Cotton:Diagnostics

COTTON PESTS AND NUTRITIONAL DISORDERS

AIR POLLUTION

‘‘‘Air Pollution:’‘‘ Plants that are exposed to sulfur dioxide experience leaf margin injury consisting first of gray green, water-soaked zones that later become brown to black. (Environmental Disorders)

ALTERNARIA LEAF SPOT (ALTERNARIA MACROSPORA)

‘‘‘Alternaria Leaf Spot (Alternaria macrospora) ‘‘‘ This disease is characterized by an exceptionally long terminal cell, or beak, which is as long as the rest of the multi-celled spore combined. The infections begin as tiny, dull brown circular spots and slowly enlarge to nearly 1 cm on true leaves, but may also be produced on cotyledons and bolls. (Leaf Disorders)

ANTHRACNOSE (GLOMERELLA GOSSYPII)

‘‘‘Anthracnose (Glomerella gossypii) ‘‘‘ This disease is characterized by spots with irregular dry margins on the cotyledons, dark lesions that severely girdle the plant near the soil surface, and lack of vigor and yellowing of the leaves in all other parts of the plant. Seedlings are commonly infected. The best form of control is to apply seed treatments. (Leaf and Crown Disorders)

AREOLATE MILDEW (RAMULARIA AREOLA)

‘‘‘Areolate Mildew (Ramularia areola) ‘‘‘ This disease is characterized by lesions that develop on leaves and are restricted to single-leaf areolae with a distinctly irregular, angular appearance. The lesions are light green to yellow green on the upper leaf surface, and a profusion of conidiophores and conidia forms frosty white to gray mildew patches on the undersurface. This disease usually appear late in the growing season as the bolls are ripening. The best forms of control is by using resistant or tolerant cultivars, and applications of fungicides. (Leaf Disorders)

AREOLATE MILDEW (RAMULARIA AREOLA)

‘‘‘Areolate Mildew (Ramularia areola) ‘‘‘ This disease is characterized by small, irregular, angular lesions that develop on leave near the end of the growing season. They are light green to yellow green on the upper leaf surface and frosty white to gray on the underneath side. Occasionally lesions develop on the bracts of the bolls and on the cotyledons. These are circular, water soaked, dark green areas that usually causes some defoliation. The best forms of control include using resistant cultivars and applying fungicides.

ARSENICALS

‘‘‘Arsenicals:’‘‘ Plants exposed to arsenicals have reddened foliage with burned leaf margins where the spray collects. These plants may weaken, die, or recover very slowly. (Chemical Injury -- Herbicides)

ASCOCHYTA BLIGHT (ASCOCHYTA GOSSYPII)

‘‘‘Ascochyta Blight (Ascochyta gossypii) ‘‘‘ This disease is characterized by pale brown circular spots that grow up to 2 mm in diameter and appear on true leaves and cotyledons. These spots group together into irregular dead areas that may infect large portions of the leaf and are surrounded by a narrow, dark brown border. On stems, the spots take the form of elongated brown cankers that may girdle the stem and kill the distal parts. (Leaf Disorders)

BACTERIAL BLIGHT (XANTHOMONAS CAMPESTRIS PV MALVACEARUM)

‘‘‘Bacterial Blight (Xanthomonas campestris pv malvacearum) ‘‘‘ This disease is characterized by round to elongate lesions on cotyledons that are initially deep green and water-soaked, later drying to brown. Lesions on hypocotyls are black, elongate cankers that often girdle and kill the seedlings. The foliar phase of bacterial blight, called angular leaf spot, may appear at any time during the season as water-soaked lesions that are angular. During the boll phase, round, water-soaked, sunken and brown to black lesions appear in the carpel wall. The best form of control is to use resistant cultivars and good sanitation practices. (Seedling Disease)

BLACK ROOT ROT (THIELAVIOPSIS BASICOLA)

