This fungal disease occurs each year, showing up first in the spring before blooms appear. It is most common on older varieties and will not kill the plants, but it will cause them to become weakened and malformed each year, thus affecting plant vitality.
Cladosporium paeoniae is the fungal pathogen that causes small, circular, red or purple spots to appear on the upper surface of young leaves just before the peony blooms. Later, the spots appear on the underside of leaves. The lower sides on infected leaves soon turn a dull chestnut brown, while the upper surfaces appear as glossy dark purple. As the host tissues mature, the lesions enlarge rapidly and may form large, irregular blotches that make affected plants unsightly. Stem and petiole lesions are short, reddish brown streaks at first. The lesions on stems near the soil line become somewhat sunken or pitted and tend to merge and darken. Spots on all plant parts remain purplish or brownish red throughout the season. Because of the diverse symptomology, red spot, leaf blotch, or measles are all names for the same fungal disease that affects all aboveground parts of peony.
Fungicides will not be effective at this time of year. If you have a problem with this disease, consider replacing plants with newer resistant varieties, or mark your calendar now to spray next year. Fungicides are used to protect new foliage as it emerges. Sprays are initiated when new growth is 2 to 4 inches tall and are continued following label directions until flowers begin to open. The addition of a spreader-sticker will help coverage. Also to help manage the problem, in the fall of each year, you should remove all old tops to ground level and destroy, bury, or remove these from the garden. Mark your calendar now, or you will likely forget to do this task.
Fungicide options are listed in either the Illinois Homeowners’ Guide to Pest Management or the Illinois Commercial Landscape and Turfgrass Pest Management Handbook 2000. For more information on this disease, consult RPD No. 631, Red Spot, Leaf Blotch, or Measles of Peonies.