Did you check the article on powdery mildew from the Old Farmers Almanac? Apparently there are different types.My squash has powdery mildew.
I successfully treated my rosemary for it one year with milk, I'll have to try it on the squash. If not I'll use the copper fungicide. It definitely stopped the blight on the tomatoes even though it rained the same day! I just hate using stuff if I dont have to.