Thank you for your squash mildew question. Here's a link to an Extension article which explains what it is and how to prevent it (at least somewhat!) I think that most people prefer an oil such as Neem oil, which is totally organic, has no toxic effect on anything or anyone after it dries, and is safe to use in vegetable gardens. (Just rinse it off before consuming the vegetable.) Since it also kills mites and insects (as well as fungi), be sure to spray it when there are no beneficial insects, such as bees, on the plant, since, rather than poison them, it suffocates them. A fungus breathes much as we do (fungi aren't plants), so that's how it works on powdery mildew.
Hope this is helpful. Be sure to follow label directions for use, and good luck!