Herbaceous peonies are rugged perennial plants valued for their drought tolerance and deer resistance. Breeders continue to introduce exciting hybrids to the marketplace featuring new color breaks, patterns and flower forms. Though carefully selected, nature works hard to expose any growing flaw.

Spring was a two-edged sword this year. Mild weather helped extend the typical bloom season for herbaceous peonies. Flowering lasted nearly a month on early and midsummer varieties at the Joyce Lockatell Memorial Garden. Central Virginia however experienced persistent rainfall during this period. Conditions were favorable for disease.

Herbaceous peonies are hosts for botrytis blight, (Botrytis paeoniae) measles, (Cladosporium paeoniae) and powdery mildew (Erisyphe polygoni). These diseases frequently appear in production settings. Fungal spores can easily jump from plant to plant. Heavy wind or rains are great modes of transmission for these pathogens. Infection from neighborhood plants or new purchases from the local garden center are entirely possible. Rather than lament over misfortune or exacting blame, proper identification and treatment can resolve the issue.

Botrytis is the most common disease of garden peonies. Symptoms appear in spring on young leaf shoots about one foot in height. Stems wilt and some die. A brown or blackish rot appears at the base of leaves and stems. Affected areas of the stalk above ground will be covered by a gray mold filled with large amounts of spores. Immediate removal, clean up and treatment is essential to combat infection.

Peony measles or "Red Spot" can be observed as mall circular discolored lesions on leaves. Spots can combine to make foliage look irregularly blotched. The under surface of these areas becomes light brown. A number of powdery mildew species can attack plants in the home and commercial landscape. Native dogwoods, lilacs, hybrid honeysuckles and zinnias are common hosts for these fungi. Herbaceous peonies have their own powdery mildew species that develops as a white film on leaves and stems. Effects of infestation are seen in late summer. Mild and wet springs are a perfect environment for the spread of powdery mildew in the home landscape.

Daconil® (chlorothalonil) and Cleary’s 3336® (thiophanate methyl) can be used individually or tank-mixed to combat both botrytis blight and powdery mildew. Immunox® (myclobutanil) is another fungicide labeled for botrytis and peony measles control. Spectracide packages it in one-quart containers for tank mixing. The Pest Management Guide from Virginia Cooperative Extension also suggests Ortho® Rosepride® (triforine) for powdery mildew control. Protective sprays should be made as new foliage emerges and matures in early spring until bloom season. A spray for Measles may be a good control after bloom season.

Good garden sanitation in October helps to mitigate future infection. Spent flowers should be removed during and after bloom season. Declining foliate in October should be cut back to the ground. Collected debris should be thrown in the garbage or burned. No composting!! Additional advice and information can be obtained from your local Cooperative Extension office. Please follow product label for mixing and applying any registered pesticide.

After receiving some recent emails on the absence of bearded iris rebloom, we would like to suggest the use of rock phosphate and green sand at planting time. Unlike inorganic fertilizers such as 5-10-5 or 5-10-10, organic formulations will persist in the soil profile for many growing seasons. Nutrition is available as needed for consistent plant growth.

Remontant or reblooming bearded irises have short summer dormancy periods. This genetic trait is the key to fall flowering. Without available nutrition, seasonal rhizome growth is seriously compromised. The absence of new growth dooms future blooms. The use of rock phosphate and green sand during installation eliminates the need to top dress iris clumps with inorganic fertilizers in early spring and summer to promote flowering. It saves effort and possible heartache. We sell Espoma Rock Phosphate in eight pound bags and Green Sand in ten pound packages. Results have been outstanding at both the Flatrock and Cosby Farm gardens. Contact us for additional information on iris and peony care.

Diseases and Pests of Ornamental Plants
Fourth Edition, by Pascal Pirone
Ronald Press pgs 372-373