Cucurbits:Diagnostics

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CUCURBIT PESTS AND NUTRITIONAL DISORDERS


Contents

ACREMONIUM HYPOCOTYL ROT (ACREMONIUM SPP)

‘‘‘Acremonium Hypocotyl Rot (Acremonium spp.) ‘‘‘ Symptoms of Acremonium hypocotyl rot of melon are first evident in the area of the peg. The infected region of the root is initially light yellow brown, but within 2-3 days develops into a dry, red brown cortical rot. Lesions on hypocotyls and roots enlarge from the peg. Sometimes the roots become impaired resulting in the death of many fibrous roots. As the first fruits approach maturity, the primary roots are corked and may exhibit dry lesions at the insertion points of disappeared secondary and tertiary roots. Older leaves show interveinal yellowing, and young leaves lose turgidity. Control of Acremonium hypocotyl rot is accomplished through shallow planting followed by irrigation rather than deep planting after preirrigation. When deep planting is practiced, fungicides have given excellent control if sprayed directly on the seed at planting.


ALTERNARIA LEAF BLIGHT (ALTERNARIA CUCUMERINA)

‘‘‘Alternaria Leaf Blight (Alternaria cucumerina) ‘‘‘ Lesions on leaves are the characteristic symptom, although fruit spots have also been reported. The first lesions appear on the older leaves, near the crown. They are initially yellow brown and small with a light green or yellow halo but expand into large, brown dry areas, which usually show concentric zonation when viewed from above. Rotation of crops, regular applications of protectant fungicides, and avoiding leaf wetness with irrigation are the most effective control measures.


ALTERNARIA LEAF SPOT (ALTERNARIA ALTERNATA PV CUCURBITAE)

‘‘‘Alternaria Leaf Spot (Alternaria alternata pv cucurbitae) ‘‘‘ Dry flecks surrounded by yellow halos appear on the leaves and enlarge into spots, which may group together to form nearly circular lesions. The lesions bear the black brown conidia of the pathogen. Severely infected leaves become yellow and senescent and then die. Satisfactory control can be achieved by reducing relative humidity in greenhouses, removing infected plant debris prior to crop establishment, and applying protectant fungicides.


ALTERNARIA ROT (ALTERNARIA ALTERNATA)

‘‘‘Alternaria Rot (Alternaria alternata) ‘‘‘ The disease is characterized initially by small, brown lesions that soon become black. The lesions are circular to oval and typically are shallow, extending 2-15 mm below the epidermis and have a spongy but firm texture. Affected areas can extend well into the flesh and are easily removed from the surrounding tissue. The best forms of control include proper culling, handling, and refrigeration.


ANGULAR LEAF SPOT (PSEUDOMONAS SYRINGAE PV LACHRYMANS)

‘‘‘Angular Leaf Spot (Pseudomonas syringae pv lachrymans) ‘‘‘ Angular leaf spot first appears as small, water-soaked lesions on leaves. The lesions usually expand until they are delimited by larger secondary veins in the leaves, which give the lesions an angular appearance. Accompanying water-soaking at high relative humidities is a clear to milky exudate, which collects in a tear on the lower surface of the lesion. The exudate dries to form a thin white crust on or adjacent to the lesion. On fruit, lesions appear as small, circular, water-soaked spots, often with a light tan center. Using resistant cultivars, pathogen-free seed, and controlling irrigation are good control measures.


ANTHRACNOSE (COLLETOTRICHUM ORBICULARE)

‘‘‘Anthracnose (Colletotrichum orbiculare) ‘‘‘ On cucumber leaves, lesions usually first appear near veins, are roughly circular, range from light brown to reddish, and can reach more than 1 cm in diameter. The leaves may be distorted, and the centers of lesions may crack or drop out, creating a shot-hole appearance. On petioles and stems, lesions are shallow, elongated, tan areas. On fruit, lesions are circular, sunken, water-soaked areas which first develop as the fruit. Chemical control and resistant cultivars are the most effective controls of the disease.


APHIDS

‘‘‘Aphids: ‘‘‘ Aphids damage cucurbits by direct feeding, by contamination with excrement, and as vectors of plant pathogens and can occur just after the formation of the first true leaf. Insecticides are not effective in preventing the spread of aphid borne viruses, but may be helpful in control the onset of infection. Refer to current recommendations for registered insecticides.


ASTER YELLOWS (AY)

‘‘‘Aster Yellows (AY) ‘‘‘ Infected plants are easily detected by the conspicuous yellowing of young leaves, the proliferation of secondary shoots, and the rigid, erect habit of the plants. Leaves are often misshapen and smaller than normal and have stiff, thick laminae. Flowers are usually malformed and greenish yellow and often have prominent leafy bracts. Fruits are small, malformed, and lighter than normal color. Application of insecticide can drastically reduce leafhopper populations but is often ineffective in reducing the disease.


