Many plants are susceptible to powdery mildew, the fungal disease that appears as a white or grayish powdery growth on leaves, stems and buds. Powdery mildew can weaken plants, compromising growth, flower and fruit production and quality.
It is typical for plants in the cucurbit family, such as squash, melons, pumpkins and cucumbers, to suffer from powdery mildew. The best treatment for powdery mildew of squash is prevention, but short of that, there are ways to keep powdery mildew from getting out of hand.

1、Plant squash varieties that are known for resistance to powdery mildew and other disease problems. Winter squash varieties that are resistant include Autumn Delight, Table Star, Celebration and Indian Brave. Resistant summer squash varieties include Patriot II, Sunglow, Ambassador and Lynx. Always plant squash in a sunny spot and far enough apart for plenty of air circulation.
2、Avoid excessive fertilizer use. Over-fertilizing encourages susceptible new growth on squash and other plants.
3、Spray squash plants with fungicide before symptoms of powdery mildew appear. Spray early with a protectant fungicide like wettable sulfur spray or with biological fungicides which contain microorganisms that destroy fungal pathogens.
4、Spray with eradicant fungicides made from horticultural oils if powdery mildew appears in spite of your preventive measures. Follow all package directions carefully and be aware that some plants and varieties are more sensitive to horticultural oils than others.
As an alternative to horticultural oils, you can mix your own fungicide. One recipe for homemade fungicide is one gallon of water mixed with four or five cloves of crushed garlic. You can also mix one gallon of water with a squirt of dish soap and 1 tbsp. of baking soda to make a fungicidal spray.
Spray either homemade formula directly on plants, taking care to thoroughly soak the entire plant, including the underside of leaves. Reapply these sprays every three or four days to treat powdery mildew.

From Network Resources