Powdery mildews are caused by 6 very closely related genera of ascomycetes. They all have one type of asexual spore or conidium which is called Oidium when it is not accompanied by a sexual stage. In California the powdery mildews usually do not produce the sexual stage which is a cleistothecium. Virtually all the powdery mildew pathogens behave alike. And they are odd.
Spores germinate on a dry leaf and are actually inhibited by free water.
Survival is by colonizing weed hosts, infections on stems and evergreen leaves or in infected dormant buds. As noted above, ascospores are rare and therefore play no important role in the life cycle. Damage is usually minor, causing some defoliation and distortion and the objectionable white, powdery mycelium. When this happens, shoots may be killed and flowers damaged. The diseases are not lethal, but can be very disfiguring.