Research conducted on Long Island by Meg McGrath of Cornell has shown that resistant varieties provided better control of powdery mildew than that obtained with fungicides applied to standard susceptible varieties.
Squash varieties with powdery mildew resistance include Butternut types: Betternut 401 and Bugle. Green acorn squash types: Autumn Delight, Royal Acorn PM, Sweet REBA, Table Star, and Taybelle PM. Specialty types: Bush Delicata – elongated green fruit with white stripes; Celebration - gold striped acorn; Harlequin – green striped acorn. There are tables showing cucumber, melon, pumpkin an squash varieties with resistance to PM and other vegetable diseases at:

Here is a recipe for a nontoxic, low-cost spray for powdery mildew that has been effective on numerous crops:
Per gallon of water, add:
1 heaping Tablespoon baking soda
1 Tablespoon dormant oil
1/2 teaspoon insecticidal or dish soap.