That's contrary to everything I know about powdery mildew here in California. Cool damp nights should deter it, because powdery mildew will not grow on a wet surface. I have some old ramblers that are susceptible to it, but the only ones that get it are the ones that are too tall for me to water from overhead. I also disagree with the statement that "warm weather will stop it." It thrives here even when we go into triple digits.
