I've found Cornell mixture to be as effective as anything for powdery mildew. If you have trouble mixing a small batch, mix a gallon and store it in a jug. Shake very thoroughly just before transferring to the sprayer.

You might also try the milk spray. I have yet to try the Cornell mixture, though I intend to. In a pinch I tried the milk spray and it worked. Just add 1 part milk to 9 parts water to a squirt of Dawn (or whatever brand dish detergent you have on hand.) I don't believe it keeps due to the milk content-but it did work, because I had powdery mildew on several of my roses and it is now gone. Blackspot is also reduced tremendously on the most susceptible. Hope this helps.

bren