If you have powdery mildew, you could use a spray mixture of milk and baking soda.

Control of Powdery Mildew can be achieved by spraying either of these solutions, in my experience. A 50/50 mixture of fat free (skim) miand water or 1 teaspoon of baking soda mixed in 1 quart of water (3 teaspoons per gallon).

Perhaps this from Growing a Greener World may be of some help as well.

http://www.growingagreenerworld.com/controlling-or-eliminating-powdery-mildew/