Yes! We recommend an increased CerarMist treatment schedule for plants already infected with powdery mildew. Spray the foliage thoroughly, being sure to coat the undersides of leaves too. Treat on day 1, 3, 5 in the first week, and then go to once per week application. If your plants have already been sprayed with a coating product (oils, waxes, etc), CerarMist will not be as effective (if at all!). CerarMist needs to be able to come in contact with the plant foliage surface.