I don't know how big your garden is, but have you tried the milk spray? (1 cup of milk to 9 cups of water, sprayed on the plants). It's said to work wonders with powdery mildew. Might help.

Also do a search on the organic gardening forum. They have a lot of homemade recipes for mildew and other problems. Many times mildew is caused by plants being too close together so they do not get enough air circulation to dry properly between waterings.

Lee