Answer: The white powder is most likely powdery mildew, a fungal disease. Powdery mildew fungi spread quickest when the humidity is high and temperatures are between 50F and 90F. To discourage the disease, plant on sites with good air drainage, give the plants plenty of space, water during the morning only, and allow the soil to dry out between waterings. Remove any affected leaves to keep the disease from spreading. For heavy infestations of powdery mildew, sulfur or copper sprays can be applied. To obtain these treatments, check your local garden center, and follow the directions carefully.
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