BACTERIAL BROWN SPOT (ERWINIA ANANAS)

‘‘‘Bacterial Brown Spot (Erwinia ananas) ‘‘‘ In honeydew melons, symptoms appear as conspicuous yellow brown, smooth, firm lesions up to 40 mm in diameter. Symptoms on melon are very similar with firm lesions that are slightly yellow brown, and up to 10 mm in diameter, extending 1-2 mm below the epidermis. Symptoms on melon fruit are obscured by the net. No control measures have been developed.


BACTERIAL FRUIT BLOTCH

‘‘‘Bacterial Fruit Blotch: ‘‘‘ The characteristic symptom of bacterial fruit blotch of watermelon is a dark olive green stain or blotch on the upper surface of the fruit. The blotch is first noticeable as a small water-soaked area, less than 1 cm in diameter, but it rapidly expands to cover much of the fruit surface in 7-10 days. As the blotch increases in size, the area around the initial infection site becomes dry. Leaf lesions are small, dark brown, somewhat angular, and often inconspicuous. Good sanitation practices and crop rotations are the best forms of control.


BACTERIAL LEAF SPOT (XANTHOMONAS CAMPESTRIS PV CUCURBITAE)

‘‘‘Bacterial Leaf Spot (Xanthomonas campestris pv cucurbitae) ‘‘‘ Small, water-soaked lesions first appear on the lower sides of leaves and are later visible as indefinite yellow areas on the upper sides. Because of their small size, the lesions can easily be overlooked. Small (2 to 4-mm), angular, yellow to beige spots are often associated with large and secondary veins. Initial lesions are small, slightly sunken, circular spots, with a beige center and a dark brown halo. Overhead irrigation should be avoided, and general sanitation practices should be incorporated.


BACTERIAL RIND NECROSIS (ERWINIA SPP)

‘‘‘Bacterial Rind Necrosis (Erwinia spp.) ‘‘‘ In watermelon, the disease causes a brown, corky, drying of the interior of the rind, which rarely extends into the flesh. There are rarely any external symptoms on watermelon. The only exception is misshapen fruit in cases of severe systemic internal drying. Growing cultivars that are less susceptible to bacterial rind necrosis is the only known control.


BACTERIAL SOFT ROT (ERWINIA CAROTOVORA PV CAROTOVORA)

‘‘‘Bacterial Soft Rot (Erwinia carotovora pv carotovora) ‘‘‘ Early symptoms of bacterial soft rot are a distinct water-soaked appearance and pronounced softening of the surrounding tissue. As the decay progresses, the fruit tissue becomes extremely soft and mushy and often collapses, losing its original form. Avoiding bruising and injury to the fruit is the first step in controlling bacterial soft rot.


BACTERIAL WILT (ERWINIA TRACHEIPHILA)

‘‘‘Bacterial Wilt (Erwinia tracheiphila) ‘‘‘ Individual runners or whole plants wilt and die rapidly. Affected runners appear dark green at first and then dry out as the wilt becomes irreversible. Plants may wilt dramatically during the heat of the day but partially recover by morning. Foliage of affected plants often appears yellow. Control of bacterial wilt depends on control of the cucumber beetle vectors. Roguing wilted plants, using trap crops, and implementing appropriate insecticide programs are the best forms of control.


BELLY ROT (RHIZOCTONIA SOLANI)

‘‘‘Belly Rot (Rhizoctonia solani) ‘‘‘ On the undersides and blossom ends of cucumber fruit, the disease produces water-soaked, tan to brown lesions, which become sunken, cratered, irregular, and dried as they enlarge. Soil may be fumigated before planting. In humid areas, artificial barriers can be placed between the soil and the fruit to prevent infection. Another effective method of avoiding the pathogen is deep plowing before planting.


BLACK ROT (DIDYMELLA BRYONIAE)

‘‘‘Black Rot (Didymella bryoniae) ‘‘‘ In cucumber, fruit rot can develop before harvest, but it usually develops during transit or storage. Before harvest, a black decay occurs, especially at the blossom end, and can extend into the pulp. On watermelon, distinct, circular, greenish tan to black spots first appear on any part of the fruit. The lesions later have black centers, and under moist conditions many pycnidia and perithecia develop near their centers. In squash, if fruit are damaged prior to or during storage, a brown to pinkish water-soaked area develops, followed by blackened areas with conspicuous fruiting bodies. Special care should be exercised to avoid rind injuries to all fruit, especially winter squash and pumpkins, as wounds provide entry for the black rot organism in storage.


BLACK ROT AND PURPLE STEM (PHOMOPSIS CUCURBITAE)

‘‘‘Phomopsis Black Rot and Purple Stem (Phomposis cucurbitae) ‘‘‘ On cucumber, symptoms appear as cankers or lesions on stems, as a blight of the inflorescence, or as a fruit rot. The pathogen also causes purple stem, a vine decline of melon, which normally occurs late in the season. There are no control measures available for the disease.


