Downy Mildew - Peronospora effusa                      Clemson University, USDA Cooperative Extension Slide Series, www.ipmimages.org HOSTS: Broccoli, cauliflower, cabbage, Cucurbits (cucumber, squash), lettuce, onion, pea, spinach. DESCRIPTION: Downy mildew is caused by a fungus which is active during cooler weather, especially temperatures around 46° F. Temperatures between 46 - 60° F. accompanied by rain or high humidity (85%) promote infection. Leaves turn yellow then black and rapidly rot in wet conditions. LIFE CYCLE: The fungus overwinters in plants, on seeds, and in the soil. First spores occur in the spring after exposure to water for several hours. CONTROLS: Cultural: Grow tolerant or resistant varieties. Allow good air circulation by thinning and controlling weeds. Manage overhead irrigation so that plant surfaces have time to dry before cool evening temperatures, or use soaker hoses, drip irrigation. Rotate your crops on a three year rotation. Chemical: Spray copper at the first sign of disease and repeat if weather is wet.
Missoula County Extension Office; Missoula, Montana 59808 - Updated for 2006