Diseases can limit cucurbit production. Effective disease management is essential in the production of high quality cucurbit vegetables. Some diseases directly attack the fruit, rendering it unmarketable. Others indirectly reduce yields by killing plants prior to harvest or cause defoliation, which reduces fruit size and quality.
Cucurbit crops are subject to biotic diseases caused by pathogens that include fungi, bacteria, viruses, and nematodes. Abiotic (non biological) diseases are caused by environmental conditions such as soil imbalances (nutrients or pH), moisture imbalances, and chemical injury (herbicides and toxic chemicals).
Effective management depends on the correct disease identification. Incorrect identification can lead to the use of the wrong management practice, wasted expense, and crop failure. For example, diseases caused by bacteria or viruses are not controlled with most fungicides. Likewise, a particular fungicide may control one fungus disease, but not another. Cucurbit growers should learn to recognize the more common diseases by their symptoms and have sufficient knowledge to select appropriate management practices for the particular disease situation. Some diseases are easy to identify in the field while others are more difficult. The OSU Plant Disease Diagnostic Laboratory offers disease diagnosis as a service to growers. Samples can be submitted to the laboratory through the County Extension office (see OSU Extension Facts L-220 Plant Disease and Insect Diagnostic Laboratory.)
Integrated Pest Management (IPM) involves the use of several different strategies and the practical use of pesticides for management of diseases and other cucurbit pests. Better and more economical control results when IPM is practiced, compared to relying on a single management practice such as pesticide application. Infection and spread of some pathogens are associated with the presence of certain weeds (alternate hosts) and insects (vectors). Therefore, weed and insect management are important components of disease control in an IPM system for cucurbits. Disease management strategies that are effective components of an IPM system are listed below.
Fungi, bacteria, and nematodes that cause soil-borne and some foliar diseases survive in the soil or on crop debris in soil between cucurbit crops. These pathogens build up to damaging levels with repeated cucurbit cropping. A 3- to 4-year rotation with non-cucurbit crops where possible is recommended.
Most cucurbits grow best on sandy loam, sand, or silt loam soils with a pH of 6.5 to 7. Growth on acid and/or poorly drained soils often results in increased incidence of Fusarium wilt and fruit rots. Maintaining records of the disease history of fields is beneficial for avoiding disease problems or implementing preventive measures. Late plantings should not be situated near early plantings where a disease already exists.
Several destructive diseases of cucurbits can be spread from infested fields to clean fields by soil and crop debris carried on equipment and workers. Equipment and boots should be washed to remove all clinging soil and debris when leaving infested fields to avoid contamination of clean fields. Clean fields can also be worked before entering infested fields.
Disease resistant varieties of cucurbit crops are available and should be planted where possible. Tables 9-14 lists the disease reactions of some cucumber, cantaloupe, and watermelon varieties grown in Oklahoma. Tables 11 and 12 list newly developed disease resistance pumpkin and squash varieties. For some diseases, resistance is the only effective control. A factor affecting disease resistance is the development of pathogen races. Races are strains of a pathogen that overcome resistance and are capable of attacking previously resistant varieties. See cultivar selection in Tables 9-14 for more information.
Fumigation increases yields, earliness, and controls soilborne diseases. However, fumigation is expensive and potentially dangerous for inexperienced applicators. For production of high-value cucurbits, such as early cantaloupes, row fumigation may be economically feasible.
Some diseases may be seed-borne or introduced into fields on infected transplants. Efforts should be taken to obtain seed and transplants from reputable sources. Seed treatment with a fungicide is effective in minimizing the potential for seed-borne diseases caused by fungi and helps ensure establishment of an adequate stand. Only healthy transplants should be used to initiate plantings.
Wild cucurbits and numerous other weed species may serve as alternate hosts or sources of infection for virus diseases. However, elimination of weed hosts over an entire cucurbit growing area is probably not practical. Maintaining good weed control in cucurbit fields is an important component of a disease management program. Heavy weed competition interferes with spray penetration and deposition onto foliage when fungicide applications are required.