BLOSSOM END ROT

‘‘‘Blossom-End Rot: ‘‘‘ Symptoms first appear as small, light brown spots at the blossom end of immature fruit. As affected watermelons grow, these spots can enlarge rapidly to form dark, sunken, leathery lesions. The lesions are generally dry and can be as large as the diameter of the fruit. A soft, secondary wet rot may develop if affected areas of the fruit are invaded by decay fungi or bacteria.


BLUE MOLD (PENICILLIUM SPP)

‘‘‘Blue Mold (Penicillium spp.) ‘‘‘ In general, cucurbit fruits with blue mold have been predisposed by temperature, bruising, and injury, and they typically have other problems. Blue mold can be effectively prevented by eliminating predisposing conditions, maintaining low temperatures in storage, and moving the produce within its reasonable postharvest shelf life.


CERCOSPORA LEAF SPOT (CERCOSPORA CITRULLINA)

‘‘‘Cercospora Leaf Spot (Cercospora citrullina) ‘‘‘ Symptoms usually appear first on older foliage as small, circular to irregularly circular spots with tan or light brown centers. The lesions gradually enlarge, spreading over the leaf surface. The best form of control is to use good sanitation practices as well as a regular fungicide spray program.


CHARCOAL ROT (MACROPHOMINA PHASEOLINA)

‘‘‘Charcoal Rot (Macrophomina phaseolina) ‘‘‘ Symptoms in melon include yellowing and death of crown leaves and a water-soaked lesion that encompasses the vine at the crown. These symptoms normally begin showing just prior to harvest. All cucurbit fruit are susceptible to charcoal rot, although occurrence is more frequent in cucumber and melon. There is no control specified for this disease.


CHOANEPHORA FRUIT ROT (CHOANEPHORA CUCURBITARURA)

‘‘‘Choanephora Fruit Rot (Choanephora cucurbitarura) ‘‘‘ The disease affects both blossoms and fruit. When flowers are invaded early, they turn soft and usually drop off the plant. Later infections cause the flowers to turn brown, and if the fruit has begun to develop, the fungal mycelium invades the fruit tissue, causing it to decay in a wet rot. No practical control measures have been developed.


CRATER ROT (MYROTHECIUM RORIDUM)

‘‘‘Crater Rot (Myrothecium roridum) ‘‘‘ The disease is characterized by shallow to deep, sunken lesions, 2-50 mm in diameter. Large lesions often penetrate to the seed cavity in melon and honeydew. Lesions on leaves are round to irregular in shape, 2-15 mm in diameter, with a tan center, brown margins, and a yellow halo, and they often form concentric rings. Abundant greenish black sporodochia may also be arranged in concentric rings on either surface of the leaf. Foliar diseases can be controlled with fungicides.


CUCUMBER BLACK ROOT ROT (PHOMOPSIS SCLEROTIOIDES)

‘‘‘Phomopsis Black Root Rot of Cucumber (Phomopsis sclerotioides) ‘‘‘ Symptoms of Phomopsis black root rot on cucumber roots are somewhat sunken and irregular grayish black spots surrounded by a darker line. Infection sites are often located at the point of attachment of lateral roots and root hairs and are generally circular in shape. Steam sterilization of greenhouse soil has been the most reliable and effective control measure. The fungus tends to recolonize both steam-sterilized and fumigated soils very quickly making annual disinfestation treatments necessary.


CUCUMBER FUSARIUM WILT (FUSARIUM OXYSPORUM PV CUCUMERINUM)

‘‘‘Fusarium Wilt of Cucumber (Fusarium oxysporum pv cucumerinum) ‘‘‘ Infection of older plants usually results in wilting of one or more runners, followed by wilting of the entire plant. The wilting symptom is accentuated by heat and drought stress and by fruit load. Vascular discoloration of the roots and stem is also common. The use of resistant cultivars and planting clean seed on land not previously cropped with cucumbers and the best means of control.


CUCUMBER MOSAIC (CMV)

‘‘‘Cucumber Mosaic (CMV) ‘‘‘ In cucumber, melon, and squash, cucumber mosaic virus causes severe plant stunting, prominent foliar yellow mosaic, malformation, prominent downward leaf curling, and drastic reduction of leaf size and stem internodes. Flowers of severely affected plants may have prominent abnormalities and greenish petals. The best forms of control include using resistant cultivars, insecticides, reflective mulches, and mineral oils.


DAMAGE BY ARTHROPODS

‘‘‘Damage Caused by Arthropods: ‘‘‘


DAMPING OFF AND ROOT ROT (PYTHIUM SPP)

‘‘‘Pythium and Phytophthora Damping-Off and Root Rot (Pythium spp.) ‘‘‘ Mature plants show symptoms of root and crown rot. Initially, feeder roots are depleted and soon after, brown lesions develop on lateral roots. Sudden wilt is another symptom when healthy-appearing plants suddenly collapse during the heat of the day. Control of root rot can be achieved by planting on raised beds to allow for maximum water drainage after each irrigation.


