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.
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Spores
of downy mildew
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