bark on the rootstock portion of the trunk of trees on trifoliate orange or some trifoliate orange hybrid rootstocks.


‘‘‘Other Bacterial Canker Diseases: ‘‘‘ The strains of the bacterium causing four other forms of the disease are distinguished from one another and from the strain causing canker A by host range, cultural and physiological characteristics, bacteriophage sensitivity, serology, and genomic analysis by restriction endonuclease digestion. The B form, cancrosis B occurs mainly on lemons. Cancrosis B produces cankerous lesions on fruit, leaves, and twigs, similar to but smaller than those produced by canker A. It grows more slowly in culture than the A strain, and it forms smaller colonies. The C form of canker is known only in Brazil, where it has been recorded primarily on Key lime. The symptoms are the same as those of canker A. A leaf disease caused by X. campestris pv. Citri develops lesions that begin as small, yellowish, raised, water-soaked areas on leaves and twigs, and they form raised pustules similar to those produced by canker A. No infection of the fruit has been reported. Another leaf disease caused by X. campestris pv. citri develops lesions that are flat with dry centers, which often become shot-holed. Control strategies for all these forms of canker are the same as for canker A.


‘‘‘Other Nematodes: ‘‘‘ Many other species of nematodes can parasitize citrus roots, but the economic importance of most of them is uncertain. A sting nematode, Belonolaimus longicaudatus Rau, has been associated with citrus and causes root tips to swell and cease elongating. Some sheath nematodes are known to parasitize citrus roots by producing apical galls on rough lemon roots, and another causes citrus roots to become swollen. Root-knot nematodes are occasionally found in citrus roots, but they rarely mature and are not generally considered to cause a problem in citrus orchards. Some stubby-root nematodes feed on root tips and inhibits root elongation. Fumigation of infested soil has enhanced tree growth.


‘‘‘Overwatering: ‘‘‘ Overwatered trees have leaves that turn yellow and drop. An abnormal number of blossoms may appear. The best form of control is to decrease the number of irrigations and avoid planting vegetation or plants around the trees that require lots of water.


‘‘‘Perchlorate Toxicity: ‘‘‘ The toxicity symptoms appear as a yellowish orange mottle at the tip of the leaf and the veins generally remain green.


‘‘‘Pesticide Spray Injuries: ‘‘‘ Some pesticide spray materials can injure leaves and fruit if they are applied incorrectly. Spray oils specified for use on citrus trees are relatively safe if applied as recommended, but injury may occur if they are applied when temperatures are very high or when the trees are under a moisture stress. Leaves adversely affected by an oil spray soon abscise. In severe cases, even the fruit may drop. Improperly timed applications of spray oils may reduce the sugar content of the fruit, delay rind coloring, and render the foliage more prone to freeze injury. The sulfur sprays or dust treatments that are still used in some countries to control the citrus rust mite can burn fruit, particularly if followed soon afterwards by an oil spray. Fungicidal sprays containing copper are widely used in many countries to control citrus diseases, and they seldom cause direct injury to fruit rind or foliage. However, they commonly cause darkening of existing blemishes, such as wind scar. In areas where pollutants such as oxides of sulfur or nitrogen exist in the atmosphere, deposits of copper fungicide can be solubilized to such an extent that direct injury occurs. In such areas, it may be necessary to add lime to the spray mix. On leaves, the injury appears on the lower leaf surface and may later extend to the upper surface. Some of the liquid copper fungicides, such as copper ammonium carbonate and liquid copper rosin, are liable to cause stomatal drying, even where supposedly no air pollution effects are involved.


‘‘‘Phomopsis canker: ‘‘‘ Phomopsis canker is caused by a fungus that survives in cankers in trees or dead wood in surrounding areas. The bark and wood develop localized dead areas near pruning wounds or other injuries and branches eventually become girdled and leaves wither. The best forms of control are to prune late in the dormant season and remove diseased branches from orchard and burn them.


‘‘‘Phomopsis stem-end rot (Diaporthe citri) ‘‘‘ Leathery, pliable, tan to brown rot, generally begins at the stem end of the fruit and fingerlike, projections of decay are not produced.


‘‘‘Phomopsis Stem-End Rot (Diaporthe citri) ‘‘‘ Phomopsis stem-end rot occurs after harvest during transit or storage. The fungus proceeds from the stem end of the fruit through the rind and central axis, and it eventually invades the juice sacs. The infected tissue shrinks and a clear line of demarcation is formed at the junction between diseased and healthy rind. No streaks or fingerlike projections of discolored rind are normally seen in the rind of fruit affected by Phomopsis stem-end rot. The fungus does not grow through the center of the fruit and it rarely reaches the stylar end by this route before reaching it by way of the rind. Surface mycelium occasionally develops on the rind if the fruit is exposed to very moist conditions, but the disease does not spread from decayed to healthy fruit in packed cartons. The control of Phomopsis stem-end rot is essentially similar to that of Diplodia stem-end rot.


‘‘‘Phosphorus: ‘‘‘ Phosphorus deficiency is difficult to recognize in citrus. Leaves of affected trees are uniformly light green to yellowish green, not unlike those of nitrogen-deficient trees. Phosphorus uptake by citrus roots is greatly enhanced by the activity of certain mycorrhizal fungi.


