Watermelon Disease Descriptions

General

Watermelons are susceptible to several diseases that attack the roots, foliage, and fruit. The most common diseases of watermelon are anthracnose, fusarium wilt, , downy mildew, gummy stem blight, root-knot nematode and virus diseases. Also, cercospora leaf spot, powdery mildew, bacterial fruit blotch, damping-off, and root rots/vine declines have been reported as a problem. Consult your local agricultural Extension Service for identification and management of watermelon diseases and the latest recommendations on the use of chemicals. Disease control is essential in the production of high quality watermelons. A preventive program that combines the use of cultural practices, genetic resistance, and chemical control as needed usually provides the best results. Cultural practices are useful for limiting the establishment, spread, and survival of pathogens that cause watermelon diseases. Many of the fungal, bacterial, and nematode pathogens survive in old crop debris and in soil. Fields should be rotated with non-cucurbit crops for at least three years to reduce pathogen levels.Grass crops are ideal for rotations where nematodes are a problem.Diseases such as anthracnose, bacterial fruit blotch, gummy stem blight, and Fusarium wilt are known to be carried on seed. This can lead to rapid disease development and spread in greenhouse transplant production and to the introduction of diseases into fields. Purchase seed from reputable sources, and apply a fungicide seed treatment prior to planting.Most foliar diseases are spread by water-splash or are favored by long periods of leaf wetness. Use drip irrigation or avoid frequent sprinkler irrigation with small amounts of water. Finally, use tillage practices that promote the rapid decomposition of old vines and melons soon after harvest. The use of disease-resistant varieties is an economical means of controlling diseases. Several varieties have resistance to Fusarium wilt. Some varieties also are resistant to anthracnose.Management of foliar diseases such as anthracnose, downy mildew, Cercospora leaf spot, and gummy stem blight may require fungicide sprays. Consult North Carolina Agricultural Chemicals Manual for a listing of fungicides approved for use on watermelon

Around the world, over 10 viruses are known to be a problem in watermelon production. Watermelon mosaic virus(WMV-2), papaya ringspot virus-watermelon strain (PRSV-W, formerly known as watermelon mosaic virus-1), and zucchini yellow mosaic (ZYMV) are the most seriously important watermelon virus diseases in the USA..Virus diseases are destructive to the watermelon crop, and are difficult to control. The major control strategies involve insecticides to eliminate the insect vectors, herbicides to remove alternate hosts, or genetic resistance. The most economical method for control of virus diseases is genetic resistance.

Anthracnose

Anthracnose is a very common fungal disease and is caused by the seed-borne fungus Colletotrichum lagenarium. Most cucurbits are susceptible. Most commercial watermelon varieties have some resistance. Depending on weather, leaf and fruit spotting may occur on young plants, especially in late plantings. Appears first on crown leaves as small, brown-black spots. During damp weather, orange-pink masses of spores develop in centers of larger leaf spots. During dry weather the spore masses turn gray. Mostly the youngest leaves are attacked. On fruit the fungus causes sunken spots on the rind. Symptoms may not be visible at harvest but may develop in transit or at the market. Small anthracnose spots can be confused with pimples disease.CONTROL: A combination of seed treatment, crop rotation and fungicide applications are necessary for control on susceptible varieties. Follow general management practices; start spray schedule at first appearance. Vines should be thoroughly covered with fungicide spray. Avoid soaking seed before planting. It is advisable to grow watermelons, cucumbers, squash or pumpkins only once every five years on the same land.

Downy Mildew

Downy Mildew is caused by the air-borne fungus Pseudoperonospora cubensis. Disease is serious on late season cantaloupes, pumpkins, squash, and watermelons. The fungus over-winters in Florida, and each year the spores are blown northward and arrive in North Carolina from May to August, if at all. Disease is most severe during periods of mild temperatures and rainy season. Typically its arrival coincides with summer rains. CONTROL: Resistant cultivars; start spray schedule at first appearance.Follow a protective spray program during periods favorable for infection and spread. Begins as small yellowish areas on the leaves. Early in the morning when moisture has been present overnight, young spots may appear as water-soaked, somewhat angular areas on the lower side. As the lesions enlarge in a somewhat angular manner they change from yellow to brown. On the lower side, purplish spore masses may be seen, and several infected leaves may die in 10-14 days. Older leaves become infected first. Downy mildew does not attack stems or fruit.CONTROL: Follow general management practices; start spray schedule at first appearance.

Fusarium Wilt

Fusarium wilt is caused by Fusarium oxysporium f. sp. niveum. The fungus may survive several years in soil. The disease is favored by warm, and sandy soils like those in watermelon-growing areas.Susceptible varieties are killed at any stage of growth. If inoculum levels of Fusarium are high, seedlings may wilt in the field.Foliage of infected plants turn yellow and wilt.Plants die soon after symptoms are observed.On the dying vine’s stem, a pinkish white cottony growth can appear near ground level. The fungus spreads outward from infected plants.There are three races of the wilt fungus. Genetic resistance has been incorporated into many of the newer cultivars for two of the more commonly occurring races. The third race has only been found in a few isolated areas of the world. There is no source of resistance to this race. CONTROL: Wilt losses are managed by following a five year rotation program with non related plants and use of resistant varieties.

Gummy Stem Blight

Gummy Stem Blight is a warm weather southern disease. It is caused by seed borned fungus Didymella bryoniae. All cucurbits are susceptible, and depending on the weather, stem and leafspotting may occur on young plants, especially in late plantings.Early symptoms are round black, wrinkled spots on young leaves and dark sunken areas on stems. Early infection usually develops from contaminated seed.Many times it produces brownish circular spots which rapidly expand into large tan lesions of round, indefinite size. The lesions may exhibit brown gummy exudate on the surface and stems may be girdled. In prolonged rainy periods most of the foliage and vine may rapidly collapse. Fruit lesions appear as small water-soaked areas, similar to anthracnose, which rapidly enlarge to indefinite size. Also, a brown streak may appear in the blossom end of the fruit. Although this disease may attack young tissue, it is primarily a disease of older tissue. CONTROL: Follow general management practices; start spray schedule at first appearance.

Root-Knot Nematode

Root-Knot is a nematode diseased that is caused by Meloidogyne spp. are usually present in agricultural fields and affect many crops. Nematode damage is visible as stunted yellowed plants. The plants develop symptoms of plant stress and when the population is very high, the plants die. Examination of the roots reveals small to moderately sized galls. The galls will be scattered over the root system. Normally nematode damage is restricted to a small area of the total field. CONTROL: Promptly disk all crops after harvest. Soil fumigation with a soil nematicide will reduce the soil population.Rotations with corn, sorghum or pasture grasses will also reduce the root knot population. The practice of summer fallow will reduce the nematode population as well.