Treating powdery mildew with fungicide involves the application of commercially sold products that include the ingredient chlorothalonil, which eliminates the mildew but leaves a white residue on the leaves. It can also be treated with less toxic options such as horticultural, neem and jojoba oils. Sulfur can also be used, as well as biological fungicides.
Fungicides meant to treat powdery mildew can be protectants or eradicants. Some serve both functions. Oils are primarily eradicants, but can sometimes work as a protectant. Apply oils to the mildewed plant with a spray bottle. Wait two weeks between the application of oil and sulfur, and don't apply oils in temperatures 90 degrees Fahrenheit or higher or during droughts.
Sulfur is purely a protectant. The most effective sulfur products are those combined with dishwashing detergent surfactants. Sulfur is also available in dust form, but it cannot be used on all plants and may cause irritation to the eyes and skin. Squash and melons should not come in contact with sulfur, and, like oils, sulfur should not be applied in temperatures 90 degrees Fahrenheit or higher.
Biological fungicides are eradicants that include the bacteria Bacillus subtilis. They kill powdery mildew without passing toxic effects to insects, pets or people who consume the produce, but they are not as effective as oils or sulfur.