Seasonal development
and life cycle—Downy mildew
Downy mildew can develop very quickly from an initial infection into an epidemic spread by airborne spores
if humidity and temperature conditions are favorable (1.5 - 7 hours of leaf wetness and 43° - 80° F). Spores
can travel long distances in moist air, but are quickly killed by dry conditions. Initial sources of disease
can be infected bulbs, sets, seeds, and plant debris.
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Spores
on onion leaf
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