Some insects transmit virus and bacterial diseases. Aphids are responsible for spreading all of the currently important virus diseases of cucurbits in Oklahoma. Unfortunately, control of aphids with insecticides is not effective in reducing levels of virus disease. However, insects that spread bacterial disease should be controlled when warranted.
Fungicide sprays may be needed for effective management of some foliar diseases caused by fungi. Most soil-borne diseases cannot be controlled by foliar fungicide application. Copper sprays are also useful in reducing foliar diseases caused by bacteria. Consult the latest edition of the Oklahoma Extension Agents' Handbook (Circular E-832) for a list of suggested chemical treatments for specific diseases. The performances of some fungicides and bactericides against important cucurbit diseases rated in the appendix. Fungicides protect healthy plants from infection, but do not cure diseased plants. Timing of the first application is critical because diseases are more difficult to control once established. Adequate spray volume achieves thorough coverage of the foliage. Applications should be made before an anticipated rain event rather than after because this affords protection during periods favorable for infection. The first application should be made before or shortly after symptoms first appear. A fungicide spray generally offers one to two weeks of protection before loss of activity occurs. Therefore, fungicides must be reapplied to maintain extended periods of protection. Consult fungicide labels for suggested spray intervals. Growers also time fungicide applications by using weather-based spray advisory or disease forecasting systems. These programs are developed for specific diseases and permit the timing of fungicide sprays to coincide with weather conditions that favor infection. Oklahoma has a spray advisory program available for timing fungicide sprays to control anthracnose.
Two broad groups of fungicides are available for control of plant diseases. Protectant fungicides are active only on plant surfaces and act like paint to protect plants from infection. Protectants tend to be broad-spectrum in activity and mode of action. As a result, development of fungicide resistance in pathogens has not been a problem. Systemic fungicides penetrate plant surfaces and move within plants to various degrees. Systemic fungicides have the benefit of being rain-fast and able to eradicate fungal infections at very early stages. Systemic fungicides generally have very specific modes of action, making them vulnerable for development of fungicide resistance. Fungicide resistance has been a problem for cucurbit diseases such as powdery mildew, downy mildew, and gummy stem blight. Fungicide resistance can lead to reduced levels of disease control and complete control failures. Strategies for reducing the risk of resistance development include tank mixing or alternating systemic and protective fungicides, alternating different types of systemic fungicides, using recommended rates, and maintaining low disease pressure. Labels for at-risk fungicides provide detailed guidelines for preventing fungicide resistance.
Excessive irrigation or frequent irrigations with small amounts of water favors spread and development of many diseases and should be avoided. Drip irrigation reduces foliage diseases because drip systems do not wet the foliage. The wetter the foliage, the greater the potential for foliar infections to occur. Drip irrigation also reduces the spread of diseases moved by splashing water or runoff.
Crops should be scouted regularly (at least once per week) for all pests and diseases. Scouting allows for early detection of diseases so timely management practices can be implemented. Regular crop inspections assess the effectiveness of management programs already implemented.
Vine debris should be incorporated into the soil by plowing or disking after harvest to hasten decomposition since many of the pathogens survive in and on debris.
Acid Yellowing of Cantaloupes
Cantaloupes do not grow vigorously in strongly acid soil. Plants grown in soils below pH 6.0 grow poorly and develop a yellow-green color known as acid yellowing, an abiotic disease. Crop production is delayed and yield can be greatly reduced.
Control of Acid Yellows: Acid soils should be avoided or corrected with lime before planting cucurbits.
Table 16. Relative importance of cucurbit diseases on specific cucurbit
crops in Oklahoma (H=high, M=moderate, L=low, - does not occur)
Anthracnose (Colletotrichum obiculare)
Anthracnose is a common and destructive fungal disease of watermelon, cantaloupe, and cucumber in Oklahoma. Pumpkin and squash are rarely affected. Periods of warm, rainy weather favor the disease. All parts of the foliage, stems, and fruit can become infected. Leaf spots appear as small yellowish areas that enlarge rapidly. On cantaloupe and cucumber, spots turn tan and are often surrounded by a yellow border or "halo." Spots on watermelon are angular and brown to black in color (Figure 14). Spots eventually dry, crack, and leaves shatter or die completely. Elongated lesions also appear on stems and petioles (Figure 15). Entire vines may be killed by the combination of leaf and stem infections. Circular and sunken spots, which range in size from 1/4 to 1 inch in diameter, develop on infected fruit (Figure 16).