‘‘‘Phymatotrichum Root Rot (Phymatotrichum omnivorum) ‘‘‘ Phymatotrichum root rot, or cotton root rot rarely appears on trees that are more than 3 years old. Infected trees suddenly wilt and die and when they are pulled from the soil, the bark of decayed roots sloughs off, and woolly, interwoven strands of the fungus are revealed on the surface of the roots. These strands are white at first and later become yellowish buff. The disease can live in the soil for many years, and it invades new areas by continued slow growth through the soil from plant to plant. Incorporating organic matter into the planting site or acidifying the soil with sulfur may help to delay root rot where the disease is prevalent, but control measures are generally not applied.


‘‘‘Phytophthora Gummosis (Phytophthora spp.): ‘‘‘ An early symptom of Phytophthora gummosis is the sap oozing from small cracks in the infected bark, giving the tree a bleeding appearance. The gumming may be washed off during heavy rain. The bark stays firm, dries, and eventually cracks and sloughs off. Lesions spread around the circumference of the trunk, slowly girdling the tree. Secondary infections kill and discolor the wood deeper than gummosis itself. Management of Phytophthora gummosis focuses on preventing conditions favorable for infection and disease development. When establishing a new orchard, carefully check the lower trunk and rootstock of new trees for any symptoms of gummosis. In addition to improving the growing conditions, you can halt disease spread by scraping off the diseased, dark bark and a buffer strip of healthy, light brown to greenish bark around the margins of the infection. Allow the exposed area to dry out.


‘‘‘Phytophthora-Induced Diseases: ‘‘‘ The most serious diseases caused by Phytophthora spp. are foot rot and gummosis. Foot rot is an injury of bark on the trunk or roots near ground level. Gummosis is a rotting of bark anywhere on the tree. Infection occurs through wounds or natural cracks in the bark. Badly affected trees have pale green leaves with yellow veins, as is typical of a girdling effect. If a lesion has ceased expansion or the fungus has died, the affected area is surrounded by callus tissue. Phytophthora causes a decay of feeder roots, which slough their cortex, leaving only the stele and giving the root system a stringy appearance. Phytophthora also infects fruit, causing a firm, light brown decay. Many problems due to Phytophthora in orchards can be avoided by beginning with Phytophthora-free nursery stock. Citrus seed should be treated with hot water at 52°C for 10 min to eliminate Phytophthora spp. Sanitary measures and certain precautions must be taken to avoid introduction of the fungus into nurseries. It is important to keep the trunk and the area under the dry and free of debris. The surrounding soil can be kept weed-free by herbicides, to avoid mechanical damage to the bark. Soil banks and insulating materials for protection from freezing can provide a favorable environment for the development of foot rot if they retain water for long periods. Foot rot and gummosis can be prevented by painting the trunk with copper fungicides or by applying systemic fungicides.


‘‘‘Phytophthora-induced foot rot: ‘‘‘ This disease is characterized by rotting of the bark, but not extending far into roots.


‘‘‘Phytophthora-induced gummosis and foot rot: ‘‘‘ This disease is characterized by rotting of bark on the trunk, which may extend into crown roots, form discrete lesions or girdles not, extending to the ground, occur only above the bud union, or extend across the bud union, the occurrence and location, of the rot depending on the susceptibility of the scion and rootstock.


‘‘‘Phytophthora Root Rot (Phytophthora spp.): ‘‘‘ As with other root diseases, Phytophthora root rot causes a slow decline of the scion top. The foliage turns light green or yellow and may drop, depending on the stage of infection. The disease destroys the feeder roots and does not spread to woody roots or the crown. The pathogen infects the root cortex or rind which turns soft and separates from the stele. The best forms of control are resistant rootstocks, irrigation management, fungicides, fumigation, and cultural control.


‘‘‘Pink Disease and Thread Blight (Corticium salmonicolor and Corticium koleroga) ‘‘‘ Pink disease primarily affects the bark of trunks and limbs of mature trees and produces drying and gumming. Under humid conditions, the causal fungus spreads rapidly along limbs, and eventually, the pathogen penetrates and kills the bark, which dries and fades to a gray color. The mycelium is usually white when actively growing and turns dark brown with age. Major limbs or entire trees are often girdled and killed. For control of pink disease, pruning out and burning affected limbs followed by sprays of copper fungicides or lime sulfur have been effective. Thread blight is seldom severe enough to require treatment although it can be controlled with copper fungicides.


‘‘‘Pleospora Rot (Pheospora herbarum) ‘‘‘ The fungus begins at wounds on the side of the fruit or at the stem end and infected tissue remains firm at first, later becoming only slightly leathery and pliable. Decayed fruit are dark brown to almost black, both inside and outside.


‘‘‘Postbloom Fruit Drop Disease: ‘‘‘ The first symptoms of the disease are water-soaked patches on petals, usually not forming until the open-flower stage, but produced earlier in severe attacks. The patches, bearing salmon pink acervuli, expand rapidly in favorable weather and soon involve all the petal tissue. The blighted petals remain firmly attached to the basal disk, turning hard, dry, and reddish brown. After flowering, fruitlets abscise at the base of the ovary, leaving the basal disk, calyx, and peduncle firmly attached to the tree. Disease control is achieved by protecting blossoms with sprays of benomyl, thiabendazole, and chlorothalonil. Treatment with protective sprays only when periods of prolonged wetness are expected to coincide with blossom peaks is more satisfactory in such situations.


‘‘‘Potassium: ‘‘‘ The symptoms of potassium deficiency in citrus are expressed more strongly in the fruit than in the foliage. Lack of potassium results in small, thin-skinned fruit with an unusually smooth peel texture. Excessive potassium uptake results in large, coarse-skinned fruit, which tend to remain partially green even after reaching maturity.