‘‘‘Black Root Rot (Thielaviopsis basicola) ‘‘‘ This disease is characterized by black infected tissue of the root system of seedlings and the portion of the hypocotyl below the soil line. The taproot is usually smaller than those of healthy seedlings and severely infected plants are usually stunted and can easily be pulled from the soil. The best form of control is to plant when soil temperatures reach 16 °C (60 °F), preferably higher. (Root Disorder)

BLUE DISEASE

‘‘‘Blue Disease:’‘‘ This disease is characterized by apical leaves that bulge slightly toward the base at the ends of the lobes and the area between the main veins. These areas also become light green but as the infection spreads and the leaf ages, the infected areas become deep, bluish green. The best forms of control include clean sanitation practices, careful destruction of crop residues, and applying insecticidal treatments to reduce the vector populations. (Viruses and Mycoplasmalike Organisms)

BOLL ROTS

‘‘‘Boll Rots:’‘‘ There are several types of boll rots that affect cotton. Diplodia boll rot develops as small, brown lesions on capsules or bracts and in moist areas, the spots expand rapidly and consume the entire boll. As the Diplodia infection progresses, the bolls turn black as the fungus compacts into black masses bearing an abundance of pycnidia and spores. Anthracnose (Glomerella) boll rot begins as small, reddish brown spots, each with a shallow central depression and as the spots enlarge, the tissue within the infected zone blackens. Larger spots show a roughly ringed area, the outer area is a dull reddish brown, the middle area is black, and the central part is usually pink from the conidial masses. Fusarium boll rot. Infection is first evident as dry lesions on the toothed margins of the bracts. As the boll ages in moist conditions, the lesions enlarge to reach the receptacle, invade the capsule, and cause a blue black to brown rot. Zanthomonas boll rot. In the boll rot phase of bacterial blight, lesions develop on the bolls and tend to be circular, dark green and water-soaked or greasy when young. These spots rapidly become sunken, brown, and dry in the center with a dark red margin. Internal boll rot. A small group of fungi, when infected into a young boll by insects, ferment the sugars that envelop the growing fibers and convert the entire lock into a decayed mass. No evidence of infection is seen until the boll opens, exposing stained, rotted lint and shriveled seed. The best forms of control is to plant disease-free seed, treat seed with fungicides, and use resistant varieties when available. (Seedling Disease)

BORON

‘‘‘Boron:’‘‘ Plants suffering from boron deficiency are bushy, the terminal buds are killed, and young leaves may be pale green. Drought-induced boron deficiency has been associated with thick, leathery older leaves with long, spongy petioles. (Nutrient Deficiencies)

CALCIUM

‘‘‘Calcium:’‘‘ Plants suffering from calcium deficiency usually are susceptible to other seedling diseases and the plant stalks are weak. (Nutrient Deficiencies)

CERCOSPORA LEAF SPOT (CERCOSPORA GOSSYPINA)

‘‘‘Cercospora Leaf Spot (Cercospora gossypina) ‘‘‘ This disease is characterized by barely visible reddish dots that increase in diameter to about 2 cm and have a narrow red margin around a white to light brown center of dead tissue. (Leaf Disorders)

CHARCOAL ROT (MACROPHOMINA PHASEOLINA)

‘‘‘Charcoal Rot (Macrophomina phaseolina) ‘‘‘ This disease is characterized by a dry rot and many tiny black sclerotia distributed throughout the wood and softer tissues. Some varieties form pycnidia at the surface of foliage and stem lesions. These pycnidia are black, roughly spherical bodies that resemble sclerotia in size, shape, and color. This best form of control to control the water supply in crops grown under irrigation, especially as daily temperatures reach seasonal highs. (Root Disorder)

CHLOROPHYLL DEFICIENCY A GENETIC PROBLEM

‘‘‘Chlorophyll Deficiency A Genetic Problem:’‘‘ Plants suffering from chlorophyll deficiency have bright, white to yellowish areas appear on only half the leaf or plant or on a single branch. Chlorophyll deficiency usually only affects an occasional plant. (Genetic Problem)

