This disease may have been developing on your peonies for a month or more, but you may have only noticed it now. 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 underside of leaves. The lower sides of infected leaves soon turn a dull chestnut brown, while the upper surfaces appear a 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 the peony. The disease occurs to some extent every year and is caused by Cladosporium paeoniae. It is most serious in large plantings, where plants are dense and grown closely together, and where the old tops are not destroyed in late autumn or early spring.
Fungicides will not help this year’s plants. To control the problem for next year, remove all old tops to ground level and destroy, bury, or remove them from the garden. Do this in the fall or next spring before new growth starts. 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 1998-1999 Illinois Commercial Landscape and Turfgrass Pest Management Handbook. Spray the plants weekly during cool, damp, overcast weather, starting when the new shoots are 2 to 4 inches tall, and continue until the flowers begin to open. Adding a spreader-sticker helps coverage. For more information on this disease, consult Report on Plant Disease No. 631.