Okay, as strange as it sounds, a milk spray will get rid of powdery mildew. I was told of it on a garden forum here, I tried it on my Papa Meilland rose that is in a bad spot, thus prone to Black Spot and Powdery Mildew, and it fixed it. Be kind to the environment .
http://www.sciencenews.org/view/gene...to_Mildew_Woes