Issue 15, August 28, 2009
Peony Leaf Spot/Blotch
This is the time when many gardeners want to know what is causing the foliage on their peony to look so puny. Red to purple foliar blotches are caused by a fungus, Cladosporium paeoniae. The disease is known as leaf blotch, measles, or just Cladosporium. Early season infection on susceptible species can be unappealing and may reduce plant vitality. Infection that is appearing just now is usually not a problem to plant growth. There are some things you can do, starting now, to help minimize this disease for the future.
This fungal pathogen may cause small, circular, red or purple spots on the upper surface of young leaves just before the peony blooms. Later, the spots appear on the under side of leaves. The lower sides on infected leaves soon turn a dull brown, while the upper surfaces appear glossy dark purple, very visible after rain. As the leaves mature, the lesions enlarge rapidly and may form large, irregular blotches that make affected plants unsightly as in the image. Stem and petiole lesions are short, reddish brown streaks initially, later becoming sunken or pitted. Spots on all plant parts remain purplish or brownish red throughout the season, making this disease easy to identify.
If Cladosporium is a chronic problem on your peonies, now would be a good time to consider replacement with disease resistant plants. Work with your nursery to find the best alternatives in the colors you prefer. One of the most helpful cultural practices is to remove leaves and stems from the site in the fall. Once leaves dry down, cut stems off to the soil line and remove all plant debris. Plants can be dug, split and spaced further apart to improve air circulation and keep humidity lower during the growing season. Pruning nearby trees and shrubs to allow more light and better air flow may help reduce infection. Finally, avoid wetting foliage when watering, or water early in the day. Fungicides are available for home garden use to help prevent peony foliar diseases, but they are not helpful at this time of 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. Chemical options are listed in the Illinois Home, Yard, and Garden Pest Guide or the Illinois Commercial Landscape & Turfgrass Pest Management Handbook. Copper and mancozeb are the active ingredients labeled for this use.
For more information consult RPD No. 631, Red Spot, Leaf Blotch, or Measles of Peonies. --Nancy Pataky
Author:
Nancy Pataky