‘‘‘Powdery Mildew (Acrosporium tingitaninum) ‘‘‘ The disease is more severe in nurseries than in orchards. Initial symptoms develop on young shoots near the center of the tree and nearest the ground. Whitish, powdery patches of mildew appear on the upper surface of leaves, usually starting from the margin or mid rib and infected young leaves may shrivel and drop. Young stems are also infected, and dieback is common. Young fruit may be attacked and drop prematurely. More recently, effective control has been obtained with bitertanol, tridemorph, triadimefon, dinocap, and benzimidazole fungicides, applied at the first signs of attack and repeated at 10-day intervals.


‘‘‘Prevention, Management, and Control of Disease: ‘‘‘ Prevention is the only effective means of reducing losses from most citrus diseases. Certification programs are operated in some countries to prevent the spread of virus diseases. Healthy, virus-free sources of bud wood are maintained for distribution to nurseries and growers and are periodically reindexed for the presence of viruses. Another important disease prevention measure is the use of resistant rootstocks. Site selection is another important aspect of disease prevention. Adequate drainage is a major prerequisite for successful citrus production, regardless of the rootstock used. Careful selection and preparation of sites for nurseries and orchards is essential, and installation of drainage systems may be necessary. The use of chemicals to control diseases is justified only if significant economic losses are anticipated and there are no practical alternatives. Chemicals are available to control certain soilborne pathogens, such as nematodes and fungi, but they are expensive and are generally applied only in seed beds and nurseries or to soil used for the production of containerized trees. Diseases may reduce fruit yields directly, by attacking the fruit or indirectly, by causing defoliation or stem injury that affects fruit development and yield. In general, chemical treatments are used mostly when fruit is destined to be marketed fresh.


‘‘‘Psorosis: ‘‘‘ Psorosis is a graft transmissible disease, presumably a virus, most often found in old citrus plantings. The most distinguishing field symptom is scaling and flaking of the bark on the scion cultivar. During early stages, patches of bark on the trunk or scaffold branches show small pimples or bubbles, which later enlarge and break up into loose scales. Gumming often appears around the margins of a lesion. In an advanced stage, deep layers of bark and the wood become impregnated with gum and die. Psorosis is transmitted in infected budwood or possibly with contaminated grafting tools. As with other graft transmissible diseases, the use of disease-free budwood is the major method for preventing damage from psorosis.


‘‘‘Puffing: ‘‘‘ Puffing, or puffiness, is a condition of unknown cause in which the peel separates from the pulp. It affects mandarin cultivars almost exclusively and may occur on the tree or in storage.


‘‘‘Tristeza (quick-decline effect): ‘‘‘ Sudden wilt and death of trees occurs on sour orange rootstock and the leaves dry up on the tree before falling. A yellow brown zone is often present at the bud union.


‘‘‘Ringspot (CRSV) ‘‘‘ Characteristic symptoms are ring spots or large, irregular yellow patterns on mature leaves, which are frequently gum-impregnated and are slightly raised on the lower leaf surface. Many isolates are associated with bark scaling of sweet orange and grapefruit. The fruit may also have ringspot symptoms. The use of virus-free bud wood and seed sources has been considered adequate to control the spread of all psorosis-like diseases.


‘‘‘Rio Grande gummosis, Phytophthora-induced gummosis: ‘‘‘ Early symptoms of Rio Grande gummosis consist of narrow cracks in the bark, which release a pale yellow gum. The water-soluble gum is produced in large quantities and may accumulate on the bark or may be washed away during rainfall. The advancing margin of infected tissue is salmon to orange and often becomes a bright pink when exposed to air. As the disease progresses, gum pockets develop beneath the bark, and the discolored, gum-infiltrated tissue spreads several decimeters through the internal wood. Wood decay is often initiated after cracks in the bark expose the sapwood. Rio Grande gummosis is most prevalent following a severe freeze. Proper pruning practices to remove wood killed by freeze injury and promote fast healing are probably the most important control measures.


‘‘‘Robinson Dieback (Diplodia natalensis) ‘‘‘ Twigs and branches of Robinson tangerine trees commonly die back, even where other cultivars in the same location are healthy. The primary cause of the disorder is probably some inherent physiological weakness that renders the tree more prone to colonization by this weak pathogen. Field observations have indicated that Robinson dieback is more likely to occur when trees are subjected to moisture stress.


‘‘‘Rosellinia Root Rot (Rosellinia spp.) ‘‘‘ Affected trees become yellow, lose their leaves, and die because of severe root loss or girdling of the trunk at the soil line. The causal fungus produces white, fan-shaped mycelial growth under the bark of affected roots. Roots of diseased trees eventually become covered with the dark brown to black mycelium of the fungus, and abundant perithecia form under humid conditions. Rosellinia root rot can be controlled by trenching around large stumps or removing them and avoiding drought conditions.


‘‘‘Rubbery Wood: ‘‘‘ Rubbery wood is a graft-transmitted disease of unknown cause and limbs of affected trees bend downward and are abnormally flexible. The trees become unproductive, and many die.


‘‘‘Rumple of Lemons: ‘‘‘ Rumple is a disease of lemon fruit. Juice quality and yield of peel oil are not affected unless picking is long delayed. The disease does not affect the foliage. Rumple appears in late summer, when the fruit begin to turn yellow. The earliest symptom is a yellow mottling of the rind that is slightly depressed, particularly between oil glands. The cause of rumple is unknown. Rumple can be avoided by picking fruit before color break.


