Powdery Mildew is abundant in arid and semi arid areas, the spores do not require moisture. It is an obligate parasite. It lands on the leaf of a plant and redirects nutrients from the plant to itself. In severe cases it covers the leaves, blocking out the sun and inhibiting the plants ability to photosynthesize. If necessary apply fungicide.
Fungicide works much better as a preventative measure, before the powdery mildew becomes a problem.
The University of Arizona , College of Agriculture , Cooperative Extension , in Maricopa County .