Michelle Grabowski, UMN Extension Educator
If your peonies look like they have been dusted with flour this summer, the likely culprit is powdery mildew. Powdery mildew is a fungal disease caused by Erysiphe polygoni. Several scientists have noted that powdery mildew on peony seems be an emerging problem in landscape plantings in recent years. Although powdery mildew is a common disease problem on garden plants like phlox and bee balm, many gardeners have grown peonies for decades without powdery mildew until recently.
Powdery mildew is unlikely to kill a peony plant. In fact the fungus can only feed on live plant cells. The powdery mildew fungus covers peony leaves and stems with powdery spores and fine fungal strands known as mycellia. Spores are spread from plant to plant on wind currents. In the early stages of infection, powdery mildew colonies look like fluffy snowflakes resting on the leaf surface. these infections quickly expand to cover the entire leaf surface in powdery white to gray fungal growth.
Powdery mildew is more common in plants that are growing in shade and have poor air movement, as these conditions favor fungal growth. Planting peonies in full sun with good air movement around plants can help reduce problems with powdery mildew. If peonies are suffering from powdery mildew there is little that can be done this summer. Preventative fungicides can be applied when the first few leaf spots appear earlier in the year. Fungicides, however, are not necessary to protect the health of the plant and many gardeners simply choose to tolerate the white coating that shows up at the end of summer.
The amount of powdery mildew that appears often varies from year to year depending on weather conditions. So having heavily infected plants this year is no guarantee that the problem will occur to the same degree in following years. As with all leaf spot diseases, it is a good idea to remove infected plant debris from the garden to prevent overwintering of the pathogen. Infected plant debris can be brought to a municipal compost facility or composted in a backyard compost pile that heats up.