I have powdery mildew on my David Phlox plants that I haven't had the previous 3-4 years that I've grown it. Is it the early heat?
powdery mildew
Surprised, since David is resistant. Have you had a lot of humidity? That's the culprit.
A solution made of one part skim milk to nine parts water sprayed once a week will get rid of it according to researchers. This also controls some other forms of white fungus I have found. At least slows it down. If you remove any leaves from an infected plant throw in trash not in the compost. Baking soda added to water in small amounts is also effective in controlling, if sprayed regularly, for fungus such as black spot etc.
http://www.gardenguides.com/779-using-milk-control-powdery-mildew-garden-pest-tip.html