There are commercial agents that will kill Powdery Mildew pretty easily, but I don't like to smoke that. I've had luck with an insecticidal soap (Safer) spray make sure to get under the leaves as well. Wait until dried and spray with water with a couple of drops of plain dish soap (no bleach additives) and now since off with plain water using a plastic sheet or tarp to keep too much of the Safer out of the soil.