If you have seen peonies, you have most likely seen this disease. It goes by many names, but red spot seems most descriptive. The disease is caused by a fungus, Cladosporium, that grows superficially on leaves, stems, and petioles, causing unsightly spotting but not significantly affecting plant vitality. It does not cause early leaf drop or stem dieback.
Small, circular, red or purple spots appear on the upper surface of young leaves just before the peony blooms. Later, the spots appear on the underleaf surface. The lower sides on infected leaves soon turn dull chestnut brown, while the upper surfaces are glossy dark purple. The mature leaf blotch phase is late in the season (occurring now) and is quite unsightly. The lesions coalesce and form large, irregular, unsightly purple blotches. Also, it is this stage that produces overwintering inoculum for next year's infection.
Peonies vary in their susceptibility to this disease. The older varieties that many of us have in our sentimental gardens are the most susceptible types. The vigorous, thick-stemmed, newer varieties available show little disease development.
As with many of the fungal leaf diseases, you cannot improve the situation once disease is present or once the new growth for the year is completed. Many of the fungicides available for disease management are preventive. They are applied to protect leaves from infection, to prevent fungal sporulation, or to otherwise interrupt the fungal life cycle.
Infected peonies cannot be helped this late in the season. Recognize the disease now and plan to manage it next year. This fall or early next spring, remove all old top growth to ground level and destroy, bury, or remove it from the garden. Just before the shoots break through the soil surface in the spring, spray the soil around the plants with mancozeb, maneb, thiophanate-methyl, or copper fungicides. Be sure to soak the soil surface area, stem stubs, and any remaining peony debris. Spray the plants weekly during cool, damp, overcast weather, starting when new shoots are two to four inches tall and continuing until the flowers begin to open. If this sounds like too much trouble, consider replacing your peonies with newer resistant varieties, or just put up with the disease. For more details consult Report on Plant Diseases No. 631.