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. Plant grapes in sunny areas with good air circulation. Removing weeds and orienting rows to maximize air movement will also speed drying of leaves. Preventative fungicide programs from bloom through harvest can be use to control downy mildew.