The weather this summer has resulted in many peonies catching the “measles” and others getting powdery mildew. Measles is a disease, also known as red spot, that causes distinct, reddish-purple spots on the upper leaf surfaces. These spots often coalesce and become large, reddish purple blotches on the upper leaf surfaces but are a light brown color when viewed from the underside of the leaves. The spots on stems will merge and form streaks that are reddish brown.
Plants infected with powdery mildew look like they have been dusted with flour. This disease is more rare in Kansas than Measles but does show up in wet, humid summers.
Sanitation is the best control for both these diseases. Remove all diseased tissue, including stems, at the end of the growing season. Actually, the foliage can be removed now with no harm to the plants as they are already essentially dormant.
Mulch that contains plant debris should also be discarded and then replaced with fresh mulch. Reducing the source of the inoculum will reduce the chances of another severe outbreak next year. (Ward Upham)