‘‘‘Citrus rust mite injury: ‘‘‘ Smooth or only slightly roughened, gray, tan, or russet brown areas appear on the rind.


‘‘‘Rusty Brown Spots (Leprosis): ‘‘‘ Leprosis develops rusty brown spots with sunken centers.


‘‘‘Salt Injury: ‘‘‘ The injury usually results from salt uptake through the roots. Absorption of salt by foliage may occur in coastal areas. In regions where the salt problem is chronic, affected trees have thin canopies of lusterless green leaves. Irregular brown dry areas may develop near the leaf tip and margins. Leaves of affected trees tend to abscise between the leaf blade and the petiole, leaving the petiole attached to the twig. Rootstocks differ to some extent in their sensitivity to high concentrations of salt. It may be feasible to reduce the salt concentration to nontoxic levels by heavy irrigation with fresh water.


‘‘‘Sap Beetles: ‘‘‘ Dried fruit and sap beetles. May also spread yeasts causing sour rots. Ripe fruit contains small black beetles that sometimes have dull yellow bands on the wings. White, short-legged grubs may be present. Eggs are laid on ripe and rotting fruit of all types. Remove and destroy fallen fruit and cull piles. Beetles can be trapped in containers of fermenting fruit.


‘‘‘Satsuma Dwarf (SDV) ‘‘‘ Satsuma dwarf virus can infect a wide range of citrus species and cultivars and has been transmitted experimentally to many non-citrus hosts. Symptoms in satsuma mandarin include stunting and narrow, boat or spoon shaped leaves. In other cultivars, the symptoms are mild or absent. The use of disease-free budwood is recommended for all new plantings and for topworking.


‘‘‘Scab Diseases: ‘‘‘ Infection of actively emerging shoot apices of highly susceptible cultivars causes much distortion. A protuberance develops on the invaded side of the leaf, and a corresponding depression forms on the opposite side. Scab pustules consist of a stroma, which contains mycelia of the pathogen plus dead host cells, and hyperplastic host cells, which contain few or no chloroplasts. Infection of very young fruit promotes the formation of relatively large volumes of hyperplastic tissue, in conical or warty outgrowths on the rind. With later infection, the rind on all cultivars responds by producing pustules raised only slightly above the normal contour of the fruit surface. If numerous enough, the pustules coalesce to form extensive scabby or scurfy areas, which may crack into platelets as the fruit expands. The scurfy type of scab symptom may be confused with wind scar. The leaf also tends to hold a film of water between itself and the rind surface and thus aids scab infection. When first formed, scab stromata range from pink to light brown. Later they become corky and turn yellowish or grayish brown or even black. Several fungicide spray treatments a year may be needed to control scab on foliage in nurseries. The more effective fungicides are those that prevent production of conidia on existing scab pustules and also provide long protectant action. Overhead sprinkler irrigation greatly enhances the risk of scab attack and should be reduced to a minimum when susceptible shoots or young fruit are present.


‘‘‘Scale Insects: ‘‘‘ In some areas of the world, armored scales, such as red scales, cause serious defoliation and dieback of twigs and branches. Some armored scales, such as the purple scale and the chaff scale, prevent normal coloring of the fruit. Green spots remain in the rind where the insects feed.


‘‘‘Sclerotinia Twig Blight (Sclerotinia sclerotiorum) ‘‘‘ Sclerotinia twig blight is a minor disease of citrus. The disease may occur on blossoms or fruit or at any location along twigs or branches, which may become girdled. At first, the lesions are soft and exude gum. Later, the bark becomes shredded into longitudinal fibrous strips. Above a lesion girdling a branch, leaves wilt, turn brown, and fall. The bark is rarely penetrated unless it is predisposed to infection by frost damage or wounds, or unless infected flower petals, fruit, or dead bark are in direct contact with healthy bark.


‘‘‘Septoria Spot (Septoria spp.) ‘‘‘ Lesions on leaves appear as raised, blister-like, black spots, 1-4 mm in diameter, surrounded by a yellow halo. Eventually, the centers of the spots become dry and pale brown. On fruit still attached to the tree, the lesions consist of small depressions or pits. The pits are light tan or buff with a narrow greenish margin, which becomes reddish brown as the fruit matures. Small, black pycnidia of the causal agent may be produced in these lesions. Infections begin when Septoria spores are spread throughout the tree in dew or rain water. Control of Septoria spot is possible by means of copper fungicides applied prior to winter rainfall.


‘‘‘Shell Bark and Dry Bark: ‘‘‘ Symptoms begin as small areas of the outer bark that die, crack, and loosen in long strips. Shelling usually moves from the bud union upward, but sometimes starts in the crotch of scaffold branches. If the disorder extends into the inner bark and cambium, the disorder is called dry bark and the affected areas dry and crack slightly but do not shell because no new tissue is generated underneath. If dry bark extends around most of the tree's circumference, food transport is severely limited and the tree will die. As a preventive measure, use only budwood from tolerant parent trees.


‘‘‘Shell Bark of Lemons: ‘‘‘ Symptoms seldom appear before the trees are 10 years old. The outer bark dies and cracks into vertical strips, starting in small patches, which may later enlarge slowly to involve more extensive areas of the trunk and major limbs. The inner bark and cambium usually remain intact. When shell bark is more severe, the inner bark may dry out and crack, allowing decay organisms to invade the cambium and exposed wood and cause more serious injury to the tree. Temporary recovery of affected trees can be promoted by severe pruning.


