Watermelon | Texas Plant Disease Handbook

Wilting or Yellowing of Plants

Fusarium Wilt Causal Agent (fungus – Fusarium oxysporum f.sp. niveum): Initially, leaves of infected plants wilt during hotter times of the day, but eventually, the wilt becomes permanent. Initially, one or a few vines of a plant are affected, but eventually the other vines wilt and the plant dies. Other agents can cause plants to wilt. For example, anasa wilt is caused by the feeding of the squash bug. The fungus causes browning of the xylem (water-conducting tissue) of the crown and runners. Races 1 and 2 of this fungus are present in several watermelon-growing areas of Texas. Most commercial cultivars have resistance to to race 1, but not race 2. The fungus persists many years in soil and so, watermelons should not be replanted into infested soils for at least five years.

Yellow Vine Causal Agent: (unknown, suspected to be a bacterium): This disease is found mainly in the Cross Timbers vegetation zone in central Texas and Oklahoma, but it has been found outside of this area, specifically, in Anderson county in east Texas. The symptoms appear two weeks before harvest. There is a general yellowing of leaves and terminal leaves of the vine are turned up. Not all plants within a field initially show symptoms. Plants eventually die. These symptoms are not diagnostic for this disease. The distinctive symptom of yellow vine is a honey-brown discoloration of the phloem in lower crown and primary root of plants. The cause has not been conclusively determined, but a phloem-limited bacterium has been consistently associated with diseased plants and a deltacephaline leafhopper is thought to transmit this bacterium. Other susceptible cucurbits include squash, pumpkin, and cantaloupe. There are no control measures available for this disease, but resistance in triploid seedless watermelon (e.g. ‘Tri-X 313’) has been identified.

Southern Blight Causal Agent: (fungus – Sclerotium rolfsii): This soilborne fungus infects the crown, resulting in wilting of foliage. The fungus also causes a fruit rot. Infection is favored in warm, moist soils. There are no control recommendations.

Charcoal Rot Causal Agent: (fungus – Macrophomina phaseolina): A water-soaked lesion develops around the crown of the plant, followed by the appearance of an amber-colored ooze at the surface. This area becomes dark brown, dried and cracked. Small black fungal structures (microsclerotia) are then formed in this area. A dark ring is visible beneath the outer layer when a cross section is cut through the canker. Infected plants have yellow crown leaves and die soon after symptoms are observed. The damage is most severe when melons approach maturity. There are no effective controls for the fungus. Altering the irrigation program to prevent water stress may alleviat the problem.

Monosporascus Root Rot/Vine Decline Causal Agent: (fungus – Monosporascus cannonballus): Infected plants usually do not develop symptoms until near harvest time. Infected plants wilt and eventually die. Sometimes, there is vascular browning. Brown lesions may be found on the roots, even on plants that are not wilting. Root lesions may be caused by other soilborne fungi, such as Pythium sp. and Rhizoctonia solani. This disease is primarily a problem on silt or clay soils of the Lower Rio Grande Valley. The fungus is indigenous to the soil and populations build up with continued cucurbit monoculture, particularly cantaloupe. Soil fumigation is an effective control, but is not usually economical. Soils with consistent problems should not be planted to cucurbits.

Damping-Off Causal Agents: (fungi – Rhizoctonia solani, Pythium spp.): Seedlings wilt and die. The lower stems have a watersoaked appearance. Damping-off is usually a problem of direct-seeded watermelons germinating in cool, moist soil, but Pythium aphanidermatum can rapidly kill new transplants in warm soils. A soil application of mefenoxam (Ridomil Gold) at the time of planting will prevent damping-off caused by Pythium species.

Nematodes: Nematodes can substantially reduce yield of watermelon. The affected plants are stunted and chlorotic. Root-knot nematodes can cause galling on roots, but there are no visible root symptoms with the reniform nematode. The root-knot nematode occurs primarily in sandy soils. Although nematicides can provide an economical control, a crop rotation with corn or sorghum and a weed-free fallow can reduce numbers of nematodes below damaging thresholds.