COLD DAMAGE TO SEEDLINGS

‘‘‘Cold Damage to Seedlings:’‘‘ Seedlings suffering from cold damage have leaves that become dry on the margins or leaf tips and usually results in death of the terminal leaves. Established plants with fully developed leaves and become exposed usually experience death of the leaves. Early symptoms on all plants consist of water soaking, followed by complete drying and subsequent defoliation of severely affected leaves. (Environmental Disorders)

CONTROL OF LEAF SPOTS

‘‘‘Control of Leaf Spots:’‘‘ The best form of control is the use of resistant varieties. (Leaf Disorders)

CRAZY TOP (ACROMANIA)

‘‘‘Crazy Top (Acromania):’‘‘ This disease is characterized by abnormal upright growth on the uppermost branches. Only a few flower buds and bolls develop and then fall off leaving a conspicuous barrenness in the top third or half of the plant. Leaves in the affected portion are abnormally small, rounded, and cupped. (Environmental Disorders)

DEFOLIANTS AND DESICCANTS

‘‘‘Defoliants and Desiccants:’‘‘ Plants that have become exposed to excessive rates of desiccants have leaves that are either frozen or killed so quickly that normal abscission does not occur and the leaves remain on the plant as if a quick-killing desiccant had been used. Desiccation is undesirable for spindle-picker harvesters because it increases the content on fine-textured trash in seed cotton and lowers quality. (Chemical Injury -- Defoliants)

DINITROANILINES

‘‘‘Dinitroanilines:’‘‘ Plants that have been exposed to dinitroanilines may have inhibited lateral root development and inaccurate incorporation depth may result in stunting and increased loss of resistance to the seedling disease complex. (Chemical Injury -- Herbicides)

ESCOBILLA (WITCHES' BROOM) (COLLETOTRICHUM GOSSYPII PV CEPHALOSPORIOIDES)

‘‘‘Escobilla (Witches' Broom) (Colletotrichum gossypii pv cephalosporioides) ‘‘‘ This disease is characterized by small, deep green leaves that have reduced lobes and are frequently wrinkled with shortened petioles. The branches usually have short, tightly compacted twigs with an excessive number of leaves. These branches are usually swollen and twisted with with shortened internodes and enlarged nodes. Affected plants frequently have both diseased and healthy branches. (Leaf Disorders)

FERTILIZERS

‘‘‘Fertilizers:’‘‘ Excessive applications of nitrogen solutions or anhydrous ammonia cause burn and death when the chemicals come in contact with young seedlings. (Chemical Injury -- Herbicides)

FUSARIUM (FUSARIUM SPP)

‘‘‘Fusarium (Fusarium spp.) ‘‘‘ This disease is characterized by discoloration of the root and stem tissues which turn brown internally and then rot. After the plants emerge, dry, brown to reddish brown lesions usually appear on the hypocotyl near the soil surface and may girdle the stem. (Seedling Disease)

FUSARIUM WILT NEMATODE COMPLEX (FUSARIUM OXYSPORUM PV VASINFECTUM)

‘‘‘Fusarium Wilt Nematode Complex (Fusarium oxysporum pv vasinfectum) ‘‘‘ This disease is characterized by wilting cotyledons leaves that turn yellow and brown and may fall off leaving bare stems in seedlings. In older plants, the first symptoms usually appear on lower leaves about the time of first flowering and inconspicuous leaves wilt and then turn yellow or brown. Over time, the entire leaf wilts and symptoms appear on other leaves which drop and plants become stunted and die. The best form of control is to use resistant cultivars, fumigate the soil, and avoid planting in infested fields. (Leaf Disorders)

GLOMERELLA (GLOMERELLA GOSSYPII)

‘‘‘Glomerella (Glomerella gossypii) ‘‘‘ This disease is characterized by dead spots in areas of the stems, leaves, or bolls of plants of all ages. It is best controlled through the use of fungicides and resistant cultivars when available. (Seedling Disease)

HAIL

‘‘‘Hail:’‘‘ Hailstorms cause injury ranging from slightly rugged leaves to complete defoliation. (Environmental Disorders)