The Anthracnose fungus winters on infested debris from previous cucurbit crops. Spores produced on this debris cause the first infections on the new crop. These spores are carried to plants by splashing rain or running water. The disease increases as a result of infection by spores produced on new lesions, which are spread in the same manner. The seed-borne fungus can be introduced into clean fields on contaminated seed or infected transplants.
Control of Anthracnose
Anthracnose is difficult to manage once it becomes established, thus sanitation practices should be employed to prevent contamination of clean fields. Plantings should be initiated with fungicide-treated seed from a reputable source or with healthy transplants. Other effective strategies include crop rotation, residue management, and fungicide spray programs. Adequate resistance to race 2, which is commonly encountered in Oklahoma, is lacking in watermelon varieties. Fungicides are most effective when a spray schedule starts before symptoms appear. Reviewing field history can help anticipate Anthracnose outbreaks. A weather-based spray advisory program (www.mesonet.ou.edu/ premium/agmodels.html) has been developed in Oklahoma to aid in the timing of fungicide applications for Anthracnose.
Alternaria Leaf Spot (Alternaria cucumerina)
This fungal disease has been minor in Oklahoma but occurs on cantaloupe and watermelon. Fruit are rarely infected, but defoliation resulting from leaf infections reduces fruit size and quality. The fungus survives in contaminated debris from old cucurbit crops for up to 2 years. Initial infection comes from airborne spores. Thereafter, spores produced on leaf spots serve to increase disease levels when long periods of leaf wetness occur.
Symptoms appear first on older leaves near the crown of the plant. Leaf spots are, at first, very small but enlarge to 1/4-inch diameter on watermelon and 3/4-inch diameter on cantaloupe. Spots become numerous and turn dark brown to black on watermelon but are light brown on cantaloupe. Key features of Alternaria leaf spots are distinct. Their margin and the zonate rings within the lesion impart a target-like appearance. Severely infected cantaloupe leaves shrivel and die, after curling upwards at the margins. Infections then spread to the outer leaves.
Control of Alternaria: Crop rotation and residue management provide partial control by delaying disease development. However, fungicide sprays are needed to limit disease development. No cucurbit resistant varieties are available.
Bacterial Fruit Blotch (Acidovorax avenae subsp. citrulli)
Bacterial fruit blotch of watermelon was first identified in Oklahoma in 1991 and has since been a periodic, but minor problem (Figure 17). Bacterial fruit blotch has also been reported to occur on cantaloupe and pumpkin in other states. However, the disease has been of major economic importance to the watermelon industry because of its impact on watermelon seed availability and price. While the disease has not become widespread in Oklahoma, it has the potential to reduce yield of marketable melons by 50%. The bacterium survives in seed, and contaminated seed provides a source for new infections and disease outbreaks. However, the bacterium can also survive in the field from year to year in crop residue and in contaminated seed left after harvest that produce volunteer plants the following season. The disease is favored by warm, moist conditions. These conditions regularly occur during greenhouse production of transplants.
Foliar symptoms of fruit blotch can be found as early as plant emergence, but the important phase of the disease is the blemishing of mature fruit. Small water-soaked areas (a few millimeters in diameter) on cotyledons or leaves may develop, but are easily overlooked. Mature leaf spots are dark brown and angular similar to bacterial leaf spot. The foliar phase of the disease does not cause leaf blight or defoliation. However, leaf spots serve as a source of bacterium for fruit infections. Fruit infections first appear as small water-soaked areas on the upper surface of melons. This water-soaked area expands to cover a large portion of the fruit surface but does not extend into the melon flesh (Figure 17). Areas within older lesions may turn brown and crack. White bacterial ooze may also exude from cracked areas and the rind may then decay from other secondary organisms.
