"If you are seeing powdery-looking patches on the foliage of your plants, you most likely have a case of the very common powdery mildew fungal disease. Here is a simple spray for controlling the spread of the fungus. It won't get rid of the fungus on leaves that already have it, but it will prevent it from spreading to the rest of the plant." Farming & Agriculture
Spray your zucchini plants once a week with a solution of 1 part milk to 9 parts water. You can use any kind of milk available. This will keep your plants from getting powdery mildew, a serious threat to zucchini plants. | eHow
Powdery mildew natural fungicide Combine: 1 teaspoon baking soda 1 quart water A few drops of liquid soap Combine in a large container and transfer some to a spray bottle. Spray the leaves of plants where powdery mildew can be seen, then repeat a week later. For best results, treat plants as soon as you notice the powdery mildew.
#OrganicGardening Remedy for powdery mildew - got it on my squash, pumpkin and zucchini plants this year. 1T baking soda 1/2 t liquid soap (Dr Bronner's is a nice organice, totally natural option) 1 Gal water. Spray weekly, preferably in am, but not when plants are in full sun. Make remedy fresh each time you use it - don't store!