About midsummer I get complaints of diseased peonies. Although cultural measures can help plant vitality then, that’s too late to apply fungicides for leaf diseases of peony. Now is the time to apply them. I am not advocating that all peony plants should be sprayed. Read on and determine if your plants are candidates for this type of help.
Peony leaf spots, leaf blotches, or measles start as small, circular, red or purple spots on the upper surface of young leaves just before the peony blooms. Later, the underside of leaves turns dull chestnut brown, while the upper surface appears glossy dark purple. With time, the lesions enlarge rapidly and may form large, irregular blotches, making affected plants unsightly. Stem and petiole lesions are initially short, reddish brown streaks. Stem lesions 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 through the season. The disease occurs to some extent every year and is caused by the fungus Cladosporium paeoniae. It is most serious in large plantings, where plants are dense and grown closely together, and where old tops are not destroyed in late autumn or early spring.
Each fall (or in the spring before new growth starts), remove all old tops to ground level and destroy, bury, or remove them from the garden. Mark your calendar now or you will likely forget to do this task. If you still see these symptoms every year on your peony and are tired of the weak, ragged appearance that results, then try a fungicide. Applications should begin when new shoot growth is 2 to 4 inches tall (now) and should be repeated per label instructions until flowers begin to open. Fungicide options are listed in either the Home, Yard, and Garden Pest Guide (new name for Illinois Homeowners’ Guide to Pest Management) or the 2001 Illinois Commercial Landscape and Turfgrass Pest Management Handbook. Since the homeowners’ revision is not yet available, I will list those options. Homeowners can use thiophanate-methyl, a systemic fungicide sold as Bonide Bonomyl (not benomyl), Dragon 3336, or Ferti-lome Halt. They could also use mancozeb, which is a protective-contact fungicide, or copper sulfate (bordeaux), also a protective-contact type of fungicide. For more information on this disease, consult RPD no. 631, “Red Spot, Leaf Blotch of Peonies.” Although some references list resistant varieties as an option, I could not find a list of such plants. Possibly nurseries have offerings.