Control of Fruit Blotch: Control of bacterial fruit blotch centers on planting seed or transplants that are free of the bacterium and on crop rotation and residue management programs. Preliminary research indicates that spraying copper fungicides in fields where the foliar infections have been confirmed may minimize losses.
Bacterial Leaf Spot (Xanthomonas campestris pv. cucurbitae)
Bacterial leaf spot is increasing in importance in Oklahoma and other Midwestern states. The disease severely affects pumpkin, but also affects squash, watermelon, and cantaloupe. The disease appears during the summer months when temperatures are high and rainfall occurs. Bacterial leaf spot can cause complete blighting of foliage and severe fruit rot when rains are frequent. While little is known about the biology of this pathogen, the seedborne bacterium probably survives in the field in crop debris and volunteer cucurbits. Most bacterial pathogens are spread mechanically and by splashing and wind-driven rain. Overhead irrigation promotes bacterial disease development.
On watermelon and cantaloupe, small water-soaked spots appear on the lower side of the leaf, and are later visible on the upper side of the leaf as diffuse yellow spots. Lesions become round to elongated and angular, tan to brown in color, and surrounded by a yellow border of varying intensity (Figure 18). Initial spots on pumpkin and squash are easy to overlook because of their small size (1/8-inch diameter). Spots are light tan and surrounded by a yellow border (Figure 19). Spots usually appear water-soaked on the lower leaf surface. Where spots merge, large areas of yellowing may develop. Leaf spots often are large and elongated when associated with leaf veins. Severe infestations may lead to complete vine defoliation. Fruit spotting can be severe on pumpkins. Fruit spots are tan, initially small (1/8-inch), circular, slightly sunken, with a dark brown, greasy border (Figure 20). Spots often enlarge to over 1/2-inch diameter, crack, and extend deep into the rind. Severely affected fruit may collapse in the field or become prematurely soft after harvest.
Control of Bacterial Leaf Spot: Crop rotation and planting clean seed help manage bacterial leaf spot. Avoid saving seed from fields where bacterial leaf spot has been identified and purchase seed from reputable sources. Resistant varieties have not been identified. Rapid incorporation of crop residue after harvest is recommended. The effectiveness of chemicals has not been demonstrated.
Bacterial Wilt (Erwinia tracheiphila)
Bacterial wilt has been of minor importance in Oklahoma, but can be severe when it occurs. The disease effects cantaloupe and cucumber the most, squash and pumpkin less frequently, and rarely affects watermelon. Cucumber beetles (stripped and spotted) spread the bacterium, and occurrence of the disease is associated with beetle feeding. Symptoms first appear as dull green, wilted areas on leaves. Individual runners and then entire plants wilt and die (Figure 21). A field diagnosis can be made by pressing the cut surfaces of a wilted stem together and observing the sticky threads that form when the stems are slowly pulled apart (Figure 22). The wilt bacteria overwinter in cucumber beetles.
Control of Bacterial Wilt: Prompt control of cucumber beetles is the only effective management strategy for this disease. See the section on management of insects and mites for more information.
Cercospora Leaf Spot (Cercospora citrullina)
Cercospora leaf spot is a common, but minor fungal disease on watermelon, cantaloupe, and cucumber, particularly in southern Oklahoma. The disease mainly affects leaves. Spots appear first on older leaves and may become numerous. Spots are small (1/8- to 1/4-inch diameter) circular to irregularly circular with white to tan centers and dark margins (Figure 23). While the disease may cause defoliation and reduced fruit quality, significant damage from this disease rarely occurs in Oklahoma.
Control of Cercospora Leaf Spot: Crop rotation and residue management programs will help delay disease development. Resistant varieties are not available. This can effectively be controlled with most fungicide spray programs targeted at other, more important diseases.
Downy Mildew (Pseudoperonospora cubensis)
Downy mildew is a fungus that infects all members of the cucurbit family, but in some areas, certain crops are affected more than others. In Oklahoma, cantaloupes and watermelons have been very prone to downy mildew attack from late July through September. While downy mildew outbreaks are usually severe, the disease is sporadic and may not appear for one or more years. The disease causes rapid vine defoliation, and while it does not directly attack fruit, fruit from defoliated vines is of poor quality. The fungus requires living plants for growth and survival and does not overwinter in Oklahoma, but rather spreads as airborne spores northward from southern production areas as the season progresses. The disease favors periods of high humidity and leaf wetness. Unlike downy mildew fungi that effects other crops, infection can occur when temperatures are warm (up to 80°F). Downy mildew spores can also tolerate several hot days and remain infective.