DODDER (CUSCUTA SPP)

‘‘‘Dodder (Cuscuta spp.) ‘‘‘ The first sign of dodder is a dense, tangled, yellow, threadlike vine intertwined with the cucurbit plant. This parasite can quickly spread down a row to cover entire areas, giving a yellow color to affected spots in the field. Controlling dodder is difficult, because it survives in the soil for such a long time. The most successful long-term strategy is an integrated approach to control. Crop rotation with nonhost plants such as wheat, barley, corn, soybean, or dry beans can also reduce the incidence of dodder.


DOWNY MILDEW (PSEUDOPERONOSPORA CUBENSIS)

‘‘‘Downy Mildew (Pseudoperonospora cubensis) ‘‘‘ Symptoms of downy mildew are first evident as small, slightly yellow to bright yellow areas on the upper leaf surface with the color less vivid on the corresponding lower leaf surface. Lesions appear first on the older crown leaves and appear progressively on the younger, more distal leaves as these leaves expand. As the lesions expand, they may remain yellow or become dry and brown. Lesion margins are irregular on most cucurbits, but on cucumber they are angular and bound by leaf veins. The best forms of control include using resistant cultivars, applying fungicides, and using good cultural practices.


FUNGAL DISEASES OF AERIAL PARTS

‘‘‘Fungal Diseases of Aerial Parts: ‘‘‘


FUSARIUM ROT (FUSARIUM SPP)

‘‘‘Fusarium Rot (Fusarium spp.) ‘‘‘ The disease can penetrate in a number of ways including, the epidermis, the stem end, and lesions on the surface. It is one of the more common preharvest and postharvest diseases of cucurbit fruits. Avoid wounding the fruit during harvest and packing, proper storage and transit temperatures, and prompt handling of melons upon arrival at the market provide some protection against postharvest decay.


FUSARIUM WILTS (FUSARIUM OXYSPORUM)

‘‘‘Fusarium Wilts (Fusarium oxysporum) ‘‘‘ The disease is characterized by wilting of the host and generally infects the roots, both by direct penetration and through wounds. The normal progression of symptoms of the vascular wilts is a general, dull green color and lack of luster of the leaves followed by yellowing of the foliage, beginning with the older leaves, and then wilting of the leaves. The best form of control is to use resistant cultivars and long-term crop rotations or fallow periods.


GUMMY STEM BLIGHT (DIDYMELLA BRYONIAE)

‘‘‘Gummy Stem Blight (Didymella bryoniae) ‘‘‘ Circular, tan to dark brown spots appear on the leaves, often first at the margins, and enlarge rapidly until the entire leaf is blighted. Circular, black or tan spots appear on the cotyledons and stems of young plants. Satisfactory chemical control may be obtained by regular applications of protectant fungicides.


INSECTS

‘‘‘Insects: ‘‘‘


LASIODIPLODIA FRUIT ROT (LASIODIPLODIA THEOBROMAE)

‘‘‘Lasiodiplodia Fruit Rot (Lasiodiplodia theobromae) ‘‘‘ The disease causes a root rot, and a vine decline in melon at the stem end. Infected tissue is somewhat soft and has a water-soaked appearance. As the disease progresses, the affected tissue becomes brown and shriveled. In the latter stages of decay, the shriveling becomes more pronounced, and black pycnidia and dark gray mycelium develop internally. Fungicide pastes applied to cut peduncles have been successful in controlling the disease. Care should be taken to reduce injury in the harvesting, handling, and packing of the fruit.


LEAFMINERS (LIRIOMYZA SPP)

‘‘‘Leafminers (Liriomyza spp.) ‘‘‘ Adult leafminers are small flies that are black and yellow. Adult females puncture the upper surfaces of leaves for feeding and egg laying. Eggs are cream-colored and oval, laid singly in separate leaf punctures. Scouting should be initiated prior to bloom in all crops, and treatments should be initiated when numerous leaf punctures or small holes are observed on the majority of leaves on the plant.


LETTUCE INFECTIOUS YELLOWS (LIYV)

‘‘‘Lettuce Infectious Yellows (LIYV) ‘‘‘ In cucurbits, the characteristic symptom is pronounced interveinal yellowing and leaf curling, and in melons and cucumbers the striking yellow color is accompanied by a green mosaic. Symptoms develop primarily on older leaves. Allowing a host-free period and stringent weed control is the best control method.


MAGNESIUM DEFICIENCY

‘‘‘Magnesium Deficiency: ‘‘‘ Symptoms of magnesium deficiency usually first appear several weeks before harvest, when the vines are growing rapidly and the fruit have begun to enlarge. Experienced observers may first notice a gray green discoloration of interveinal tissue in crown leaves. The interveinal discoloration expands, and the tissue turns brown or tan over time. In advanced cases of magnesium deficiency, much of the tissue disintegrates, leaving what resembles leaf skeletons in the crown of the plant.


MANGANESE TOXICITY

‘‘‘Manganese Toxicity: ‘‘‘ Evidence of manganese toxicity usually appears shortly before harvest. Crown leaves of affected plants with heavy fruit loads have an unhealthy, pale green cast. Close inspection of affected leaves reveals tiny lesions, approximately the size of small pinholes, surrounded by yellow halos. The lesions occur in clusters between the veins of affected leaves.


