Downy mildew, caused by the fungus Pseudoperonospora cubensis, was observed in pumpkin and melon fields throughout Illinois in the past two weeks. This disease affected pumpkin field severely in 2001, but no detection of the disease was made in 2002. Downy mildew affects cantaloupes, cucumbers, gourds, muskmelons, pumpkins, squash, and watermelon. Downy mildew can reduce yield, fruit quality, and harvesting time.
Symptoms of downy mildew vary with the host and the environmental conditions. The first symptom is usually the appearance of indistinct, pale green areas on the upper leaf surface. In this stage the disease resembles the mosaic mottling. The pale green areas soon become yellow in color and angular to irregular in shape, bounded by the leaf veins. As the disease progress the lesions may remain yellow or become brown and necrotic. During moist weather the corresponding lower leaf surface is covered with a downy, pale gray to purple mildew. Leaf symptoms can be used to diagnose downy mildew in the field in most cases. On watermelons, yellow leaf spots may be angular or non-angular, and they will later turn brown to black in color. Often on watermelons, an exaggerated upward leaf curling will occur.
Control of downy mildew on cucurbits is achieved primarily by the use of resistant varieties and/or fungicide spray programs. Fungicide sprays are recommended for all cucurbits. However, resistant varieties of cucumbers allow for fewer spray applications. Squash, pumpkin, cantaloupe, and non-resistant cucumber varieties are very susceptible and should be sprayed every five to seven days. When nighttime temperatures are between 55° and 75°F and relative humidity is above 90%, conditions are ideal for infection. Ridomil Gold Bravo, Bravo Ultrex, Bravo Weather Stick, Manex, Maneb, fixed copper, Aliette, Cabrio EG, Quadris, and Flint are effective against downy mildew. However, since powdery mildew is also present in the cucurbits fields, spray application of Cabrio EG, Quadris, or Flint plus a fixed copper should control downy mildew and powdery mildew, and reduce the occurrence of the bacterial spot. Please follow the label directions carefully.