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.
On fruit, most infection occurs during two periods in the growing season. The first is when the berries are about the size of small peas. Little infection occurs during hot summer months. As nights become cooler in late summer, a second infection period may develop. Cultural practices that improve air circulation are recommended to reduce the threat of disease.