Healthy peonies rarely suffer from diseases if they are grown in locations with good soil drainage, full sunlight and adequate air circulation. Stressed peonies, however, are susceptible to fungal infections that cause leaf spots.
The Cladosporium paenoiae fungus causes peony leaf blotch. It overwinters in infected leaf litter on the ground and matures during the spring, producing spores that infect new foliar growth through splashing water. The disease is more prevalent when peonies are clustered in dense plantings, according to the University of Illinois Extension. Other diseases that cause spots on peony leaves include botrytis blight, caused by the Botrytis cinerea fungus, and Phytophthora blight, caused by the Phytophthora cactorum fungus. A variety of other fungal pathogens also cause leaf and stem spots on peonies.
Round purple or red spots on the upper sides of the leaves characterize peony leaf blotch. These spots, which range from 1/25- to 1/8-inch in diameter, grow as the host tissue matures. The lesions grow together to form irregular-shaped blotches on leaves and stems. Botrytis blight creates irregularly shaped leaf spots, diseased buds, discolored flowers and fuzzy gray fungal spores. Peonies infected with Phytophthora blight have black, leathery plant parts and diseased stems. Other leaf infections create cankers, which are diseased areas on stems, as well as leaf spots.
Remove the tops of infected peonies during the fall and cut the stems back to ground level to prevent the spread of leaf spot diseases, recommends the University of Minnesota Extension. It also suggests removing fallen leaves, buds or flower parts to prevent infections from recurring the next spring. Space plants so that they have adequate air circulation, and water them from the base rather than on the leaves to help control leaf spot infections. The application of an appropriate fungicide at timed intervals during the growing season protects peonies from becoming infected.