Bacillus subtillus (sp?) Works well but requires multiple applications, especially if there is a bad infestation. There are sulphur and copper based fungicides that work well too, but I can't vouch for their ecogrooviness.
If I want to get rid of a.powdery mildew infestation I would get some potassium bicarbonate and mix it with
water until I had.water with a pH of 10+. I would spray that heavily on the affected plant twice, 3 days apart. And then i would start spraying b. Subtillus once a week/10 days for a month or so. Some brands of subtillus that ive used are Serenade and Companion (I'm personally a bigger fan of Companion but I think Serenade is available at like home depot type places nationwide)