Answer: Beebalm is highly susceptible to mildew. The mildew appears depending on the weather each year, some years are worse than others. The best defenses are to plant the beebalm in spot with good air circulation, thin the plant in the spring so it does not grow quite so dense, and select a variety with noted resistance to it. In the early stages you may be able to rinse it off the plant with water from the garden hose and clip off affected leaves. Treatments really need to be done prior to it appearing in order to be effective.
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