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Powdery mildew is a disease that can affect a wide range of plants. It forms a very distinct-looking white powder, generally on top of infected leaves. In severe cases, it can also be found on the undersides of leaves, buds, fruit, stems, Roses, peonies, squash, crabapples, fruit trees, ninebarks, and many other plants host this disease. Powdery mildew will appear in mid to late summer when conditions are especially hot and dry since it cannot grow on wet leaves. Ironically, high humidity also helps it spread. The fungus overwinters in leaf debris around infected plants.
Many plants are bred to be resistant to powdery mildew, so selecting disease-resistant varieties for our landscapes will help avoid the disease long term. In areas of high humidity where plants are packed together, pruning to increase air circulation will also help. If the disease is already present on a desirable plant, don’t water overhead and consider using Bonide’s “Copper Fungicide” or Neem oil. Both of these products are safe for edibles. Applications will have to be made two weeks apart throughout the growing season for good control.