This fungal disease caused by Graphiopsis chlorocephala (previously named Cladosporium paeoniae) is commonly known peony blotch, peony measles, red spot and leaf blotch disease.
Symptoms first appear as reddish, brown to purplish dots on the topside of the foliage with brown spots on the underside. Stems are often streaked with purple or red. The spots first appear early in the season when flower buds appear and progresses over the season. Eventually the spots merge together and appear on buds, pets and seed heads. Although this disease is unsightly, it’s not a killer. Dwarf and red peonies are more prone to this common disease, however there are cultivars there are resistant cultivars available.
Control: Remove infected foliage as soon as they appear, but disinfect your pruners after each leaf is clipped off to reduce spreading the disease even more. Discard the infected growth and do not compost them.
Prevention: Since this disease overwinters, remove all foliage from all your peonies and discard them. Make sure you disinfect your pruners between each peony. Thoroughly clean up all debris beneath and around the plants. Avoid overhead watering and keep foliage dry so apply water the base of the plant, not the top. Avoid crowding peonies as lack of air circulation also contributes to this disease. You can use a sulphur of copper fungicide to help control this peony measles, but correct maintenance is essential.