INSECT INJURY

‘‘‘Insect Injury:’‘‘ Plants that are infected by thrips often resembles phases of the seedling disease complex on young plants. Aphids can cause severe stunting, delayed fruiting, and reduced yields. (Insect Problems)

LANCE NEMATODES (HOPLOLAIMUS COLUMBUS AND GALEATUS)

‘‘‘Lance Nematodes (Hoplolaimus columbus and galeatus) ‘‘‘ Plants infected with lance nematodes are severely stunted and yellow. Almost complete defoliation has been reported in cotton infected by lance nematodes in low-moisture conditions. (Nematode Disorder)

LEAF CRUMPLE

‘‘‘Leaf Crumple:’‘‘ This disease is characterized by downward curling of the leaves, mosaic found in the veins, reddish spots that occasionally appear in yellow areas that are in between the veins of senescent leaves and bracts and flowers tend to be irregularly shaped and stunted. (Viruses and Mycoplasmalike Organisms)

LEAF CURL

‘‘‘Leaf Curl:’‘‘ This disease is characterized by thick, lower surfaces of veins on the leaves and as these areas grow together, they cause the leaves to curl. From the underside, affected veins appear abnormally dark green and opaque. New leaves that form on infected plants are usually extemely distorted, small and curled. These plants do not produce a very high yield. (Viruses and Mycoplasmalike Organisms)

LEAF DESICCATION

‘‘‘Leaf Desiccation:’‘‘ Plants suffering from leaf desiccation do experience regrowth and it is generally more abundant. Leaf desiccation begins to develop soon after a sudden rise in temperature and light intensity, but leaf and vascular discoloration are not evident. (Environmental Disorders)

LIGHTNING

‘‘‘Lightning:’‘‘ Plants that have been damaged by lightning have isolated, oval to round spots of dying or dead tissue. The extreme heat results in rapid desiccation so that leaves remain intact. (Environmental Disorders)

MAGNESIUM

‘‘‘Magnesium:’‘‘ Plants suffering from magnesium deficiency have reddish purple leaves with veins that remain green. This symptom may be confused with normal maturity of aging leaves in late season and with spider mite injury. Lower leaves are infected first. (Nutrient Deficiencies)

MANGANESE TOXICITY

‘‘‘Manganese Toxicity:’‘‘ Plants that are subject to manganese toxicity have crinkled upper leaves that are dwarfed, have severely shortened internodes, and overall stunting of the entire plant. (Nutrient Disorders)

MORPHOLOGICAL CHARACTERS

‘‘‘Morphological Characters:’‘‘ This condition is characterized by a red leaf color, okra-type leaves, frego bracts, absence of nectaries, leaf cupping and puckering. (Genetic Problems)

MOSAIC

‘‘‘Mosaic:’‘‘ Characteristic symptoms of mosaic are similar to leaf curl, but leaf spots may also occur. (Viruses and Mycoplasmalike Organisms)

NITROGEN

‘‘‘Nitrogen:’‘‘ Plants suffering from nitrogen deficiency have smaller, yellow leaves that eventually turn red and slower growth of the overall plant and development of fruit. (Nutrient Deficiencies)

OTHER NUTRIENT DEFICIENCIES

‘‘‘Other Nutrient Deficiencies:’‘‘ Plants suffering from zinc deficiency may become stunted and have thick, brittle, cupped leaves. Plants suffering from manganese deficiency have yellowish gray to reddish gray upper leaves with green veins. (Nutrient Deficiencies)