MEASLES

‘‘‘Measles: ‘‘‘ Measles initially appears on fruit surfaces as small, water-soaked areas, 1-3 mm in diameter. Different areas of the fruit surface may be affected, but symptoms generally occur on the upper surface and often under leaves that are in close proximity to fruit. Leaves, petioles, and stems also develop symptoms of measles, with spots that are initially water-soaked and 1-3 mm in diameter. Later the centers of these spots become dry and take on a tan color. The best control can be through reducing irrigations in the fall-harvested crop.


MELON FUSARIUM WILT (FUSARUIM OXYSPORUM PV MELONIS)

‘‘‘Fusarium Wilt of Melon (Fusarium oxysporum pv melonis) ‘‘‘ Symptoms include general yellowing of the foliage and sporadic occurrence in the field. In young seedlings, a hypocotyl rot and damping- off may occur. In older plants, the older leaves turn yellow, and one or more runners wilt. Mature plants often wilt severely and rapidly late in the season, because of stress due to the fruit load. The best form of control is to use resistant cultivars.


MICRODOCHIUM BLIGHT (MICRODOCHIUM TABACINUM)

‘‘‘Microdochium Blight (Microdochium tabacinum) ‘‘‘ The disease is characterized by the production of light tan to "bleached," sunken, spindle-shaped lesions, primarily on the main stems, petioles, main leaf veins, and peduncles and sometimes on leaf blades. On fruit, the fungus causes white, tan, or silver tasseling on the upper surface. Individual lesions are less than 0.5 cm long but often group together to form a continuous dry, scabby surface. The disease is readily controlled with protectant fungicides.


MITES (TETRANYCHUS SPP)

‘‘‘Mites (Tetranychus spp.) ‘‘‘ Spider mites damage cucurbits by puncturing cells of the leaves, mainly on the lower surface. They extract plant juices and chlorophyll, interrupting the normal production of photosynthate. An early sign of infestation is stippled areas on foliage. As feeding progresses, leaves become yellow. Use recommended insecticides for control.


MOLYBDENUM DEFICIENCY

‘‘‘Molybdenum Deficiency: ‘‘‘ The first signs of molybdenum deficiency appear shortly after young plants recover from transplant shock, or about the time when direct-seeded melons begin to vineand plants are light green. Their leaves become pale green or slightly yellow between the veins. As symptoms progress, interveinal leaf tissues exhibit a distinct yellowing, and marginal leaf tissues assume a dry appearance. Most symptomatic leaves occur near the crown of the plant.


NET SPOT (LEANDRIA MOMORDICAE)

‘‘‘Net Spot (Leandria momordicae) ‘‘‘ The pathogen produces lesions that appear within 5 days after infection as white leaf spots with reddish margins on both surfaces of the leaf. The veins and veinlets in the white center of the lesion acquire a brownish red discoloration and a ringlike form, giving the lesion a netlike appearance 10-15 days after inoculation. There are no control measures available.


NUTRITIONAL DISORDERS

‘‘‘Nutritional Disorders: ‘‘‘


PAPAYA RINGSPOT W (PRSV-W)

‘‘‘Papaya Ringspot-W (PRSV-W) ‘‘‘ Most susceptible cucurbit species respond to infection with very prominent foliar symptoms and severe plant stunting. Foliage of infected plants often exhibits green mosaic, malformation, puckering, blisters, distortion, and narrow laminae. Where available, the use of resistant cultivars is the most effective control.


PHYTOPHTHORA CROWN ROT (PHYTOPHTHORA CAPSICI)

‘‘‘Phytophthora Crown and Root Rot (Phytophthora capsici) ‘‘‘ Initial symptoms include a sudden, permanent wilt of infected plants without a change in color. Often plants die within a few days of the first symptoms or after soil is saturated by excessive rain or irrigation. The roots and the stem near the soil line turn light to dark brown and become soft and water-soaked. Infected stems collapse quickly, and ultimately the roots are completely destroyed. Avoiding fields with heavy, poorly drained soils is the best form of controlling the disease.


PHYTOPHTHORA FRUIT ROT (PHYTOPHTHORA CAPSICI)

‘‘‘Phytophthora Fruit Rot (Phytophthora capsici) ‘‘‘ The initial symptoms of Phytophthora fruit rot are a water-soaked or depressed spot especially on the underside of the fruit, where it is in contact with the ground. Symptoms also develop on the upper surface of the fruit, especially following rain or irrigation which provides splashing water for fungal dispersal. The fungus produces a white, yeast-like growth, that contains many sporangia, especially under moist conditions. Rotation of crops and management of soil moisture are two important methods of control.


