I have battled powdery mildew on my rose bushes for years, and find that a light mist of neem oil once a week really REALLY works to prevent it.
Never spray neem to the point of runoff, just mist lightly, dry, and repeat a few days later.
The glossy film that builds up acts as a prophylactic against a wide variety of molds, diseases, and pests.
Serenade is B. subtilis, a beneficial bacteria- both Serenade and Neem can be used in organic gardening and are COMPLETELY non-toxic to humans. Word.