If the weather is too hot and sunny for wettable sulfur, you may want to try a new organically-approved systemic fungicide that you can get at Home Depot or Lowes:
ORGANOCIDE PLANT DOCTOR, which is a clear odorless, non-oily liquid of mono-potassium salt of phosphorous acid. This is totally unlike the smelly fish-oil "Organocide 3-in1." Though Plant Doctor is labeled only for anthracnose on mango bloom, I read elsewhere that this active ingredient is also effective on powdery mildew. As there is usually also some anthracnose around, and since most fungicides work best as preventives, before you even see any symptoms, you will not be out-of-line using this product.