PICKLEWORM (DIAPHANIA NITIDALIS)

‘‘‘Pickleworm (Diaphania nitidalis) ‘‘‘ Primary damage is due to burrowing within fruits, which renders them unfit for consumption. Often, the only outward sign on an infested fruit is a small entrance hole. In heavy infestations, larvae may burrow into stems and kill plants. Chemical treatments are the best form of control, but must start when eggs or small larvae are first noticed.


PINK ROOT (PHOMA TERRESTRIS)

‘‘‘Pink Root (Phoma terrestris) ‘‘‘ Pink root of cucurbits is characterized by pink to red lesions on secondary and tertiary roots. The disease causes no aboveground symptoms in melon and watermelon plants. There is no control measures available for the disease.


POWDERY MILDEW (SPHAEROTHECA FULIGINAE)

‘‘‘Powdery Mildew (Sphaerotheca fuliginae) ‘‘‘ Whitish, talcum-like, powdery fungal growth develops on both leaf surfaces and on petioles and stems. Symptoms usually develop first on older leaves, on shaded lower leaves, and on abaxial leaf surfaces. Older, fruit-bearing plants are affected first. Infected leaves usually wither and die, and plants senesce prematurely. Resistant cultivars and fungicides on the undersides of the leaves and the lower canopy are used to manage powdery mildew.


PYTHIUM COTTONY LEAK (PYTHIUM SPP)

‘‘‘Pythium Cottony Leak (Pythium spp.) ‘‘‘ Generally, Pythium species produce brownish, water-soaked lesions which rapidly develop into watery, soft, rotted areas. The rotted areas quickly become covered with cottony extramatrical mycelium, particularly under humid conditions and on the portion of the fruit directly in contact with the soil. Cucurbits should be planted in well-drained sandy or sandy loam soil with sufficient ditching to allow rapid drainage after heavy rainfall.


RED ROT (EPICOCCUM NIGRUM)

‘‘‘Red Rot (Epicoccum nigrum) ‘‘‘ Symptoms of red rot include a bright red discoloration of the rind with no aerial mycelium or fruiting bodies. The texture of the decayed area is similar to that of healthy tissue. Decayed and healthy tissue are not easily separated. The growth of the fungus is greatly retarded at 5?C making proper storage temperatures an important role in controlling the disease.


RENIFORM NEMATODE (ROTYLENCHULUS RENIFORMIS)

‘‘‘Reniform Nematode (Rotylenchulus reniformis) ‘‘‘ Spotty areas of unthrifty, yellow plants within a field are symptoms of damage caused by the reniform nematode. The nematode causes extensive root damage in cucurbits, resulting in stunted top growth, yellowing of foliage, and wilting under moisture stress. Heavily infected plants with poor root systems develop symptoms of nitrogen, potassium, and manganese deficiencies due to limited mineral uptake. Leaf tissue of infected plants has small, compact cells, resulting in reduced leaf size. Crop rotation, fumigant nematicides, and planting resistant cultivars are the best forms of control.


RHIZOPUS SOFT ROT (RHIZOPUS STOLONIFER)

‘‘‘Rhizopus Soft Rot (Rhizopus stolonifer) ‘‘‘ The initial symptoms of Rhizopus soft rot of cucurbits are a water-soaked appearance and softening of the tissue. As fruits become completely colonized, they often collapse from the weight of other fruit. The diseased tissue is wet, soft, and somewhat pliable, and large portions of it can be removed intact. Internally, the diseased tissue in most cucurbits is characteristically water-soaked and darker than the surrounding healthy tissue. Rhizopus soft rot can be controlled most effectively by minimizing injury and bruising of the fruit and by proper cooling and temperature maintenance during shipping and storage.


ROOT KNOT NEMATODES (MELOIDOGYNE SPP)

‘‘‘Root-Knot Nematodes (Meloidogyne spp.) ‘‘‘ Root-knot nematode damage is usually associated with patches of stunted, yellow plants within a field that have reduced size and number of leaves, excessive wilting in warm weather, and poor fruit quality and yield. Usually a formation of galls or swellings on the roots develop. Where available, plant resistance is the most economical and environmentally safe method for control of root-knot nematodes.


SCAB (CLADOSPORIUM CUCUMERINUM)

‘‘‘Scab (Cladosporium cucumerinum) ‘‘‘ On leaves and runners, pale green, water-soaked areas are the initial symptoms. These spots gradually turn gray to white and may become "shot-holed" in appearance. A yellow halo appears around the lesion. Scab can produce the greatest damage on fruit. Spots first appear as small, sunken areas similar to insect stings. A sticky substance may ooze from the infected area, especially on fleshy fruit. Using resistant cultivars, rotating the crops, and using protectant fungicides are recommended measures for control.


SCLEROTINIA ROT (SCLEROTINIA SCLEROTIORUM)

‘‘‘Sclerotinia Rot (Sclerotinia sclerotiorum) ‘‘‘ Infection tends to occur in dead tendrils and petioles or through withered flowers still attached to developing fruit. The disease is favored by low temperatures and extended wet periods. Because the disease is relatively minor in cucurbits, no control procedures have been developed.


