Connecticut Garden Journal: Tips for reducing, controlling, and preventing powdery mildew disease
As the nights become cooler in late July and August, powdery mildew disease starts making its appearance on many vegetables, flowers, trees and shrubs.
This fungal disease thrives when humidity goes up and temperatures cool in late summer. There are a number of different fungi that cause the disease. It can attack plants such as lilacs, crabapples, bee balm, phlox, tomatoes, squash, zinnias, and many other plants. Beside the white, powdery growth on the leaves, powdery mildew can also cause leaf rolling or deforming and yellowing or greying of the leaves.
While powdery mildew disease usually doesn't kill plants, it can reduce production of vegetables and be unsightly in a garden. Luckily, there are many ways to reduce the infection. First, you can move to a drier climate (just kidding). But better would be to space plants further apart to increase air flow and cause the leaves to dry out, clean up infected plants well and grow resistant varieties. Some good powdery mildew resistant varieties include 'Marshall's Delight' bee balm, 'Volcano' and 'Candy Store' phlox, 'Butterbaby' butternut squash, 'Purple Dome' aster and 'Indian Summer' crabapple.
Start looking for diseased leaves now as it gets started. Pick, remove and destroy the leaves. If you need to spray to control powdery mildew, select organic pesticides and spray now before the diseases starts spreading quickly. Bio Neem oil, Bacillus subtilis (Serenade) and sulphur sprays all will reduce the spread, but not kill established disease. Even a baking soda mixture can help. I've sprayed a 1 tablespoon of baking soda, ½ teaspoon of liquid soap and 1 gallon of water mixture to prevent the disease.