By Ken Johnson
Peonies are a common sight in many yards. They are long-lived, with many plants living for over 100 years, and require relatively little maintenance and have few problems. One problem that has become more prevalent over the years is powdery mildew. Powdery mildew appears as white, powdery growth that covers the foliage of the plant.
While powdery mildew is unlikely to kill the plant, in fact the fungus only feeds on live plant material, it can make them unsightly. 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. Spores are spread from plant to plant on wind currents.
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 peony plants are crowded, they could be divided in the fall and spaced farther apart to increase air circulation. Try not to handle or work in plants while the foliage is wet, because this will spread the spores. Also try to avoid overhead watering, as splashing water will also spread the spores.
Preventative fungicides can be applied when the first few leaf spots begin to appear. However, this will only protect unaffected foliage; once leaves become infected with powdery mildew, little can be done. Fungicides, however, are often not necessary to protect the health of the plant, and many gardeners simply choose to tolerate powdery mildew infections on their peonies.
The amount of powdery mildew that appears on plants will often vary from year to year depending on weather conditions. Therefore, having heavily infected plants one year is no guarantee that the problem will occur to the same degree in subsequent years. As with all leaf spot diseases, it is a good idea to remove and destroy infected plant debris from the garden to prevent overwintering of the pathogen.