Powdery mildew is a scourge of summer for squash, pumpkins, and other cucurbits. Organic growers should always start with varieties with some degree of resistance to powdery mildew – seed catalogues 
often call partial resistance “tolerance”. Although resistance will generally not be complete, efforts to manage powdery mildew with organic-acceptable products will be more productive if growers start with a variety that can put up a fight on its own than one that is highly susceptible.
Dr. Meg McGrath, Cornell University, has summarized field research results throughout the US for organic-approved products tested against various diseases of vegetables and herbs. Her summary for zucchini powdery mildew research in NY includes the following:
Best results are obtained when these products are used preventatively or at the very first signs of powdery mildew, usually in mid-July in Ohio. If you wait until powdery mildew has progressed to the stage you see in the photo above, it will probably be too late to get it under control.