Non-Pathogenic Causes: Agents that can cause seedling death that resemble damping-off include the seed corn maggot and lightning damage. Wind movement of young plants on plastic mulch can cause damage to the stems, leading to wilt. This problem disappears once vines are able to anchor to something. Anasa wilt, caused by the squash bug, can be mistaken for Fusarium wilt. Watermelon growing next to brush are more prone to this problem. Herbicide injury can cause symptoms that resemble those caused by pathogenic fungi. Glyphosate can cause yellowing. Trifluralin can cause scattered wilting in a field and affected plants will also exhibit discoloration or swelling of the crown area.

Leaves Deformed, or with Spots or Mold

Bacterial Fruit Blotch Causal Agent: (bacterium – Acidovorax avenae subsp. citrulli): Infected seedlings initially develop water-soaking on the lower surface of cotyledons and leaves. These areas then become necrotic. The pathogen can also cause non-distinctive, necrotic leafspots on transplants. On leaves of more mature plants, the pathogen can cause large, circular brown lesions, usually near the edge of the leaf, but this is not reliable for diagnosis. The unique symptoms are: irregular, rust-colored lesions that run along leaf ribs, while on the underside of the leaf, the leaf rib is water-soaked. Leaf lesions can serve as a source of bacteria that can infect fruit. In the field, leaf lesions are often difficult to find and may be confused with lesions caused by pathogenic fungi. Where this disease has occurred, watermelons and other cucurbit crops should not be replanted in the field for at least one season, crop debris should be plowed under, and volunteer plants destroyed. Cucurbit weeds such as citron melon should also be controlled. Copper-based fungicides should be applied on a regular schedule at the time of flowering or earlier, if transplants are used.

Anthracnose Causal Agent: (fungus – Colletotrichum orbiculare): Symptoms appear first on crown leaves as small, brown-black spots usually after vines begin to “run”. These lesions are also visible on the underside of leaves. During damp weather, orange-pink masses of spores develop in centers of larger leaf spots. During dry weather the spore masses turn gray. All but the youngest leaves are infected. Lesions can coalesce, causing the leaves to die. The disease can be seed borne and symptoms can sometimes be seen on young transplants. A combination of seed treatment, crop rotation and fungicide applications are necessary for controlling this disease on susceptible varieties. Protective spray applications should be made when vines start to run and should be continued at 7 to 10 day intervals during periods of humid or rainy weather. Schedules may be lengthened to 14 days during dry weather.

Gummy Stem Blight Causal Agent: (fungus – Didymella bryoniae): Early symptoms are round black, wrinkled spots on young leaves and dark sunken areas on stems. Early infection usually develops from contaminated seed and can be visible on transplants. Often, infected transplants will have lesions on the the stem. On older leaves in the field, irregular brown to black spots develop between leaf veins. Leaf lesions will expand, eventually resulting in the death of leaves. As the disease progresses, older stems near the crown, most commonly near a leaf petiole or tendril, split and a light brown gum oozes from the watersoaked areas. The crown may also be infected and produce a brown ooze. Infected runners die, leaving fruit exposed. vegetables are not usually affected. Protective fungicide applications are needed for control. Resistance to benomyl and thiophanate-methyl fungicides have occurred in some growing areas, so these fungicides are not recommended.

Downy Mildew Causal Agent: (fungus – Pseudoperonospora cubensis): The disease is most severe during periods of cool temperatures and rain. Irregular yellowish to brown spots, often vague in outline, appear on upper leaf surface near the crown. Brown spots later become more distinct on both sides of the leaves. The underside of leaves exhibit a brown to gray fungal growth on wet mornings, which disappears during dry weather. This growth is best seen early in the morning, or, alternatively, suspect leaves can be removed and kept several hours in a plastic bag with a moist paper towel to provide high humidity that encourages sporulation. Spots enlarge rapidly and turn black and the entire leaf withers and dies. Leaf petioles of diseased leaves stand upright although the leaf is almost dead. There can be a rapid, extensive loss of foliage with this disease. The disease is favored by cool (60oF night and up to 77oF day) temperatures, along with rain or continuous high humidity. This disease can occur in a field miles away from other cucurbits, indicating that the spores can travel great distances. The fungus develops and spreads rapidly during cool, moist weather conditions. Excellent control of this disease can be achieved with mefanoxam (Ridomil Gold), which must always be used simultaneously with a protectant fungicide such as mancozeb or chlorothalonil, to prevent the emergence of resistant strains. It is important that these sprays are applied on a preventative basis, before the disease becomes established in a field. Some growers report that they only use chlorothalonil or mancozeb to effectively manage downy mildew, but, the key to their effectiveness is regular, preventative applications.

