Fungal diseases and viruses are the most common diseases that affect watermelon plants. These diseases may infect the foliage, roots or fruit of watermelon. The use of fungicides can fight outbreaks of fungal diseases, but maintaining good cultural practices will prevent many diseases from developing and surviving in your garden.
The fungal disease anthracnose forms small, dark-brown spots on the leaves of watermelon plants. Orange or pink spores form in the center of large leaf spots during wet weather, while gray spores form during dry weather. Sunken spots may appear on the rind of the fruit. Remove infected vines and dispose of them away from the garden. Keep the area clean from fallen debris, as this disease can overwinter in the garden. Avoid overhead watering and rotate watermelon crops to prevent the disease. Apply fungicides at the first sign of the disease, according to the directions on the product.
Powdery mildew affects watermelon plants during warm, humid weather. This fungal disease causes yellow leaf spots and leads to early ripening of the fruit. Plants infected with powdery mildew will produce small fruits in low quantities. To prevent this disease, avoid planting watermelon in shaded locations and do not over-fertilize. Apply sulfur fungicides as soon as symptoms appear.
Downy mildew thrives in wet conditions, causing small green or yellow leaf spots. A layer of purple fuzz may grow on the undersides of the leaves. Foliage eventually turns brown and curls upward. Space plants properly to provide air circulation and reduce wet conditions. In the early stages of the disease, apply fungicides to infected plants. Use different fungicides if repeated applications are necessary, as downy mildew may grow resistant to some chemicals. In severe cases, remove and discard infected plants.
Fusarium wilt survives in warm, sandy soils for up to two years. Foliage of infected plants turns yellow and wilts. A pinkish white fuzzy layer may appear near the ground. Look for disease-resistant cultivars, such as Klondike Blue Ribbon, and rotate crops to prevent the disease from surviving in the soil.
Gummy Stem Blight
Gummy stem blight causes browning of the edges of watermelon leaves. As the disease progresses, circular brown spots and a brown, gummy substance appear on the rest of the leaf and the stem. This fungus may spread via infected seed or enter the plant via open wounds. Use insecticidal soaps to fight pests that may create open wounds. Rotate crops so that two years pass before you plant watermelon in the same location.
Watermelon plants infected with viruses often display mosaic patterns of green or yellow on their leaves. Viruses often attack young growth, causing new leaves to appear distorted. Watermelon plants infected early in the growing season may produce no fruit. Late-season infections may result in small, discolored fruit. Viruses usually transfer to watermelon plants via aphids. If you detect aphids on your watermelon plants, apply pesticides. Control perennial weeds to prevent the survival of viruses in your garden.