This is the season for widespread appearance of leaf blotch of peony. You may know it as red spot, measles, or some other nickname. A fungus, Cladosporium paeoniae, is the pathogen.
Symptoms appear about bloom time as small red or purple spots on upper leaf surfaces, giving the appearance of measles (to those of you who remember that disease). Early symptoms may go unnoticed. These spots do not enlarge until the leaves age and become less resistant. I find this interesting because usually young plant leaves are more susceptible to fungal infection and become resistant with age. At this time of year, you may see leaves with large purple–brown blotches, as seen in this image.
If you are a gardener and the disease just appeared, damage is minimal and management is easy. You need to cut back, rake, and remove all aboveground parts before new growth resumes in the spring.
Commercial growers have a bit of a marketing problem when spots like this appear on their plants. The close proximity of plants, along with the practice of letting tops remain in the field, can allow this disease to become a problem. In addition, many red varieties and some dwarf varieties are susceptible. The fungus infects in warm, wet spring weather. For this reason, fungicides may be sprayed in the spring when new growth is 2 to 4 inches tall and repeated until flowers begin to open. Commercial growers can control Botrytis with these sprays as well. The fungus overwinters on plant debris, which is why it is so helpful to remove all aboveground plant parts before new growth emerges.
For more information, refer to Report on Plant Disease (RPD), no. 631, “Red Spot, Leaf Blotch, or Measles of Peonies,” available in Illinois Extension offices or on the Web at http://www.ag.uiuc.edu/~vista/abstracts/a631.html. (