OVERVIEW OF DISEASE CONTROL

‘‘‘Overview of Disease Control:’‘‘ Verticillium Wilt: Verticillium wilt is controlled most effectively by growing tolerant or resistant varieties and using cultural practices known to reduce the disease. Fusarium Wilt-Nematode Complex: Varieties resistant to both Fusarium wilt and root-knot nematodes should be grown when both pathogens are present. Varieties resistant only to Fusarium wilt can be grown when nematodes are absent. There is a need for complete resistant of cultivars for root-knot nematodes. Bacterial Blight: Growing resistant cultivars is the most effective control for bacterial blight. Phymatotrichum Root Rot: The soil containing most roots of the cotton plant should be managed for disease control. Deep plowing after harvest, to keep the soil dry for a period, reduces the number of fungal propagules in the field. Complete spreading of barnyard manure of residues from other crops used in the rotation is also beneficial. Ascochyta Blight: Rotating crops and plowing under diseased plant residues deep enough to prevent their return to the surface the following year reduce carry-over of inoculum in the soil. Southwestern Cotton Rust: Use of fungicides has been the only practical way to control southwestern cotton rust. Leaf spots: Control of leaf spots is possible through acid delinting, coating planting seed with fungicides to prevent seed transmission, maintaining the crop in a vigorous condition throughout the year, avoiding insect and mechanical injury, and preventing the buildup of pathogens in cotton fields. Boll rots: Boll rot is controlled primarily with practices that reduce inoculum levels and promote dryness within the leaf canopy. Also avoiding rank growth, boll damage, and infested seed, all of which intensify boll rots are good control measures. (Disease Controls)

PHENOXY HERBICIDES

‘‘‘Phenoxy Herbicides:’‘‘ Plants exposed to phenoxy herbicides are severely distorted, puckered, and veins and leaves are strapped. Plants seldom die, because exposure is usually low. (Chemical Injury -- Herbicides)

PHOMOPSIS LEAF SPOTS (PHOMOPSIS SPP)

‘‘‘Phomopsis Leaf Spots (Phomopsis spp.) ‘‘‘ This disease is characterized by slightly sunken, grayish, water-soaked lesions on stems and leaves. As the disease progresses, these lesions become covered with conidial fruiting bodies that produce and liberate an abundance of spores in varying shapes. (Leaf Disorders)

PHOSPHORUS

‘‘‘Phosphorus:’‘‘ Plants suffering from phosphorus deficiency have abnormally dark green leaves, are dwarfed, and experience delayed fruiting and maturity. (Nutrient Deficiencies)

PHYLLODY

‘‘‘Phyllody:’‘‘ This disease is characterized by stringlike or straplike leaves and infected plants are sterile and exhibit floral greening. The only control measure available at this time is costly insecticidal suppression of the vector. (Viruses and Mycoplasmalike Organisms)

PHYMATOTRICHUM ROOT ROT (PHYMATOTRICHUM OMNIVORUM)

‘‘‘Phymatotrichum Root Rot (Phymatotrichum omnivorum) ‘‘‘ This disease is characterized by slight yellowing or bronzing of young leaves, which is followed by sudden wilting and death of the plant. Infected leaves are dry and become brown but remain attached to the plant. Underground, the roots are encompassed by brown, threadlike strands, and a white, cottony growth appears on the root near the soil surface. The best form of control is to plow deep keeping fields clean of grasses susceptible to cotton crops. Also, in some situations, resistant cultivars are available. (Root Disorder)

POTASSIUM

‘‘‘Potassium:’‘‘ Plants suffering from potassium deficiency have yellowish white spots that turn light, yellowish green, with yellow spots developing between veins. As these spots age and turn brown, the leaves take on a rusty appearance, the leaf tips and margins curl down, and the leaves eventually dry and shed prematurely, beginning with the lower leaves. (Nutrient Deficiencies)

POWDERY MILDEWS (SALMONIA MALACHRAE AND LEVEILLULA TAURICA)

‘‘‘Powdery Mildews (Salmonia malachrae and leveillula taurica) ‘‘‘ This disease is characterized by scattered, circular patches of white mycelium, usually on the upper leaf surfaces which causes the leaves to curl slightly, turn yellow, and drop. Heavy growth of the fungus on upper leaf surfaces late in the season gives the field the appearance of being covered with a light blanket of snow. There are no control measures available for the disease at this time. (Leaf Disorders)

PYTHIUM (PYTHIUM SPP)

