Powdery mildew is a fungus. It usually manifests itself as a white powdery substance on plant surfaces. Leaves may be distorted or twisted.
Fungus spores are carried on the wind. When the spores make contact with a plant, they will start to germinate if the temperature is between 42 F and 71 F degrees. (Spores cannot survive temperatures of 90 F or more, or water.) Optimum conditions for powdery mildew are cool, damp nights followed by days that are warmer and less humid.