Powdery Mildew on Peonies
Peonies are long lived, low maintenance plants. But in recent years many gardeners are finding the leaves of their peonies covered with a white powdery substance. The culprit is powdery mildew and seems to be an emerging problem on peonies in the landscape.
Fortunately, this fungal disease will not kill otherwise healthy and established plants. It does, however, affect their appearance.
Reduce the risk by growing peonies in full sun with good air circulation. Remove infected plant debris in fall to further reduce the risk of infection next season.
Since the disease occurrence is also influenced by weather, a bad infection this year does not mean you will have problems next year. If this is a continual problem that you want to treat, you will need to plan ahead. Apply an organic fungicide at the first sign of the disease and repeat as recommended by the label.
A bit more information: Botrytis and phytophthora blights are common disease problems on peonies. Proper sanitation and dryer spring weather will reduce the risk of these diseases.