Yes! We recommend an increased 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), Plant Protectant will not be as effective (if at all!). Plant protectant needs to be able to come in contact with the plant foliage surface.