Powdery mildew

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Powdery mildew, a harmless fungus infection, on leaves of beebalm (Monarda fistulosa). Notice that the plants bloomed heavily despite the infection and that neighboring plants of unrelated species are unaffected. Beebalm almost always gets powdery mildew.

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Powdery mildew on coralberry (Symphoricarpos orbiculatus). Again, despite the infection, this vigorous shrub is quite healthy and about to bloom on schedule. Coralberry rarely gets powdery mildew, but we’re having a rather wet growing season.

Powdery mildew, like most of the diseases that infect plants, is caused by a fungus, or rather a large group of related fungi in the order Erysiphales. It unsightly but completely harmless.

What should you do about powdery mildew? Absolutely nothing. It is a cosmetic problem and will not harm your plants. To prevent it or reduce the incidence, you can use some commonsense horticultural practices, such as preventing overcrowding or watering in the morning only. If you feel you must treat it, a mild spray of milk, diluted 1:10 with water, has been shown to be effective. But remember that almost everything you do in the garden may have unintended consequences, such as changing the pH of the soil or killing butterfly eggs.