Bee,

A recent organic gardening discovery is that hydrogen peroxide, sprayed on undiluted at the grocery store 3% strength, will knock out powdery mildew. Do a thorough application and wait a few days to see that it clears. Leaves hit too hard by the mildew that are past the tipping point may still die, but it should clear younger leaves. There is a commercial product called Plant Wash (I've never seen it and don't know the cost) is said to be as good or better.

Up to the last few years organic gardeners typically used potassium bicarbonate mixed with a little dish soap in water, or in a pinch, baking soda in that recipe, to treat powdery mildew. It had iffy results, at best it might slow its spread. The discovery that hydrogen peroxide knocks out many plant fungal and bacterial problems is big news. But keep in mind that repeated applications might be overkill and can kill beneficial bugs also if you're not careful. The store strength is the best one to use--hydrogen peroxide is available at higher concentrations for industrial applications, but there is a risk of chemical burns and injury from breathing it in.

SRS