Downy Mildew of Grapes Plant Pathology Downy Mildew of Grapes Fruit showing signs of downy mildew have been arriving in the Plant Disease Clinic.
Infected berries are usually brown and wrinkled and are covered with the whitish fungus.
The undersides of infected leaves also show fluffy, white to grayish fungal growth. Pale-yellow spots are evident on the upper surface of leaves. The fungus Plasmopara viticola causes downy mildew. The disease is favored by factors that increase the moisture content of soil, air, and the plant. Rain is the principal factor in promoting disease.