Powdery mildew. Just another garden problem. While it may not kill your plants, it certainly stresses them out, reducing the productivity and makes them really ugly. In spite of the “mildew” moniker, it’s actually a fungus, spread by airborne pathogens. And it is not the result of cold wet weather, but humidity and temperatures between 60 and 80 degrees. It overwinters in plant debris, so be sure to thoroughly clean up around and areas where it was in the garden. I’ve found two effective remedies, the first being Potassium Bicarbonate, mixed 1-2 tablespoons per gallon of water, sprayed thoroughly on the tops and undersides of the plants. The second, product, Safer’s 3 in 1 Garden Spray. One home remedy that is being studied, scientifically studied, is using a 20-50% solution of whole milk and water. In other words, you can try spraying a mix of half milk and half water. I’ve not tried the milk spray, but I can attest to the success of the Safer 3 in 1. Worked miracles on my peonies.