I get powdery mildew every year I garden in San Diego. It hits my tomatoes, cucumbers and zucchini badly. I had used neem oil in the past to try to prevent powdery mildew, but I just found this article today with another prevention method. They suggest using a milk and water spray to prevent powdery mildew. I will be using this method on my zucs and cucs this year. I will try to remember to post results.