I think it is powdery mildew, a fungal disease. I spray mine with potassium bicarbonate (bi-carb old fashioned product) and remove all infected leaves. I have only tried bi-carb. I know other members have other sprays they use too so hopefully others will chime in as I am newer to this. Using Bi-Carb slowed it down for me but it does continue to a certain degree. Prevention is key with powdery mildew on cucurbits. I was told to spray once a week and after it rains to avoid getting it. Once you have it, spray the same. Avoid watering the leaves when watering the garden and avoid watering in the late evening. I hope this helps!

http://gardening.about.com/od/gardenproblems/a/PowderyMildew.htm