Symptoms of downy mildew are variable depending on the host crop. The oldest leaves near the crown are usually attacked first. On cucurbits other than watermelon, symptoms appear as pale green then yellow spots on leaves, which eventually become light brown, angular spots confined by leaf veins (Figure 24). Under humid conditions, a fine layer of fungal growth may be seen on the underside of spots. Leaf spots on watermelon are typically irregular in shape and dark brown to black in color (Figure 25). Infected leaves quickly die, curl inward, and remain attached to upright petioles. Severely infected fields appear as if they had been frosted. Petioles and stems remain intact for some time following defoliation.
Control of Downy Mildew: Control downy mildew by planting resistant varieties of cucumber and using fungicide sprays. Early plantings for July harvest often escape infection while late plantings for harvest in August and September are most vulnerable. Because the potential for rapid vine defoliation, sprays should be initiated on a preventive basis for vulnerable plantings or soon after nearby outbreaks of the disease have been reported. A disease-forecasting program is available on the Internet at the following address: www.ces.ncsu.edu/depts/pp/ cucurbit. This program tracks the long-distance movement of the fungus from reported sources of diseased fields and may be beneficial for anticipating downy mildew outbreaks.
Fusarium Wilt (Fusarium oxysporum)
Fusarium spp. are persistent soilborne fungi that increase in soils repeatedly cropped to cucurbits. Different strains of the fungus attack cantaloupe or watermelon. Within each strain, races also occur that attack different varieties. However, symptoms are similar on both crops. Seedling infection causes damping-off and seedling blight. Infection of older plants results in wilting of one or more vines (Figure 21). Elongated brown lesions (dead areas) may develop along stems near the crown. Spore masses, which appear as pink mold, also develop on these lesions in wet weather. Affected vines and entire plants eventually wilt and die. A reddish-brown discoloration of the inner vascular tissue (xylem) is apparent when wilted stems are cut near the crown (Figure 26). Roots of severely affected plants are often decayed and appear shredded.
Control of Fusarium Wilt: Introduction of wilt fungi into clean fields from infested equipment or contaminated seed should be avoided. Control this by using resistant varieties (Tables 9, 10, and 14), soil fumigation, or both. Fields with a known history of wilt problems should be avoided. Only long crop rotations (6-year minimum) will rid a field of the wilt fungi, but shorter rotations will help limit increase of the fungus and development of races capable of attacking resistant varieties.
Gummy Stem Blight and Black Rot (Didymella bryoniae)
Gummy stem blight has been a minor disease in Oklahoma where it occurs periodically in the eastern part of the state. The name "gummy stem blight" refers to the stem phase, and black rot to the leaf and fruit phases of this fungal disease. All cucurbits are susceptible. Symptoms of the disease vary widely depending upon the host plant. The disease favors cool to warm, rainy weather, high humidity, and wet soils. On watermelon, the disease confines itself to the foliage although seedling blight and fruit rot do occur. Leaf spots are circular to irregular in shape and dark brown to black in color (Figure 27). Tiny dark specks (fruiting bodies) may be seen imbedded within the spots when viewed through a hand lens. Heavily spotted leaves are killed.
The fungus causes a gummy stem blight of cantaloupe. Symptoms first appear on the stem nodes of plants nearing harvest. Affected nodes appear water-soaked and may exude sap that dries to form amber-colored gum deposits (Figure 28). Fruiting bodies of the fungus may also form on infected nodes and appear as tiny brown specks. As infections progress, vines beyond the infected nodes yellow, wilt, and die.
The fungus causes water-soaked spots on fruit that can become a problem in storage. Gummy exudates may develop within these spots. On most cucurbits, black rot symptoms appear as water-soaked areas that later become sunken and black in color. On butternut squash, the fungus causes a large area of superficial bronzing with distinct center rings.