‘‘‘Pythium (Pythium spp.) ‘‘‘ This disease is characterized by lesions that develop on hypocotyls and may range from pinpoint discolored spots to large dry areas that are often slightly sunken. On severely damaged seedlings, the hypocotyl may be girdled by a soft, water-soaked lesion or by a firmer, light brown dry area extending down to the plant collar. Affected seedlings often die, either by toppling over or remaining upright as they age. (Seedling Disease)

RENIFORM NEMATODES (ROTYLENCHULUS RENIFORMIS)

‘‘‘Reniform Nematodes (Rotylenchulus reniformis) ‘‘‘ Plants infected with reniform nematodes have destroyed small feeder roots and jellylike egg masses dot the remaining roots. The association of the nematode with seedling blight organisms decreases plant mortality and causes poor resistance, often resulting in weedy areas in infested fields. (Nematode Disorder)

RHIZOCTONIA LEAF SPOT (RHIZOCTONIA SOLANI)

‘‘‘Rhizoctonia Leaf Spot (Rhizoctonia solani) ‘‘‘ This disease is characterized by light brown, irregular spots of varying size that appear between the large veins and are bordered by a dark purplish ring. As the fungus spreads within the leaf tissues, the zone surrounding the original spot becomes yellow, leaves become ragged and perforated, and dead areas within older spots crack and fall out. (Leaf Disorders)

RHIZOCTONIA SOLANI (RHIZOCTONIA SOLANI)

‘‘‘Rhizoctonia Solani (Rhizoctonia solani) ‘‘‘ This disease is characterized by filaments that penetrate the epidermis and colonize soft tissues of the cotyledons, hypocotyl, and roots of the seedling. Tissues in lesions appear sunken and are usually tan to reddish brown. The surface of lesions is covered with a microscopic network of yellow to brown hyphae closely pressed to the epidermis that penetrates and infects the soft tissues of the seedling. (Seedling Disease)

ROOT-KNOT NEMATODES (MELOIDOGYNE SPP)

‘‘‘Root-Knot Nematodes (Meloidogyne spp.) ‘‘‘ This disease is characterized by distinctive spindle-shaped galls or knots on the roots, usually on the lateral roots. The plant may become yellow due to reduced efficiency in water and nutrient transport. The best form of control is to use resistant cultivars. (Nematode Disorder)

SCLEROTIUM STEM AND ROOT ROT (SCLEROTIUM ROLFSII)

‘‘‘Sclerotium Stem and Root Rot (Sclerotium rolfsii) ‘‘‘ This disease is characterized by an abundance of light tan to brown sclerotia in the soil. Under moist conditions, the fungus may ascend the outside of the stem an inch or more, producing a conspicuous superficial collar of cottony white mycelium. (Root Disorder)

SOIL COMPACTION

‘‘‘Soil Compaction:’‘‘ Plants growing in compacted soils have shallow root systems and are stunted and wither in dry weather. (Environmental Disorders)

SOIL CRUSTING

‘‘‘Soil Crusting:’‘‘ Plants growing in crusted soils have curled, thickened hypocotyls on germinating plants. (Environmental Disorders)

SOIL MOISTURE EFFECTS

‘‘‘Soil Moisture Effects:’‘‘ Plants with low soil moisture result in partial germination and poor, delayed emergence of seedlings. Wilting, slow growth, and reduced fruiting are common on older plants. Prolonged drought may cause leaf margin burn and shedding of leaves and fruit. Excess moisture also complicates resistance problems by promoting seed rots and seedling diseases. Waterlogged soils are poorly aerated and cause stunting and reddening of foliage. (Environmental Disorders)

SOUTHWESTERN COTTON RUST (PUCCINIA CACABATA)

‘‘‘Southwestern Cotton Rust (Puccinia cacabata) ‘‘‘ This disease is characterized by the development of pale green lesions four to six days after inoculation, which subsequently develop into bright yellow pycnial pustules usually found of the upper leaf surfaces. As the disease progresses, aecial clusters fade to a light yellow, and pycnia darken to brown and in severe cases, the leaves may curl, drop and stem and fruiting branches may break and become brittle. The best forms of control are to use resistant cultivars and chemical control. (Leaf Disorders)