‘‘‘Sloughing: ‘‘‘ Sloughing is a progressive browning and disintegration of the peel, which remains soft and moist and then falls away, or sloughs off, leaving the flesh exposed. It occurs only on pink and red grapefruit, and only on fruit that is harvested too early.


‘‘‘Slow Decline: ‘‘‘ The citrus nematode does not kill trees but may greatly reduce their vigor. Trees whose roots are attacked by high populations of the nematode may have leaf yellowing, sparse foliage, and small fruit. Affected trees do not respond to fertilization, and they succumb to water stress earlier than healthy trees. Roots infected by significant numbers of the citrus nematode may appear coarse and dirty, because of the adherence of soil particles and debris to egg masses, but this feature is not diagnostically reliable. Care should be taken to use planting material free of the citrus nematode. Infested orchard sites that are to be planted with new trees may require preplant fumigation with methyl bromide, metam-sodium, dichloropropenes, or other materials, if approved. Postplant application of nematicides such as aldicarb, fenamiphos, and oxamyl, where approved for use, have been effective in reducing nematode populations and increasing yield and fruit size. Currently, all commercially acceptable rootstocks that are resistant to the citrus nematode incorporate trifoliate orange germplasm.

==SNAILS  ==

‘‘‘Snails: ‘‘‘ Snails form holes in leaves that could be confused with shot-hole symptoms of some biotic disease. Similarly, the sunken, irregular marks caused by snails on fruit might be attributed, at first glance, to some other agent.


‘‘‘Soft Scales: ‘‘‘ Various soft scales including citricola, brown soft, or black scale Soft scales rarely appear on fruit, but are more often found on leaves or twigs. Fruit and leaves may be covered with honeydew and sooty mold and the tree vigor may be reduced. Natural enemies usually provide effective control. Keep ants out of trees because they protect scales from natural enemies. Oil sprays also effective if treatments are timed to target a new brood right after hatching.


‘‘‘Sooty Blotch and Flyspeck (Gloeodes pomigena and Leptothyrium pomi) ‘‘‘ In humid regions, sooty blotch and flyspeck can spoil the appearance of citrus fruit, with superficial fungal colonies forming on the rind. These blemishes represent growths of the fungi themselves. Sooty blotch consists of a branching network of thick-walled dark hyphae and flyspeck has a less conspicuous hyphae but forms pycnidia that are visible as black specks.


‘‘‘Sooty Mold (Capnodium citri) ‘‘‘ Sooty mold is a black, removable fungal growth which appears on the surface of leaves, stems, and fruit after trees become infested with honeydew-excreting insects. The mold deposits are usually heavier on the upper surface than on the lower surface of leaves. The mold deposits may delay fruit coloring and can be difficult to remove in the packinghouse from cultivars with a rough rind. Honeydew-excreting insects include aphids, soft scales, mealybugs, and whiteflies and they need to be controlled. Where circumstances permit, spray oil can be applied to loosen the mold deposits, thereby facilitating their subsequent removal by wind and rain.


‘‘‘Tristeza: ‘‘‘ This disease is characterized by stunting or decline of trees on sour orange rootstock. Honeycombing may occur on the inner side of the bark below the bud union.


‘‘‘Sour Rot (Endomyces geotrichum) ‘‘‘ The initial symptoms of sour rot are similar to those of green mold and blue mold. Lesions first appear as water-soaked, light to dark yellow, slightly raised spots. Following exposure to high relative humidities, the lesion may be covered with a yeasty, sometimes wrinkled layer of white or cream-colored mycelium. Sour rot can be reduced by harvesting the fruit carefully to minimize injuries and by preventing contact between the fruit and the soil. Immediate storage of packed fruit at 10°C or less delays disease development. Postharvest treatment with sodium o-phenylphenate or guazatine provides some control of sour rot.


‘‘‘Sphaeropsis Knot (Sphaeropsis tumefaciens) ‘‘‘ Knots, or galls, usually form laterally on small branches or twigs. They are approximately spherical, usually 23 cm in diameter, and larger in some cases. Buds are often formed on the galls, and they grow out to produce witches' brooms. Old galls become cracked and dark brown to black, and branches subtending them frequently die. Other galls can form on citrus, caused by mutations, woody gall, or unknown agents, and they are not easily differentiated from Sphaeropsis knot without isolation of the causal fungus. Control of Sphaeropsis knot can be obtained by pruning and burning affected limbs.


‘‘‘Spider mite injury: ‘‘‘ Spider mite injury leaves lusterless, grayish leaves with pale, yellow stippling on the upper surface. Irrigating the soil near infected trees may offer the only form of control.


‘‘‘Spray Injury : ‘‘‘ If pesticide sprays, in particular herbicides and fungicides, are applied incorrectly, damage to fruit may occur. A persistent oil film interferes with physiological functions of the tree, reducing sugar production and color development of the fruit or, in severe cases, resulting in fruit and leaf drop. Growth regulators, such as 2,4-D, cause leaf curling and yellowing if applied at the wrong concentration or the wrong time. Fungicides containing copper may cause a pitting of leaves and fruit. Postemergence and preemergence herbicides can injure citrus by direct contact with foliage or fruit or by uptake through the roots. When roots take up too much of a preemergence herbicide, symptoms appear above ground mainly on the leaves. Bromacil damage causes extensive vein clearing, leaving a net pattern of bleached. Simazine results in a clearing of interveinal areas, not unlike symptoms caused by nutritional deficiencies. When too much diuron is taken up, major leaf veins turn yellow, and the interveinal areas remain green. Depending on the amount absorbed, any of the three herbicides may turn entire leaves brown. Localized damage on leaves or fruit may occur when a contact herbicide is sprayed onto the tree skirts. If paraquat is accidentally sprayed on or drifts onto citrus trees, leaves may drop. Glyphosate may deform new growth, darken foliage, and abort flowers.