Control of Gummy Stem Blight: Crop rotation and residue management should be implemented to reduce levels of the fungus. Fungicide sprays are needed where gummy stem blight is a problem because resistance to the disease is not available. Proper handling, curing, and storage of fruit control black rot after harvest.
Mosaic Viruses (CMV, PRSV, WMV, ZYMV)
Several types of mosaic viruses infect cucurbits in Oklahoma. Symptoms caused by the different mosaic viruses are usually similar. More than one virus may be present in plants exhibiting symptoms. The most susceptible crops in Oklahoma are pumpkin and squash, but mosaic symptoms are common in other cucurbits. Damage from mosaic viruses can be extensive if a large proportion of plants are infected early in the season. Damage results from stunted plant growth, reduced fruit set, and abnormal fruit development.
Plants can be infected at any stage of growth. Symptoms are most striking on the new growth of young, rapidly growing plants (Figure 29). Leaves are dwarfed, misshapen, puckered, pale green in color, and/or mottled with areas of light and dark green color (Figure 30). In crops such as squash and pumpkin, infected plants remain stunted throughout the season and may fail to set fruit. Stunting is usually less severe in watermelon, but vine growth may be abnormal and upright. Fruit from virus-infected plants may be small in size, deformed and knobby, and may develop unusual color patterns such as the conspicuous green areas that form on yellow squash (Figure 31). Virus infection of older plants usually results in less dramatic symptom development and damage.
Mosaic viruses that commonly infect cucurbits are usually spread by aphids, cucumber beetles, or mechanically. Aphids acquire the virus by feeding on virus-infected crops or on weeds that harbor viruses and act as virus reservoirs. Aphids migrate to cucurbit crops and rapidly infect plants during their probing and feeding activities. Aphids other than the melon aphid, a significant cucurbit pest, are usually responsible for initial virus infection in fields. Generally, only aphids carrying the virus and moving from field to field initially infect a few plants. Melon aphid activity within fields contributes to secondary spread, which can be extensive. Aphid populations and virus levels generally increase as the growing season progresses making late plantings particularly vulnerable. See the section on management of insects and mites for more information.
Control of Mosaic Viruses: Effective control of mosaic virus diseases relies on the use of resistant varieties (Tables 11 and 13). However, resistance to one or more of the viruses is lacking in many cucurbit crops including watermelon and pumpkin. Traditional resistance derived through conventional plant breeding is available in cucumber, squash, and a few cantaloupe varieties. Transgenic resistance using biotechnology is now available in squash. Aphid control using insecticides does not reduce the virus spread, because insecticides do not kill the aphids before they infect plants. Reflective mulches that repel aphids are partially effective. Weed management should be practiced in and around cucurbit fields. Cucurbit plantings, particularly those planted late in the season, should not be situated near or downwind of other fields or areas infected with the virus.
Phytophthora Blight (Phytophthora capsici)
Phytophthora blight is a devastating fungal disease of summer squash and pumpkin in many areas of the eastern U.S. In Oklahoma, the disease has been observed in the Arkansas River Valley in eastern Oklahoma. It has severely damaged processing squash during periods of high rainfall. The fungus forms resistant spores that survive in the soil for more than a year in the absence of host plants. In saturated soil, the resting spores germinate to produce secondary spores that move in water, germinate, and infect plant stems and fruit. Symptoms appear after a rain in low areas of the field. Seedlings rot near the soil line and damp-off. During fruit set, the disease causes a crown rot of pumpkin and squash, which causes affected plants to rapidly wilt, fall over, and die. Initial symptoms of crown rot are elongated, water-soaked or greasy, spots on the stem base near the soil line. The spot expands to 2 to 3 inches in length and turns tan to light brown in color (Figure 32). Following additional rains, the disease quickly spreads with water-runoff to kill large areas of plants or entire fields. Disease progress ceases during dry periods. In addition to pumpkin and squash, the fungus causes fruit rot on watermelon, cantaloupe, and cucumber. However, the vines of these other crops are resistant to infection. Fruit symptoms begin as small, water-soaked spots that expand to form large, soft, sunken areas with white to gray fungal growth.
