How to Manage Pests

Pests in Gardens and Landscapes

Seasonal development and life cycle—Downy mildew

Damp, moist conditions and cool weather are ideal for the development and spread of downy mildew. Spores, produced only under conditions of subdued light or darkness and a near-saturated atmosphere, are dispersed by wind and, in cool, moist conditions, they can survive several days. Free moisture on the lettuce leaf is necessary for spore germination and infection, but moisture is not required for vegetative growth once the fungus is within the leaf. Areas with year-round lettuce culture and favorable climate often have the most serious outbreaks. Dry, desiccating winds and clear, warm days inhibit the growth and spread of downy mildew.

Spores of downy mildew
Spores of downy mildew


Statewide IPM Program, Agriculture and Natural Resources, University of California
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