Peonies are eye-catching plants that most commonly suffer from fungal diseases, as well as from some viral and bacterial infections. The most common fungal disease for peonies is botrytis blight (gray mold), caused by over-watering or by excessively wet and rainy conditions. Plants affected by this disease begin to rot from the ground up, leaves wilt, and blackish-brown spots appear, along with gray mold and browning buds.
Two common fungal infections are phytophthora blight, which is similar to botrytis blight, and southern blight (crown rot or white mold). Southern blight causes the crown and stem of the plant to rot. Fluffy white webbing appears, and the plant wilts. Verticillium wilt, powdery mold, leaf blotch and root rot are other fungal diseases of peonies.
Peonies also suffer from viral infections. Lemoine disease, although rare, is considered the most serious viral disease for peonies. No cure is available. All infected plants must be destroyed and all garden tools sanitized. Other viruses include ring spot virus, mosaic disease, crown elongation and leaf curl. These can all interfere with plant growth, cause foliage discoloration and in some cases cause plant death.
Bacterial infections that can affect peonies are crown gall, caused by Agrobacterium tumefaciens, and bacterial blight, caused by Xanthomonas hortorum. Crown gall causes large growths at the stem along the soil line. Infected plants should be destroyed. Bacterial blight is less serious and causes leaf spots surrounded by red or yellow rings.