Issue 5, May 23, 2016
So far, it's been a beautiful year for peonies. The tree peonies bloomed earlier, and the herbaceous peonies are starting to open. While it's easy to enjoy these lovely plants while they're flowering, they're also easy to forget about later in the year when the colorful blossoms are gone, and all that's left are the leaves.
Peonies get a number of foliar diseases in summer and fall. The most common in Illinois are Botrytis blight, measles (aka red blotch or leaf blotch), and powdery mildew.
Peony plants showing symptoms of Botrytis blight, measles, and powdery mildew. Photo credit: Diane Plewa
Botrytis blight is caused by a few related species of the fungus Botrytis, including B. cinerea. B. cinerea has a very wide host range and may infect a large number of ornamental plants. Botrytis blight can affect all above-ground parts of the plant; symptoms are most often found on leaves and buds. Botrytis blight on peony causes reddish-purple spots on leaves and shriveled, discolored buds. Stems and flower petals may also become discolored, wilted, or shriveled. Brown or gray fuzzy masses may be seen on the surface of affected tissue with a hand lens; these are dense thickets of spores produced by the fungus, which will blow or splash away to initiate new infections. This disease is favored by cloudy, rainy weather, and usually shows up in spring or early summer.
Measles, or red blotch or leaf blotch, is sometimes confused with Botrytis blight. Measles is also caused by a fungal pathogen (Cladosporium paeoniae) and symptoms are superficially similar. Initial infection causes small, reddish spots on the leaves and stems, which may expand and coalesce to form large, irregular reddish-purple blotches. Warm, wet weather favors this disease, and like Botrytis blight, it usually shows up in spring or early summer.
Peony powdery mildew usually shows up later in summer. It's a common disease found on a wide variety of plants. While the fungal pathogens that cause powdery mildew on various hosts are related, they're not the same species. Each species is fairly host-specific, so the powdery mildew pathogen affecting your oak tree is unlikely to cause disease to your phlox or bee balm. Powdery mildew can affect both leaves and stems, producing a white film over the surface of the plant tissue that can be smudged off with fingers. As the infection progresses, fruiting structures are produced that can be seen with the naked eye. They appear as tiny black specks in the white film.
None of these diseases are considered a threat to an otherwise healthy plant, but they can drastically reduce aesthetic appeal. Rarely, on plants that are heavily stressed due to other pest pressures or poor environmental conditions, these diseases can be lethal. Happily, management of all of these pathogens is fairly straight-forward.
Cultural management practices include planting peonies appropriately by avoiding heavily shaded areas and spacing plants to avoid over-crowding. Watering at the base of the plants and early in the day can also help reduce some disease issues. Most importantly, good sanitation should be practiced. Spent blooms should be cut from plants, infected leaves and stems removed, and all above-ground plant tissue cut and removed in autumn when the plants begin to senesce.
Fungicides are available and can be helpful to suppress heavy infestations, but are rarely needed most years. Copper, copper + mancozeb, copper hydrozide, copper sulfate, and mancozeb are labeled for use against Botrytis blight and measles; neem oil, potassium bicarbonate, sulfur, tebuconazole, and triticonazole are labeled for use against powdery mildews. (Diane Plewa)