‘‘‘Spreading Decline (Radopholus citrophilous) ‘‘‘ When the burrowing nematode feeds on fibrous roots, it causes a cessation of apical meristem activity. An infested tree may have 50% fewer functional feeder roots than a healthy tree. Affected trees have an undernourished appearance, with sparse foliage, dead twigs, and small leaves and fruit. Cultural practices that enhance plant growth may minimize the damage to trees caused by nematode feeding. Infested trees and also adjacent healthy trees should then be pushed out and burned in place. Infested sites must not be replanted immediately after clearing.


‘‘‘Stem-End Rind Breakdown: ‘‘‘ Stem-end rind breakdown is characterized by a darkening and collapse of the rind at the stem end of the fruit. A narrow ring of normal rind usually remains close to the button and occurs mostly on small, thin-skinned fruit. It is associated with excessive moisture loss, especially between harvesting and waxing. Reducing the time between harvesting and waxing, and maintaining a high humidity help to avoid stem-end rind breakdown. Harvested fruit should be kept shaded and protected in transit to reduce heating and water loss


‘‘‘Stem pitting (tristeza): ‘‘‘ Stem pitting creates vertical depressions on grapefruit trunks and occurs when the bark is peeled on twigs of declining sweet orange or grapefruit trees.


‘‘‘Stubborn: ‘‘‘ Stubborn can infect most citrus species and cultivars and a wide range of non-citrus plants. All non-citrus hosts are herbaceous except those in the Rutaceae and Rosaceae. Stubborn is rarely lethal in citrus, but trees affected while young are often severely stunted. The internodes are shortened, and the foliage is dense and abnormally upright. Leaves may be cupped and abnormally thick, and they frequently have variable yellow patterns, which may resemble those of a nutritional deficiency. A mottled appearance and associated yellowing of the leaf veins may also be observed. Fruit from stubborn-affected trees are usually few and frequently are lopsided or acorn-shaped and small. They often do not color at the stem end as they mature, and seed are frequently aborted. Symptoms of stubborn are expressed most distinctly under warm conditions, and even reactive hosts may remain symptomless in cool weather. In areas where stubborn or its vectors are not endemic, control is achieved by the use of disease-free budwood. Infected young orchard trees should be removed and replaced with healthy replants.


‘‘‘Tristeza: ‘‘‘ Stunting is characterized by stunting or decline of trees on sour orange rootstock, resulting from early infection of trees or from the propagation of trees from infected budwood.


‘‘‘Stylar-end breakdown: ‘‘‘ This disease is characterized by brown lesions that develop on the fruit and firm drying occurs at the stylar end of Tahiti lime. Stylar-end breakdown of Tahiti lime has also been referred to as stylar-end rot, but it is not induced by a decay organism. The breakdown begins with a water-soaked tan lesion at the stylar end of the fruit which rapidly progresses and may cover half of the fruit. The affected area later dries, contracts, and becomes sunken. Stylar-end breakdown occasionally appears before picking, particularly at high temperatures following rainy weather. More commonly, however, it appears after harvest.


‘‘‘Sudden change in temperature: ‘‘‘ Sudden change in temperature or low moisture or fertility causes older fruit to drop. Make sure that the fertilizer and water are adequate for the change in temperature.

==SULFUR  ==

‘‘‘Sulfur: ‘‘‘ Symptoms of the deficiency in citrus are similar to those of nitrogen deficiency, with the leaves uniformly pale green. However, in contrast to nitrogen deficiency, sulfur deficiency produces symptoms on young leaves only.


‘‘‘Sunburn: ‘‘‘ Intensive sun radiation may blemish mature fruit. The side of fruit continuously exposed to the sun develops brownish, leathery areas and some fruit become slightly lopsided. Sunburned bark appears dried and often cracks and sheds. Application of whiting agents can reduce sunburn but is usually not economical.


‘‘‘Sunscald: ‘‘‘ Normally, the bark of citrus trees can withstand the direct rays of the sun and high temperatures. Recently planted trees and trees that have been pruned severely or have suddenly lost their leaves are particularly prone to injury. Insufficient soil moisture during periods of high temperatures can increase the risks of sunscald. Wood-rotting organisms soon enter the tree through sunscalded bark and aggravate the injury. In some parts of the world, the stems of newly planted trees are routinely wrapped with a protective material to protect the trees from sunscald until they are well established. When leaves are suddenly reoriented so that their lower surface becomes exposed to direct sunlight, they may become sunscalded, particularly on days with strong, dry winds.


