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.