Peonies are herbaceous perennial flowering plants that grow to 2 to 4 feet tall. They prefer cool climates, full sunlight and rich organic soil. Peonies are susceptible to several fungal blights.
Two types of blight commonly infect peonies. Botrytis paeoniae causes botrytis blight or gray mold in peony plantings, while Phytophthora cactorum causes phytophthora blight. Both pathogens are soil-borne fungi that spread through splashing water and infected plant tissue.
The Botrytis pathogen attacks young shoots. Young foliage turns black, while buds die without opening. Infected leaves develop spots, and blossoms are brownish or discolored. A gray mold grows over the infected parts of the plant. The Phytophthora cactorum fungus causes leathery brown or black diseased areas on the leaves, buds and shoots. The Phytophthora pathogen can invade the crown or roots, causing plant death.
The University of Wisconsin Extension website recommends controlling blight by removing and destroying diseased plant parts and by removing stalks, leaves and fallen plant debris at the end of the season to lessen the likelihood of blight the next spring. It suggests preventing botrytis blight by spacing plants to provide adequate air circulation, and lessening phytophthora blight outbreaks by planting peonies in well-drained locations using sandy soil. Planting disease-resistant cultivars limits the likelihood of disease; in addition, spraying plants with an appropriate fungicide at timed intervals protects peonies from becoming infected.