I’ve long admired Herbaceous Garden Peonies (Paeonia spp.). These long-lived perennials provide beautiful displays with their short, shrubby growth; dark glossy foliage (some species); and showy (gaudy?) blooms, even though the blooms may not last long. It was not unusual to come across clumps of these non-natives growing on the sites of long-abandoned homesteads in the mountains of my native West Virginia.
The taxonomy of these natives of Asia, Europe, and the western U.S. remains in flux. However, it’s generally accepted that Paeonia is the only genus in the family Paeoniaceae. Plant taxonomists also recognized that there are two natural forms, “woody” (= tree peonies), and herbaceous (= garden peonies). This Alert focuses on the herbaceous types. It's an important distinction relative to management.
Online descriptions of garden peonies commonly include statements such as “rarely troubled by pests or diseases,” and “a trouble-free nature.” Unfortunately, these statements are only partially correct, at least in Ohio.
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.
The lack of insect feeding damage to the flower buds may be attributed to ants drawn to buds by nectar flowering from specialized floral nectar glands. They are not there to assist with the grand opening of the floral display; that’s a myth. The ants drive off insects seeking to eat the buds and the nectar is payment for the ant’s protective services.
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.
Peonies are also susceptible to up to 14 species of Botrytis including a gray mold fungus specific to peonies, B. paeoniae. Indeed, research conducted at Washington State University as part of a Ph.D. thesis revealed that there may be as many as 9 additional undescribed Botrytis species found on peonies.
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."
Unfortunately, web searches may yield reports with images that clearly show peony leaf blotch but are mislabeled "Botrytis blight," or images of Botrytis infections that are blamed on the leaf blotch fungus. These fungi have very different disease cycles. Of course, it's not unusual to find both diseases on the same peony plant.
The occurrence of powdery mildew on peonies adds to the challenge of making an accurate "field" 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 this species can infect several species of flowering plants used in landscapes.
Of course, you should never depend on a field diagnosis as the final word on a disease diagnosis. If you want to know exactly which disease(s) is at work, sending samples to our OSU C. Wayne Ellett Plant and Pest Diagnostic Clinic can provide answers for making an accurate diagnosis.
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.
Management Using the Disease Triangle
The Disease Triangle illustrates the three conditions that must be met for a plant disease to develop: the pathogen must be present, the plant host must be susceptible to infection, and environmental conditions must be present that support infection and disease development. Removing only one of these conditions will prevent disease development.
Various web reports on peony leaf blotch recommend planting less susceptible varieties. However, I have found no publications presenting data from non-biased plant trials that assessed disease susceptibility. There are anecdotal accounts that susceptibility varies among the different peony varieties, and I've observed this in peony plantings. Of course, other factors may be responsible for varying levels of infections such as micro-environments acting to increase or decrease infections within the plantings.
Some disease suppression may be achieved by environmental management such as avoiding overhead irrigation. However, it's difficult to manage natural overhead irrigation in the form of rainfall. This challenge is illustrated by the images used in this Alert. All of the peonies appearing in the pictures in this Alert are being grown where there is no overhead irrigation and most of the peonies are in full sun.
Suppression of the pathogen by fungicidal applications can be effective; however, success is generally problematic. Multiple applications are required over a significant portion of the growing season and heavy rainfall events may require the shortening of the intervals between applications. Relying on fungicides alone is not likely to be successful for home gardeners and can even present a serious challenge for landscape management professionals.
Removal of plant pathogens through sanitation is one of the effective management strategies for both of these garden peony diseases. This approach focuses on getting rid of infectious tissues that harbor the fungi throughout the growing season or over the winter.
To Do: In the Fall (right now!)
1. Cut, remove, and destroy all of the top growth down to the soil line.
2. Rake, remove, and destroy all mulch and plant debris that was beneath the infected plants.
3. 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
1. Remove and destroy bloom buds, flowers, and stems showing signs of Botrytis infections. "Dead-heading" spent flowers is also recommended.
2. Selectively prune plants to improve air circulation which will enhance leaf and stem drying.
3. Avoid overhead irrigation; use drip irrigation if available.