Squash, pumpkins, cucumbers and other cucurbits can be damaged by the fungal disease, powdery mildew. The fungus spreads by airborne spores which do not require free water to germinate and infect leaves. Overhead irrigation should be avoided though, because high humidity favors powdery mildew.
Planting varieties that are resistant to powdery mildew is the best way to avoid the disease. Some fungicide products, including horticultural oil, neem oil and Serenade can help control powdery mildew when the disease first appears.
For more information see UC Pest Note #7406 “Powdery Mildew on Vegetables” : http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7406.html
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension, August 2012