Downy Mildew

Downy Mildew

Learn how to identify and control downy mildew naturally — without resorting to toxic sprays!

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Downy mildew (Plasmopara viticola) affects many plants and appears as yellow to white patches on the upper surfaces of older leaves. On the undersides, these areas are covered with white to grayish, cotton-like fungi. These “downy” masses are most often noticed after rain or heavy dew and disappear soon after sunny weather resumes. As the disease progresses leaves may eventually turn crisp and brown and fall off even though the plant has ample water.

Downy mildew occurs in cool, moist weather usually in early spring or late fall. Spore production is favored by temperatures cooler than 65˚F. and by relative humidities approaching 100%. This disease overwinters on plant debris and in the soil. Fungal spores can be carried by insects, wind, rain or garden tools.

Treatment

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, or use a soaker hose, 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 (see Fall Garden Cleanup).
  • Choose resistant varieties whenever possible.
  • Downy mildew is comparatively easy to control on most plants when the foliage and fruit are kept protected by a copper spray. Begin treatments two weeks before disease normally appears or when weather forecasts predict a long period of wet weather. Alternatively, begin treatment when disease first appears, and repeat at 7-10 day intervals for as long as needed.
  • The systemic action of Organocide® Plant Doctor moves throughout the entire plant to treat common disease problems. Mix 3/4 tsp per gallon of water and apply to foliage. Spray to run-off, as required for disease control.

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