Hi Laura, I’m contacting Kathryn, our Master Gardener on this problem and she knows the perennials much better than I ever will. She might have some suggestions on how to help you.

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Hi Laura:

Powdery mildew is a pain. I would remove severely infected leaves. ( you probably already know not to put diseased plant material in compost). Wipe off the other leaves. You can make up a spray of a teaspon of baking soda, 12 ounces of water, and a teaspoon of dishwashing liquid and use that as a contact spray. Neem oil (available at Lowes/Home Depot) can also be used if you don’t want to make up a “home brew”. With powdery mildew, you have to think about cultural conditions and whether anything in the environment is promoting it. Well, we have hot, steamy summers here. That’s a contributory factor. Does the plant need pruning to get more air around it. Is it sitting near irrigation where it gets sprayed too much? Is the plant getting enough sun, does it get too damp at night….etc. Some plants like roses, don’t like to be watered from the top, so try watering from the bottom, at the base of the plant.

Hope that helps, Kathryn