It think powdery mildew is not really related to bathroom mildew.
I don't know about the leaves, but I was told many years ago that keeping rose roots well watered prevented powdery mildew. And from experience, this seems to be true. I only get powdery mildew when I let a rose dry out too much. Since many of mine don't get any overhead water other than our pathetic rainfall, this seems to be case for my roses. It probably helps that I have great air circulation.
FWIW, I found the same to be true for african violets. If I keep them from drying out, they don't get powdery mildew.