Alternaria Leaf Spot Causal Agent: (fungus – Alternaria cucumerina): Lesions are round to irregular target spots on older leaves. The symptoms are first observed at the crown of the plant. As the spots enlarge, concentric rings are formed in the lesion. The disease is favored by continuous wet conditions. The fungus is controlled with a 2 year cucurbit-free rotation, destruction of the previous crop residue and application of fungicides as needed. At the end of the growing season, plow under crop residue.

Cercospora Leaf Spot Causal Agent: (fungus – Cercospora citrullina): Leaf spots have a dark brown center and a yellow halo. Infected leaves are first observed at the crown of the plant. When the disease is severe, foliage loss will restrict fruit development and result in sunburn of fruit. The fungus is controlled with fungicides. Applications should begin soon after runner development.

Myrothecium Leaf Spot Causal Agent: (fungus – Myrothecium roridum): This fungus causes small, dark brown circular lesions on leaves. These lesions can coalesce. The disease is favored by wet weather conditions that favor the development of other foliar pathogens and can be controlled by the protectant fungicides used to control these pathogens. The disease occurs infrequently and has been observed mostly in the Lower Rio Grande Valley.

Powdery Mildew Causal Agent(s): (fungi – Sphaerotheca fuliginea, Erysiphe cichoracearum): The first symptom is a white to gray dusty material on the upper leaf surface. The fungus quickly spreads to completely cover the leaf surface. Once the leaf is covered, it dies and has a crisp texture. This disease is favored by high humidity, but not free water on leaves. The fungus is best controlled with fungicide applications when symptoms are first observed. It tends to occur on older leaves and later in the growing season, so control is not usually warranted. Two systemic fungicides, Benlate (benomyl) and Bayleton (triadimefon), are no longer effective because of pathogen resistance.

Leaf Mosaic Causal Agents: (viruses -Watermelon Mosaic Virus, Papaya Ringspot Virus, Zucchini Yellow Mosaic Virus, Cucumber Mosaic Virus): Leaves of infected plants have a mosaic pattern, i.e., areas of light and dark coloration. This mosaic may be in association with raised areas on leaves. Leaves may also be distorted in shape. Diagnosis of the mosaic viruses can not be made with certainty from symptoms alone. Sometimes plants may be infected by more than one type of virus. Losses from virus infection are more significant if young plants are infected. Most of the young fruit is aborted and those that are not aborted are malformed. The viruses listed above are aphid-transmitted and can overwinter in weeds. Virus control by insect control is not usually feasible. Weed control in and around the field may help to reduce losses, except for virus introduction via winged aphids.

Tobacco Ring Spot Virus: Tiny brown spots surrounded by yellow halo may appear on young leaves as a “stippling” effect. Symptoms produced on vegetables are more important than leaf symptoms, though. The virus can overwinter in wild hosts and in infected seed. It is spread from infected plants by the dagger nematode (Xiphinema spp.), grasshoppers, tobacco thrips, tobacco flea beetles and several other insects.

Squash Leaf Curl Virus: This virus is transmitted by whiteflies and is a problem only in south Texas. Symptoms of the disease are crumpled leaves with yellowed, mottled areas. Leaves have shortened petioles that cluster around the vines. Squash is also susceptible to this virus. Severe yield losses are associated with infection of young seedlings, usually when whitefly populations are high. Infections of older plants do not affect yield. Another whitefly-transmitted virus recently identified in south Texas is the cucurbit yellow stunt disorder virus, which causes a yellowing of leaves. The squash leaf curl virus is economically important only in fall-grown watermelons. To alleviate the impact on yield, transplants should be used, rather than direct-seeded plants. Fields should be plowed under after harvest to minimize carryover of the virus to subsequent crops. The virus can persist in whiteflies for 3 weeks after feeding on infected plants, so there should be a 4 week break between melon crops.

Herbicide Injury: Growth deformation on foliage can be caused by 2,4-D or dicamba. This deformation can resemble symptoms caused by some mosaic viruses.

