Symptoms: A fungal disease, 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 mostly in cool, moist weather, usually in early spring or late fall. Spore production is favored by temperatures cooler than 65 degrees 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.
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.
Tip: The systemic action of Organocide Plant Doctor moves throughout the entire plant to treat fungal diseases. Mix 3/4 tsp per gallon of water and apply to foliage. Spray to run-off, as required for disease control.
Photo Credit: Plant Health Australia