‘‘‘Symptomless and Quiescent Infections by Fungi: ‘‘‘ Citrus leaves, stems, and fruit are commonly invaded by some fungi that never produce disease symptoms or do so only after a long incubation period. Colletotrichum gloeosporioides (Penz.) Sacc. is a common invader of citrus leaves, stems, and fruit, and the infection usually remains symptomless. Spores of the fungus germinate on the surface of the host and form appressoria, which become firmly attached to the cuticle. A narrow infection proceed no further than the cuticle or may grow intercellularly to a depth of as many as three cells before becoming quiescent. The avirulent strain of Guignardia citricarpa Kiely is another fungus that produces symptomless infection of citrus leaves and fruit. In the drier inland citrus-producing, a species of Septoria is reported as a common symptomless invader of fruit rind. Such infection can lead to Septoria spot, especially if the infected rind is first exposed to low temperatures or frost. Some of the fungi that cause serious diseases of citrus may also produce symptomless infections. For example, the black spot fungus, seldom produces black spot symptoms on infected leaves, and even on fruit the infection may remain symptomless. Infections by the greasy spot pathogen can remain symptomless throughout the life of a leaf, even when the hyphae have penetrated as far as the air spaces of the spongy mesophyll.


‘‘‘SYMPTOMS ON FRUIT: ‘‘‘ Many pathogens and environmental conditions damage citrus fruit. Adverse weather including frost, cold and wet, wind, and sunburn may also injure fruit. Other disorders are caused by pesticide injury or new mutations that produce chimeras.


‘‘‘SYMPTOMS ON LEAVES AND TWIGS: ‘‘‘ Several pathogens and various environmental conditions, toxic chemicals, and genetic disorders may cause leaves and twigs to wilt, dieback, or become discolored without affecting other parts of the tree. Most of these diseases do not cause serious long-term losses in citrus, but changes in management practices can often reduce their occurrence and improve the general health of trees.


‘‘‘SYMPTOMS ON ROOTS: ‘‘‘ Three diseases commonly affect citrus roots: Phytophthora root rot, dry root rot, and Armillaria root rot. Because of their impact on the root system, the first visible symptom of all these diseases is usually a yellowing of leaves and a slow decline of the scion top.


‘‘‘SYMPTOMS ON THE TRUNK: ‘‘‘ Various diseases and disorders produce symptoms on the trunk of citrus trees. Common symptoms are shelling and scaling of the bark and development of lesions and gumming. Serious damage to the trunk disrupts transport of water and nutrients and results in overall decline of the tree.


‘‘‘Tangerine and Murcott Collapse: ‘‘‘ Under a heavy fruit load, trees often decline rapidly in the fall and winter, as the fruit reach maturity. The first visible symptoms are leaf wilt and yellowing, followed by defoliation and fruit drop. Dieback of branches and feeder roots then occurs. In extreme cases, the tree does not recover. The collapse apparently occurs as a result of starch depletion in root and shoot tissues. This type of tree decline can be prevented by hand-thinning the excess fruit while they are still young or by light pruning or hedging in the spring to remove some of the bloom or fruit-bearing shoots.


‘‘‘Tatter Leaf and Citrange Stunt (TL-CSV) ‘‘‘ Yellow spots or mottling occur on some citrange leaves and may be associated with leaf deformities. Stems of citrange plants may also be deformed and have a zigzag growth pattern associated with yellow areas on the stem. However, when infected latent hosts are grafted to rootstocks of trifoliate orange or trifoliate orange hybrids, a bud-union abnormality can develop, and affected plants are stunted and yellow. The basic control recommendation at present is to use virus-free budwood for propagation and topworking and use a sterilant such as sodium hypochlorite on pruning tools to avoid transmission of the virus as a contaminant.


‘‘‘Fertilizer burn, Phytophthora-induced foot rot, Herbicide injury, Termite injury: ‘‘‘ Termite injury is characterized by dead or missing bark around the trunk, particularly at the soil level.

==THRIPS  ==

‘‘‘Thrips: ‘‘‘ They puncture young epidermal cells, and gray scars form on the rind as a result and the injury appears similar to wind scar. Thrips also feed on young leaves, causing distortion and gray streaks.


‘‘‘Trichoderma Rot (Trichoderma viride) ‘‘‘ Diseased fruit become cocoa brown, and the infected peel remains leathery and pliable. Rotted fruit have a characteristic coconut-like odor, which distinguishes Trichoderma rot from other rots. Under humid conditions, clumps of white hyphae appear on the surface of the fruit; later the fruit is covered by coarse, white mycelium and masses of yellow-green to dark green spores. The pathogen is unable to penetrate sound fruit directly, but juice from decayed fruit can damage the peel of neighboring fruit, allowing the fungus to invade at the point of injury. Warm bath treatments (40°C) of borax, sodium carbonate, and sodium o-phenylphenate followed by a treatment with thiabendazole or benomyl should provide effective control of this wound pathogen. Rapid cooling of the fruit is helpful, because the pathogen does not spread from fruit to fruit at 10°C or develop in fruit that are already infected if they are stored at 4°C.


‘‘‘Tristeza: ‘‘‘ In general, mandarins are especially tolerant to the infection. Sweet orange, sour orange, rough lemon, and Rangpur lime are usually symptomless but may react to some severe isolates. Stunting, stem pitting, leaf cupping, veinclearing, yellowing, and reduced fruit size are common symptoms. Veinclearing and stem pitting in Mexican lime are diagnostic. Virus infection in the scion causes drying in the phloem of the sour orange rootstock immediately below the bud union. Stem pitting may sometimes cause a bumpy or ropy appearance of the trunks and limbs of larger trees. The best forms of control are to suppress or eliminate the virus where low levels of infection exist and use virus-free budwood sources.