SPIDER MITES

‘‘‘Spider Mites:’‘‘ Plants suffering from spider mites have a rusty discoloration and the leaves eventually turn red, beginning with the leaf veins. (Insect Problems)

STING NEMATODES (BELONOLAIMUS LONGICAUDATUS)

‘‘‘Sting Nematodes (Belonolaimus longicaudatus) ‘‘‘ This disease is characterized by severe stunting and yellowing of the plant followed by death of young plants. Roots that have been infected have small, dark, sunken lesions that become severely stubbed and knobby within two to three weeks after infection. (Nematode Disorder)

SUBSTITUTED UREAS

‘‘‘Substituted Ureas:’‘‘ Plants exposed to substituted ureas are stunted and develop yellow spots or white discolorations on the leaves. Young seedlings are frequently killed. (Chemical Injury -- Herbicides)

SULFUR

‘‘‘Sulfur:’‘‘ Plants suffering from sulfur deficiency are stunted and have yellow top leaves with normal green lower leaves. (Nutrient Deficiencies)

TERMINAL STUNT

‘‘‘Terminal Stunt:’‘‘ Diseased plants have stunted terminal growth of the stems and branches and terminal leaves are puckered, twisted, and frequently spotted. The xylem of the main stem is usually discolored. Squares, blooms, and immature bolls are shed concurrently with the foliar symptoms. (Viruses and Mycoplasmalike Organisms)

THIELAVIOPSIS (THIELAVIOPSIS BASICOLA)

‘‘‘Theilaviopsis (Thielaviopsis basicola) :’‘‘ This disease attacks roots and lower stems, causing death of cotton plants of all ages, including seedlings. (Seedling Disease)

TRIAZINES

‘‘‘Triazines:’‘‘ Plants exposed to triazines experience yellow to whitish discoloration between the veins of leaves, stunting, and sometimes death occurs when they are not applied properly.

TROPICAL RUST (PHAKOPSORA GOSSYPII)

‘‘‘Tropical Rust (Phakopsora gossypii) ‘‘‘ The infected growing tissues, primarily leaves, spread from old to new tissues. In some cases it can also cause premature defoliation. Spores develop on leaves and appear as oval, corky pustules and then become circular. Spores on pedicels and branches are elongated. The best form of control is to use resistant cultivars, plant pathogen-free seed, and remove diseased crop residues from fields. (Leaf Disorders)

VEIN CLEARING

‘‘‘Vein Clearing:’‘‘ The first symptom is a vein clearing, followed by vein banding and eventually a spot. Young leaves tend to cup downward and infected plants are stunted. (Viruses and Mycoplasmalike Organisms)

VERTICILLIUM WILT (VERTICILLIUM DAHLIAE)

‘‘‘Verticillium Wilt (Verticillium dahliae) ‘‘‘ There are several different strains of verticillium wilt. SS-4 strain. It is first characterized by prematurely yellow cotyledons that quickly dry out. Irregular yellow areas appear between the principle veins and on the margins of the first true leaves, resulting in a spotted appearance. As the plants age, yellow and dry spots occur in the upper leaves. T-1 strain. The first symptom is often a downward curling of the terminal leaf, frequently followed by pronounced leaf enlargement and a general yellowing of several upper leaves, which soon fall. Besides extensive defoliation, infection by the T-1 strain can cause fruiting branches and bolls to drop. Excessive moisture in the form of rain or irrigation and heavy applications of nitrogen favor the disease. High plant densities in most cases and high levels of potassium reduce disease incidence and severity. The best form of control is to use resistant cultivars where available. (Leaf Disorders)

WINDBLOWN SAND

‘‘‘Windblown Sand:’‘‘ Plants suffering from windblown sand have green, water-soaked or black leaves and hypocotyls. On the windward side of plants with less damage, affected leaves and stems turn light tan to bronze. Defoliation may occur and leaf growth may be delayed for three to five weeks. (Environmental Disorders)