These long-lived perennials provide beautiful displays with their short, shrubby growth; dark glossy foliage (some species); and showy blooms, even though the blooms may not last long.
It is true that garden peonies appear to be immune to four-legged plant pests; deer and rabbits don’t eat the foliage. Although they are not immune to six or eight-legged pests, insects and mites seldom bother garden peonies to any significant extent.
However, beautiful peony displays are sometimes marred by several diseases with the most notorious being Peony Leaf Blotch caused by the fungus Graphiopsis chlorocephala (formerly Cladosporium paeoniae). The fungus is also responsible for producing other diseases on peonies with different common names depending on the symptoms.
Leaf blotch occurs when infections produce large, shiny, brown, or purple leaf lesions. Peony Red Spot and Peony Measles occur when fungal infections produce distinct red to reddish-black spots on the stems. Typically, the measles symptoms appear before the leaf blotch symptoms with the stem lesions expanding as the season progresses.
The gray mold fungi may infect newly emerging shoots in the spring covering them in a fine, velvety gray mold. The Botrytis can also infect flower parts later in the season to produce disease symptoms labeled “bud blast” with flower buds failing to open and “flower blight” with opened flowers collapsing and becoming blackened. Fungal infections can also move down the stems to produce a “shoot blight.”
The occurrence of powdery mildew on peonies adds to the challenge of making an a clear diagnosis. Various online university resources identify the fungus behind powdery mildew on peonies as Erysiphe polygoni. Although it’s common for powdery mildew fungi to be host-specific, apparently the species that effect peonies also infect several species of flowering plants used in landscapes.
The good news is that these diseases are not considered to be killers of garden peonies. Although symptoms tend to accelerate as the season progresses, plants apparently have enough time to produce and store enough carbohydrates to support regrowth the following season.
The bad news is that these diseases can seriously detract from the aesthetic value of peonies in landscapes. Worse, unless something is done to break the year-to-year disease cycles, the increasing abundance of inoculum in the form of infectious plant debris in the spring means infections start early in the season and can quickly escalate as the season progresses.
To Do: In the Fall (right now!)
- Cut, remove, and destroy all of the top growth down to the soil line.
- Rake, remove, and destroy all mulch and plant debris that was beneath the infected plants.
- Distribute new mulch for the winter to a depth of no more than 2 – 3″. This will suppress the release of fungal spores next spring from infectious debris that may have been missed during the fall clean-up.
To Do: In the Spring
Protect new shoots using an appropriately labeled fungicide. The product label must include the site (e.g., landscape, nursery, etc.) and make certain peonies are not listed as being sensitive to the product. This is an added protective measure and requires just one or two applications. I have heard several anecdotal accounts of peony leaf blotch being successfully managed without these fungicidal applications in the spring. However, these applications should be considered if there were heavy Botrytis infections this season.
To Do: During the Growing Season
- Remove and destroy bloom buds, flowers, and stems showing signs of Botrytis infections. “Dead-heading” spent flowers is also recommended.
- Selectively prune plants to improve air circulation which will enhance leaf and stem drying.
- Avoid overhead irrigation; use drip irrigation if available.