They say powdery mildew is worse in hot summers when plants go short of water. Unlike grey mould and tomato blight, powdery mildew is supposed to like drought not damp. I think the spores are so widespread that you can't really eliminate them by treatment or leaving plots fallow - it's more a case of not providing the conditions in which the fungus will thrive.

My cucumbers etc always seem to get a fair bit of powdery mildew whatever I do with them, though. Oh well, they crop anyway.