Downy Mildew
Downy mildew is a widespread, serious disease of beans, crucifers,
cucurbits, maize, potato, tomato, rice but most common with grapevines.
Previously considered a fungus, the downy mildew pathogen called
Plasmopara viticola, is now classified an algae. Initial leaf symptoms
are light green to yellow spots, called “oil spots” because they may
appear greasy. Under humid conditions, white, downy spore masses can be
seen on the lower leaf surface. These spores are wind dispersed. The
lesions eventually turn brown as the infected tissue dies. Severely
infected leaves drop prematurely, which can reduce winter hardiness of
the vine. Infected flower clusters dry up or become covered with white
spores under humid conditions. Infected berries turn a mottled
dull-green or reddish purple and readily fall from the cluster. Although
berries become resistant to infection within three weeks after bloom,
the rachis remains susceptible for several weeks longer.
The two types of downy mildew:
Right: powdery mildew
Left: downy mildew
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