Answer: If you can rub the powdery substance off, it's a fungal disease called powdery mildew. It will spread so be careful not to touch any other plants or you'll introduce the disease.
You can purchase fungicides to eliminate the problem or you can make your own fungicide: Baking Soda Fungicide
Mix 4 teaspoons (about 1 rounded tablespoon) of baking soda and 1 tablespoon of horticultural oil into one gallon of water. Spray lightly on foliage of plants afflicted with black spot, powdery mildew, brown patch and other fungal diseases. Avoid over-using or pouring on the soil.
Best wishes with your garden!
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