Powdery mildew is a problem for me every year. I live in an apartment that runs their sprinklers several times throughout the night from about 9 p.m. to 6 a.m. Since these run at night when it's cool, it never fails that mildew starts to grow. Not to mention that it never fails that we get rainy days that are cool.
I've tried just about everything over the years. Several people mentioned Neem Oil, so I finally picked some up at the store recently when I found it on sale. I mixed up some and put it in a spray bottle and went to work. I gave everything that had mildew on them a good spraying and everything else that could get mildew got a light spray. And then I waited to see what happened.
A week later and there was maybe 30% the amount of mildew on the leaves as there had been previously. I was so excited, as some of the leaves were just completely white and fuzzy. I've got to do another spraying, as the bottle recommended every week until it's cleared up and every 14 days to help prevent mildew.
Hopefully within a few weeks the plants will all nice and healthy again. Having this mildew on it makes it hard for the plant to photosynthesize, and it can die.