‘‘‘Tristeza Disease Complex (Tristeza virus): ‘‘‘ Susceptible rootstock/scion combinations infected with the virus show light green foliage, poor growth flushes, and some leaf drop. The trees may produce a heavy crop of small fruit because the girdling at the bud union prevents starch transport to the roots. Diseased young trees bloom early and abundantly and begin producing fruit 1 to 2 years before healthy ones do. By the time the aboveground symptoms appear, many feeder roots have died. Management of the tristeza complex depends largely on preventive measures, such as using tolerant rootstocks and tristeza-free propagation material. Where tristeza is widespread, you may want to remove infected trees only when they become unproductive.


‘‘‘Twig Dieback: ‘‘‘ Many factors may contribute to the damage, for example, a poor root system, a mild scion- rootstock incompatibility, or weather stress. The stress causes gum formation in the conducting tissue of the twigs, aggravating the water transport problem. Leaves and twigs may die immediately, and leaves dry up on the tree, or they may die some weeks later, often at the time of another hot period. Another type of dieback affects limbs and branches and usually follows a dry period the preceding fall. No gum forms in the conducting tissue, and leaves do not drop until twigs or leaves are completely dry.


‘‘‘Vein Enation (Woody Gall): ‘‘‘ The symptoms include enations on the veins and swelling or galls on the stems of rough lemon and Mexican lime. Galls typically form near thorns or in association with wounds. The only control recommended where the disease is endemic is to avoid propagation on susceptible rootstocks.


‘‘‘Bud-union incompatibility, Some virus diseases: ‘‘‘ Virus diseases are characterized by overgrowth at the bud union of some scion-rootstock combinations and some young stem or twig abnormalities.

==WATER  ==

‘‘‘Water injury, Phytophthora-induced root rot: ‘‘‘ Symptoms of chronic water injury are subtle with smaller trees and thin canopies. These trees have shallow roots, and if the water table drops, the trees may soon develop a wilt, leaf drop, and dieback due to moisture stress. A disagreeable odor may be detected when the root zone is dug and the bark may slough, even on the larger roots.


‘‘‘Whiteflies: ‘‘‘ Whiteflies appear when branches are disturbed. The fruit and leaves also become covered with honeydew and sooty mold. Natural enemies usually control the problem but ants must be eliminated.


‘‘‘Wind Injury: ‘‘‘ High winds during the emergence of a growth flush cause tattering and distortion of leaves. Newly planted trees are especially vulnerable to wind and any associated sandblasting. Young fruit are easily damaged when leaves, twigs, or thorns rub against the rind, especially during the first 3 weeks after petal fall. Most injury is caused by the edges of older leaves rubbing against the fruit. Windbreaks can successfully reduce the incidence of wind scar if planted on the side of the orchard from which the strongest sustained winds are directed during the first 1-2 months after petal fall. High winds also accentuate problems caused by drought, excessive heat, and freezes. Hot, dry winds combined with drought can promote desiccation of leaves, causing them to die so suddenly that they remain attached to the twigs. Wind abrasion scars the fruit, but a ring does not form around the stem.


‘‘‘Witches'-Broom: ‘‘‘ A witches'-broom disease that seriously damages small-fruited acid lime trees is apparently caused by a mycoplasmalike organism.

==WOOD ROT  ==

‘‘‘Wood Rot: ‘‘‘ Wood rot, also known as heart rot, is a condition of the roots, trunk, or limbs in which the wood is dead and in various stages of decay. Among the many fungal organisms that colonize dead wood are some that produce large sporophores. Abiotic factors that can induce wood rot include water injury, sunscald, and freeze injury. Wood-rotting organisms can also invade trunks and limbs through mechanically induced wounds. Control of wood rot is essentially preventive. Surgical removal of dead bark and wood is usually impractical and is unlikely to improve the chances of tree survival. Pruning paints used to cover exposed cuts are of doubtful value for preventing wood rot or for hastening healing.

==YEASTS  ==

‘‘‘Yeasts: ‘‘‘ Yeasts brought into figs by insects, especially fruit flies and small beetles, entering eyes Yeast causes ripe fruit to sour and ferment. It is difficult to control but it is necessary to destroy infested fruit.


‘‘‘Ringspot: ‘‘‘ Yellowish ring spots, blotches, or etchings occur on young or old leaves and a drying of young shoots.


‘‘‘Yellow Vein: ‘‘‘ Yellow vein is a viruslike disease discovered in several limequat trees. It was transmitted experimentally by grafting to a range of citrus cultivars, but there is no evidence of natural spread. A synergistic interaction of the agents causing yellow vein and vein enation has been reported.


‘‘‘Zebra Skin: ‘‘‘ Zebra skin occurs primarily on tangerines picked early in the season and degreened with ethylene. A darkened rind and drying rind occur simultaneously over some of the underlying fruit segments giving a zebra-like pattern on the fruit. Fruit picked while wet from an orchard that had been under moisture stress are particularly prone to zebra skin.

==ZINC  ==

‘‘‘Zinc: ‘‘‘ In many areas, particularly where citrus is grown on neutral and calcareous soils, zinc must be supplied routinely to the trees as a foliar spray. Symptoms of zinc deficiency appear on new growth and include a distinctive yellow leaf pattern, small leaves, and short internodes. The yellowing develops in interveinal areas of the leaf, leading to a striking light green to yellow mottle against an irregular dark green background. An effective foliar treatment restores the normal dark green color of the leaves but is too late to increase leaf size. When the deficiency is mild, the symptoms usually develop only on a few widely scattered terminals.