Downy Mildew Control



Downy mildews are a group of closely related pathogens that can cause major damage in the nursery, greenhouse, and landscape in the form of leaf spots, blights, and distortions. Although similar in name, do not confuse downy mildews with powdery mildews. Powdery mildews are true fungal pathogens that produce white, flour-like colonies — usually on upper leaves. Downy mildews, on the other hand, are a completely different kingdom of organisms, more closely related to algae than to fungi. The distinction between powdery mildews and downy mildews is important, because the fungicides effective against one are not usually effective against the other — although, as with every rule, exceptions do exist.
HOST PLANTS: Common in Grapes and other vine-growing fruits and vegetables.

SYMPTOMS: 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.

FAVORED ENVIRONMENT: The pathogen overwinters in infected leaves on the ground. In spring, spores are carried by rain splash to new leaves, where they require a film of water for infection. Lesions appear 5 to 17 days after infection. The disease can spread rapidly under warm conditions with frequent rain or dew. Use the 10-10-10 rule to decide when to start scouting for downy mildew: at least 10 cm (4 in.) of shoot growth, 10 mm (0.4 in.) rainfall and temperatures of 10°C (50°F) during a 24-hour period.

DOWNY MILDEW CONTROL: The best way to prevent downy mildew is to avoid the environmental conditions that favor the disease. Prune or stake plants and remove any weeds to improve air circulation. Water in the early morning hours (avoiding overhead watering if possible) to give the plants time to dry out during the day. Keep the ground under infected plants clean during the fall and winter to prevent the disease from spreading. Remove and destroy any plants with serious infection. Choose resistant varieties whenever possible.

Downy mildew is comparatively easy to control on most plants when the foliage and fruit are kept protected by organic fungicides. If you catch the infection early, apply a copper spray/dust to diseased and surrounding plants every 7-10 days until harvest.

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