A very common natural remedy for powdery mildew is diluted baking soda in water...sometimes people add a drop of liquid soap to it or horticultural oil but if I were you I would just try maybe a half a teaspoon of baking soda in a quart of water with no soap or oil....you can spray it every few days to check if it is burning the leaves or anything. that's what I use on my roses and it does kill the mildew. Good luck.

Just google natural remedy for powdery mildew.