Control of Phytophthora Blight: Phytophthora blight is difficult to control. Consistently effective fungicide programs have not been identified. Practice crop rotation so that cucurbits and peppers are not grown for at least 3 years. In areas with a history of Phytophthora blight, select well-drained fields and form firm raised beds constructed to eliminate any potential standing water or drainage back up. Pre-plant and post-emergence applications of a labeled fungicide, such as mefanoxam (Ridomil Gold), may be beneficial when used in conjunction with drainage management.
Powdery Mildew (Sphaerotheca fuliginae)
Powdery mildew is a common fungal disease of cantaloupe, cucumber, squash, and pumpkin. Recently, powdery mildew has become more important on watermelon. The disease attacks the foliage causing premature defoliation. The fungus requires living plants to grow and survive. Therefore, the primary source of the fungus is spores dispersed over long distances from infested fields. The disease increases under a wide range of temperatures, but is arrested above 100°F. Under moderate temperatures, the disease increases rapidly under low-light conditions and high humidity. Rainy weather is not necessary for powdery mildew development. Mildew symptoms first appear on the oldest leaves as small areas of whitish, powdery growth. The powdery growth may occur on either or both sides of a leaf. Areas of powdery growth eventually expand and cover most of the leaf surface (Figure 33). Affected leaves eventually wither and die. Non-typical symptoms may appear on watermelon. The non-typical symptoms develop on watermelon during mid-summer when the development of powdery growth is sparse and confined to the lower leaf surface. Yellow blotches appear on the upper leaf surface, which eventually turn brown. The disease can be difficult to diagnose under these conditions without the aid of a microscope. Powdery mildew can cause extensive defoliation that leads to increased sunburning, premature ripening, and poor quality fruit.
Control of Powdery Mildew: Control of powdery mildew is best achieved with resistant varieties. Resistant varieties have been available in cantaloupe and cucumber (Tables 10 and 13), and more recently in pumpkin and squash (Tables 11 and 12). Fungicide programs, begun prior to or shortly after the first appearance of symptoms, are beneficial on susceptible cucurbits where the disease begins early in the season. However, repeated use of fungicides can quickly lead to the development of fungicide resistance and control failure.
Rind Necrosis
Rind necrosis, also called "bacterial rind necrosis," is a sporadic problem of watermelon in Oklahoma. The condition can be devastating when it occurs because loads of watermelons that appear externally healthy are rejected during marketing when rind necrosis is discovered. Symptoms of rind necrosis are brown, corky areas on the internal areas of the rind that are revealed when melons are cut open (Figure 34). The affected areas range from single spots about 1/4 -inch in diameter to the entire rind. Except for the outwardly bumpy appearance of some affected melons where rind necrosis is severe, there are no external symptoms. Several bacteria have been isolated from rind necrosis. However, the same bacteria are found in healthy melons. It is suspected that these bacteria cause rind necrosis when triggered by some unknown environmental conditions. Rind necrosis is considered by many to be a physiological disorder of unknown cause.
Control of Rind Necrosis: There are no known control strategies other than to grow varieties that are less susceptible to rind necrosis. While calsweet is a known problem variety, there is little information available on the reaction of varieties to rind necrosis.
Root-Knot Nematode (Meloidogyne spp.)
Nematodes are microscopic roundworms that feed on plant roots. All cucurbits are susceptible to damage from root-knot nematodes in soils where high populations occur. Sandy soils are the most prone to nematode problems. Symptoms are pale green to yellow foliage, uneven growth along rows, plant stunting, and wilting during hot days. Affected plants have a knobby and poorly developed root system (Figure 35). The symptoms vary greatly with soil type, weather, and the level of nematode infestation. Nematode damage may also increase levels of Fusarium wilt and various root rots.
Control of Nematodes: Achieve control by rotating with non-host crops (grasses) for at least 2 years. Laboratory analysis of soil samples is encouraged for fields where nematodes have been a problem or are suspected.
