Powdery mildew is most often stimulated by a discrepancy in humidity compared to soil moisture. Warm, damp air (high dew point) and dry soils. Lack of good air circulation is also a contributor. The pathogen is actually sensitive to water (rain, irrigation), which will kill off the spores.

If it's July, something in my garden has powder mildew. gardengal has it, this dry combined with high humidity has turned my monarda white with it. If it gets to the point of threatening the life of a perennial I use Bordeaux mixture. Annuals are on their own.