Powdery mildew generally begins to be a problem in the garden during the summer months, or into fall. Discolored patches on leaves of white, gray or black may be signs of the pathogen. For example, the white powdery covering of powdery mildew is actually the fungus spreading across leaf surfaces.  

Plants suffering from powdery mildew look as if they have been dusted with flour.  

A wide variety of plants are affected. Some plants that are particularly susceptible are lilacs, phlox, bee balm, squash, roses, grapes and zinnias.

Powdery mildew thrives in hot weather, especially with cool nights. Drought-stressed plants are more susceptible to powdery mildew. Unlike most fungal diseases, it is actually less of a problem in rainy weather.  

Control by providing good air circulation by spacing plants correctly, by watering in the morning and mulching to prevent drought stress,  and grow resistant cultivars.  Spraying powdery mildew off with water has been shown to be very helpful.  Discard any plants that are severely infected. Spray with sulfur, copper soap fungicide or neem.