Janie, get some baking soda or potassium bicarbonate in water and a little (teaspoon) dish soap and spray that regularly over the stuff with powdery mildew.

From the Dirt Doctor Site

Potassium Bicarbonate Fungicide
Mix 4 teaspoons (about 1 rounded tablespoon) of potassium bicarbonate into one gallon of water. Spray lightly on foliage of plants afflicted with black spot, powdery mildew, brown patch and other fungal diseases. Potassium bicarbonate is a good substitute for baking soda. There are commercial EPA registered as well as generic products available.

(To look around for more of these kinds of recipes, visit the "Information Center" link on the site and then search alphabetically.)

SRS