SEPTORIA LEAF SPOT (SEPTORIA CUCURBITACEARUM)

‘‘‘Septoria Leaf Spot (Septoria cucurbitacearum) ‘‘‘ Under moist conditions, lesions first appear as small, dark brown, water-soaked spots, measuring 1-2 mm in diameter. Under dry conditions, they are circular or occasionally irregular in shape and beige to nearly white. A narrow brown border surrounds the lesion and, with age, the tissue may crack. A two-year rotation and protectant fungicides are the best forms of control.


SOUTHERN BLIGHT (SCLEROTIUM ROLFSII)

‘‘‘Southern Blight (Sclerotium rolfsii) ‘‘‘ The first symptoms of the disease are yellowing and wilting of individual runners beyond the point of infection. Disease development is favored by warm, moist conditions. Nestlike, cottony white mycelium and tan to brown sclerotia almost always appear on the stem and on infected fruit, where the fruit is in contact with the soil. The fungus rarely grows much beyond the original point of infection in cucurbits. The best forms of control include deep plowing to bury plant material and sclerotia, raising the pH to about 7.0, rotating with a nonhost, and avoid planting cucurbits after a crop in which southern blight was present.


SQUASH BUG (ANASA TRITIS)

‘‘‘Squash Bug (Anasa tritis) ‘‘‘ Damage results from the withdrawal of plant fluids. Moderate feeding by a single adult can result in rapid death of seedlings and death in older plants may result from heavy feeding. Prior to the advent of modern pesticides, hand picking of squash bugs and clean culture were suggested as the best methods of control. In small plantings with only moderate populations of squash bugs, these methods are efficient and cost-effective. In large plantings, often the only alternative for control is the application of insecticides.


SQUASH FUSARIUM CROWN ROT (FUSARIUM SOLANI)

‘‘‘Fusarium Crown and Foot Rot of Squash (Fusarium solani) ‘‘‘ Usually, the first symptom noticed in the field is wilting of the leaves. If the soil is removed from around the base of the plant, a very distinct dry rot of the crown and the upper portion of the taproot is evident. The rot develops first as a light-colored, water-soaked area, which becomes progressively darker. The fungus is generally limited to the crown areas of the plant. The best forms of control include a 4-year rotation and planting fungicide-treated seed.


SQUASH LEAF CURL (SLCV)

‘‘‘Squash Leaf Curl (SLCV) ‘‘‘ The characteristic leaf curling, sometimes together with a mild mosaic is a common symptom. A bright yellow mosaic or spot accompanied by leaf curling or enations on the undersides of leaves and severe stunting occurs. Infected squash plants are severely stunted and do not produce additional foliage after infection occurs. Infected cucurbits often produce an abundance of flowers that either are delayed in opening or are shed soon after blooming, and they rarely set fruit. Weed control, soft insecticides, and resistant cultivars combine for the best control.


SQUASH MOSAIC (SqMV)

‘‘‘Squash Mosaic (SqMV) ‘‘‘ Generally, infected plants respond with a variety of symptoms, including green veinbanding, mosaic, spotting, blisters, ring spots, and protrusion of veins at the leaf margin. Plants are often stunted, producing malformed and mottled fruits. The best forms of control include using virus-free seed and applying insecticides to control the beetle vectors.


SQUASH VINE BORER (MELITTIA CUCURBITAE)

‘‘‘Squash Vine Borer (Melittia cucurbitae) ‘‘‘ Burrowing in vines and stalks can cause wilting of the plant. A telltale sign of borer infestation is holes in vines or at the base of the petioles, from which greenish frass is extruded. Heavy infestations can considerably reduce yield. Chemical control with recommended insecticides should begin when eggs are found on foliage. The field planting should be turned under as soon as the crop is harvested.


STRIPED CUCUMBER BEETLE (ACALYMMA VITTATUM)

‘‘‘Striped Cucumber Beetle (Acalymma vittatum) ‘‘‘ Only cucurbit plants are attacked. The principal damage results from adults feeding on young seedlings. These beetles are also important as vectors of pathogens which causes bacterial wilt. Newly germinated seedlings should be monitored regularly for beetles, and recommended insecticides should be applied as a foliar spray.


TARGET LEAF SPOT (CORYNESPORA CASSIICOLA)

‘‘‘Target Leaf Spot (Corynespora cassiicola) ‘‘‘ The first symptoms appear on older leaves as angular yellow spots. On plants in the field, the spots enlarge and become circular, with light brown centers and dark brown borders. As the lesions age, they turn gray and drop out, giving the leaves a "shot-hole" or shredded appearance. In the greenhouse, lesions appear as small spots which are elongated. Good sanitation is essential for control. Also using resistant cultivars and a regular fungicide spray program will help in maintaining good control.


THRIPS

‘‘‘Thrips: ‘‘‘ Injury is caused by adults and nymphs. Thrips feed by piercing the cells of bud, flower, and leaf tissues and then sucking plant juices from the feeding sites. They also feed on pollen. Feeding causes silver colored leaves, discoloration of flowers and buds and can result in fruit abortions. Small plants can be stunted and deformed. Insecticides are recommended for control and should be applied when thrips are present and damage is evident. Management of weeds within the crop is also recommended.