Fruit Affected

Bacterial Fruit Blotch Causal Agent: (bacterium – Acidovorax avenae subsp. citrulli): Fruit symptoms start as water-soaked spots that rapidly enlarge. Symptoms are generally seen in the light portions of striped fruit [PICTURE 58]. The lesions later turn brown and crack. The pathogen is introduced to a field by planting contaminated seed or transplants. Within the field, the bacteria spread to healthy plants via rain or overhead irrigation, especially under conditions of high humidity and temperature. The fruit are most susceptible to infection around the time of pollination. As the fruit matures, it becomes resistant to infection. Once introduced into a field, the bacteria can persist on volunteer watermelon or citron plants, or in their seed. Cantaloupe and honeydew melons are also susceptible to this pathogen. Where this disease has occurred, watermelons and other cucurbit crops should not be replanted in the field for at least one season, crop debris should be plowed under, and volunteer plants destroyed. Cucurbit weeds such as citron melon should also be controlled. Copper-based fungicides should be applied on a regular schedule at the time of flowering or earlier, if transplants are used.

Anthracnose Causal Agent: (fungus – Colletotrichum orbiculare): The fungus produces sunken spots on the rind of fruit, that are often black because of the production of spores. Symptoms may not be visible at harvest, or they might be slight, but decay may develop further in transit or at the market. Small anthracnose spots can be confused with pimples disease.

Bacterial Rind Necrosis Causal Agent: (bacterium – Erwinia spp.): The first symptoms are numerous brown, dry, hard, discolored areas in the rind. These areas enlarge and may merge to form extensive necrotic areas. The disease rarely extends into the flesh of infected melons in the field. The disease occurs sporadically and is thought to be caused by bacteria that are naturally present in fruit, but the reasons for symptom development are not understood. Since usually there are no external symptoms, a small percentage of diseased melons could make the crop unmarketable, as diseased melons cannot be culled. There are no control measures.

Belly Rot Causal Agent(s): (fungi – several species): The fruit decays at the area in contact with soil. The rot begins as a small water soaked area that quickly spreads. In many cases, the area will be surrounded by a profuse growth of white mycelium. One of the fungal species associated with belly rot is Pythium aphanidermatum, which rapidly colonizes the fruit. Another species involved is the southern blight fungus, Sclerotium rolfsii, which produces seed-like resting structures. The disease is most severe during periods of rainfall and high temperature, when the soil remains wet for long periods of time. To control, use well-drained soils, or plant on plastic mulch.

Lasiodiplodia Fruit Rot / Diplodia Stem-End Rot Causal Agent: (fungus – Lasiodiplodia theobromae): Watersoaking is seen at the stem end and the tissue is soft. When harvesting, a large portion of the peduncle should remain attached to the fruit. The disease is of minor importance.

Pimples Causal Agent: (suspected virus – Tobacco Ring Spot Virus): Small bumps or pimples develop on fruit, which are most noticeable on dark rind varieties. Other fruit symptoms are ringspots and necrotic, concentric rings.

Blossom End Rot Causal Agent: (physiological – water stress): Browning and shriveling occur at the blossom end of melon, followed by a secondary decay caused by microorganisms that progresses inward. It is caused by a calcium deficiency in the fruit and occurs in plants subjected to drought stress. Cultivars can differ in their susceptibility to the problem. Watering operations that avoid drought stress can reduce or eliminate the problem. Excess nitrogen fertilizer may also contribute to blossom-end rot.

Bottle Neck Causal Agents: (physiological – several): Bottle neck or otherwise misshapen fruit can be caused by pollination problems or fluctuations in watering. Cultivars can differ in susceptibility. Increasing the bee population by bringing in bee hives and placing them near the melon field may alleviate the problem.

Sunburn: The lack of foliage to protect the fruit can result in sunburn. This often occurs as a consequence of loss of foliage caused by fungal diseases such as downy mildew.

Fruit crack/scarring: Cracks, scarring, and pitting can be caused by mechanical damage when vegetables are young, Insects can also cause such injury. Animals, such as wild hogs and racoons, can cause more substantial damage.

Splitting: Splitting is caused by an erratic watering pattern, particularly the excessive accumulation of water. The inside of the fruit is under great pressure and the fruit splits open when lightly tapped. This condition is weather related and there probably is a cultivar interaction as well.

Cross-stitch: There are parallel cuts extending into the rind, which are perpendicular to vascular bundles. The cause is unknown, but it is not suspected to be infectious.