ARE THE LEAVES of your lilac, zinnia and phlox turning white? The dry summer we had this season was the perfect environment for breezes spreading the disease-causing spores of powdery mildew.
Powdery mildew is a common name for a group of fungal diseases that affect many species of fruit, vegetables, lawns (especially Kentucky Blue) and ornamental plants. The white color you see on leaves is caused by the spread of fungal threads (mycelium) across the leaf surface. This gives the well-known powdery appearance. Warm, dry days followed by cool, humid nights is the optimum condition for the disease development.
When spores land on a leaf and conditions are right, fungal threads send a structure into the leaf to extract nutrients. Compounding the stress is the fact that the white covering on the leaf blocks light, therefore inhibiting photosynthesis (the food making). Plants usually decline in vigor during this time; however it is rarely lethal to the plant.
Don’t confuse powdery mildew with downy mildew. Downy mildew can cause a powdery covering, but is an entirely different disease and is usually found on the underside of a leaf. Its management is different.
Management of powdery mildew begins by choosing resistant varieties of plants when available. Because powdery mildew is frequently more of a problem in low-light areas, be sure your plants are getting enough light. Give your plants adequate room to grow. Spacing allows better air circulation. Remove and discard any infected material. Throw it in the trash. This disease can over-winter on plant debris. Some of the powdery mildews are inhibited by free moisture on the leaf. If other diseases aren’t an occurrence on a particular plant, such as lilac, wetting down the plant during hot, dry days may prevent the spores from taking hold on the plant.
You wouldn’t want to do this with vegetables. Wet leaves on tomatoes can spell disaster. Antitranspirant materials applied once a month starting in June may prevent this disease.
Finally, at the first sign of powdery mildew, a spray with a horticulture summer oil will prevent the spread of disease.
Remember that with plant diseases, once present, a disease can’t be cured, only managed.
Interesting enough, in California a species of lady beetle eats powdery mildew!