TOBACCO RINGSPOT (TRSV)

‘‘‘Tobacco Ringspot (TRSV) ‘‘‘ Newly infected leaves usually exhibit very bright yellow mosaic, ring spots, drying, distortion, and in some cases enations. During the first phase of infection, fruits tend to abort or remain small and become spotted and distorted. Intense cultivation and a methodical weeding program drastically reduce the presence of the vectors and the presence of the disease.


TOMATO RINGSPOT (TmRSV)

‘‘‘Tomato Ringspot (TmRSV) ‘‘‘ Infected plants react with yellow mosaic, reduction in leaf size, shortened internodes, proliferation of flower buds, and prominent ring spots on the discolored fruits. Generally, symptoms are very prominent in the initial, acute stage, which is followed by a chronic stage in which growth is mildly affected. Intense cultivation and eradication of weeds drastically reduce the presence of the virus in vectors.


ULOCLADIUM LEAF SPOT (ULOCLADIUM CUCURBITAE)

‘‘‘Ulocladium Leaf Spot (Ulocladium cucurbitae) ‘‘‘ Immature lesions are dark brown and 1-2 mm in diameter. As the lesions age, the central area becomes beige, surrounded by a dark brown ring and a circular brown halo, 6-7 mm in diameter. Several of these lesions may group together to form larger, irregular spots, but they retain their light beige centers. The best forms fo control include using resistant cultivars, fungicidal sprays, and destroying old vines from cucumber fields.


VERTICILLIUM WILT (VERTICILLIUM DAHLIAE)

‘‘‘Verticillium Wilt (Verticillium dahliae) ‘‘‘ Initial symptoms include a loss in luster in part or all of the crown leaves. Stressed leaf areas become yellow and later develop dry sectors. Eventually all leaves in the crown area collapse, as the symptoms advance along the runner. Fruit become exposed, and soluble solids are reduced. Adequate control may be achieved by avoiding sites where susceptible crops have been grown or delaying planting until temperatures increase.


VINE DECLINE

‘‘‘Monosporascus Root Rot and Vine Decline: ‘‘‘ The first symptom often observed in melon fields is stunting of young plants. Yellowing and death of leaves progress rapidly toward the ends of the vines, and most of the canopy may be killed within 5-10 days of the onset of foliar symptoms. Stem lesions are generally lacking in this disease, but aboveground symptoms are otherwise easily confused with those of other vine declines. Belowground symptoms include root lesions, root rot, loss of smaller feeder roots, and, under extreme conditions, death of the taproot. The best forms of control drip irrigation, mulching with plastic, maintaining good field drainage, and long-term crop rotations with nonsusceptible hosts.


WATERMELON FUSARIUM WILT (FUSARIUM OXYSPORUM PV NIVEUM)

‘‘‘Fusarium Wilt of Watermelon (Fusarium oxysporum pv niveum) ‘‘‘ This disease is characterized by a dull, gray green appearance of the leaves followed by yellowing of the foliage, generally starting with the older leaves near the crown and advancing outward. The leaves quickly lose turgor and wilt. The most consistent and reliable symptom is vascular discoloration, which is revealed in longitudinal or cross sections of infected roots and stems. The best control measures are plant clean, high-quality seed of wilt-resistant cultivars, plant on new land whenever possible, and if new land is not available, do not plant watermelons on the same land for a minimum of 5-7 years.


WATERMELON MOSAIC (WMV)

‘‘‘Watermelon Mosaic (WMV) ‘‘‘ Foliar symptoms include green mosaic, leaf rugosity, green veinbanding, yellow rings, and malformation. These symptoms often are very prominent in some winter and summer squashes, but affected leaves develop to nearly normal size. Fruits are not distorted, but some of their coloration is adversely affected by green spots, particularly on yellow fruit. The best forms of control include using resistant cultivars, applying mineral oil sprays to interfere with virus transmission, and insecticides to control aphid populations and slow virus spread.


WHITEFLIES

‘‘‘Whiteflies: ‘‘‘ Whiteflies damage cucurbits by direct feeding damage, by contamination with excrement on honeydew, and by inducing physiological or apparently phytotoxic disorders. Whitefly monitoring should begin at the time of seedling emergence. Yellow sticky traps can be used to trap adult whiteflies moving into a field and give an indication of when to initiate plant sampling for nymphs. Other components of a management plan include consideration of natural enemies, including predators, parasites, and pathogens.


ZUCCHINI YELLOW MOSAIC (ZYMV)

‘‘‘Zucchini Yellow Mosaic (ZYMV) ‘‘‘ The virus incites yellow mosaic, severe malformation, blisters, extreme reduction in the size of leaf lamina, drying, and severe plant stunting. Malformation is the greatest symptoms that will be observed. Resistant cultivars offer the only resistance that provide consistent control.