Yellow Vine
Yellow vine is a serious problem for cucurbit production in Oklahoma. The disease has been particularly severe on watermelon and cantaloupe in southern Oklahoma along the Red River. However, yellow vine has been observed at varying levels in many parts of the state on all cucurbits except cucumber. Early-planted cucurbits have been the most severely damaged. Symptoms of yellow vine generally appear during fruit set and become severe about 2 weeks before harvest. Leaves near the crown of the plant or entire plants turn pale green and then bright yellow (Figure 36). Yellowing and leaf death generally progresses outward from the crown. On watermelon, remaining leaves on the outer portions of the vine may appear dwarfed and unusually upright. The key diagnostic features for yellow vine include a lack of external root or crown symptoms, and a light brown discoloration of the food-carrying vascular tissue (phloem) visible in cross sections of the lower stem and roots (Figure 26). Eventually, entire plants are killed. The distribution of the disease in affected fields is patchy, and edges of the field are often severely affected.
The disease is complex because identification of a causal pathogen has proven difficult. Yellow vine has been confused with Fusarium wilt that causes a brown discoloration in the inner, water-carrying vascular tissue (xylem). Disruption of the phloem accounts for the yellowing symptom for yellow vine while the xylem disruption accounts for the wilting associated with Fusarium wilt. The presence of a bacterium (Serratia marcescens) not normally a cause of disease in plants is closely associated with symptoms of yellow vine. However, the bacterium has been found in symptomless plants and efforts to reproduce the disease by inoculating plants with the bacterium have failed. Weekly insecticide sprays have provided some degree of disease control on squash, suggesting an insect cause or vector. Squash bug has been found to carry the bacterium. In addition, row covers that exclude insects have prevented infection of plants with Serratia and development of yellow vine symptoms. In addition, caging plants with squash bugs carrying the bacterium has resulted in a low level of disease development.
Control of Yellow Vine: At present, there are no satisfactory control measures available. Triploid (seedless) watermelon varieties may have some resistance to the disease. Controlling squash bug is beneficial. See the section in management of insects and mites for more information on how to control squash bug.
Animal pests can cause major damage to vine crops, particularly watermelons and to a lesser degree cantaloupes. Field mice and rats can cause extensive stand reductions by feeding on seeds before they germinate. Mice will often go along the row, digging out and eating newly planted seed before they emerge from the soil. A high mice population can destroy newly planted seed in an entire field in one or two nights. Rodent control should begin long before the seed is planted, rather than waiting until after it has been eaten. Fence row sanitation and brush control around fields well in advance of planting time will reduce the rodent populations, as well as the overwintering sites for insect pests such as cucumber beetles.
Raccoons and coyotes are highly attracted to ripe watermelons and cantaloupes.
Coyotes are especially troublesome in watermelon fields. Propane or carbide
guns, loud radios, and/or lights at night can provide short-term deterrence
of coyotes. County trappers can be contacted through the County Sheriff's Office
or animal
damage control for assistance with coyote control. Shooting can be the last
resort, when crop damage persists. Raccoons usually cause less damage than coyotes.
Crows are the main bird pest of cucurbits, especially watermelons and cantaloupe. They tend to move from fruit to fruit creating holes that make the melons unmarketable. Strings stretched across the field with aluminum pie-plates or aluminum strips can be an effective daytime repellent. Bright colored wind-socks hung from stakes flap in the wind and can deter entering birds. Propane or carbide guns can be employed for repelling birds. Crows may be hunted during specified seasons.
Chemical animal repellents have not been widely used in vine crops because of poor deterrence, impractical application over large areas, and the high cost per acre.
Producers that need assistance with managing animal pests can contact the Cooperative Oklahoma Wildlife Services Program at 405.521.4039. This program is a joint effort with the Oklahoma Department of Agriculture Wildlife Services Division and USDA/APHIS Wildlife Services. Websites related to managing wildlife follow.
Oklahoma Department of Agriculture Wildlife Services Division at: www.state.ok.us/~okag/wls.htm
National Wildlife Research Center at: www.aphis.usda.gov/ws/nwrc
Wildlife Services Fact Sheets at: www.aphis.usda.gov/