Peonies are showy garden perennials that are usually healthy when grown in good areas under favorable conditions. They need full sun and good air circulation, along with adequate drainage and water. However, overwatering or too much rain within a year's time can cause peonies to suffer from disease. These plants can suffer from fungal, viral and bacterial diseases, with fungal infections being the most common problem.
Common Fungal Diseases
Verticillum wilt is a common fungal disease involving peony shoots wilting, although the basal parts look normal. It's a destructive fungus that's almost impossible to kill because it lives in the soil. Any affected peonies should be removed and destroyed before replanting other plants.
Botrytis blight is another fungal disease that generally occurs in cloudy, rainy weather. The disease produces fuzzy fungal spores after excessive watering or rain.
Cladosporium paeoniae is a fungal diseases causing red stem spots, leaf blotches and measles. The fungal disease known as Rhizoctonia is characterized by crown rot.
Another fungal disease, Phytophthora cactorum, also causes crown rot, besides stem and root decay.
White Mold Fungal Disease
Sclerotinia sclerotiorum is a soil-borne fungus causing stem rot in which the entire plant or a portion of it wilts. The part of the stem that's infected becomes stringy and dry, turning a light tan color. Under humid conditions white fluffy mycelium usually appears, giving the disease its name of white mold. White mold can be diagnosed by slicing a stem lengthwise and then noting different sizes of abnormally shaped black sclerotia which are hardened masses of threaded materials, inside the stem's tan area.
Any infected plant parts should be removed, while not dropping the sclerotia, which helps the plant survive for many years. Non-susceptible plants can be replanted, however plants should be spaced wider apart to improve air circulation for minimizing the possibility of any infection from this fungus.
Viral and Bacterial Diseases
Peonies are susceptible to two viral diseases: Lemoine disease and Mosaic disease. Lemoine disease involves deformed or abnormal roots, with smaller fleshy roots that have bead-like swellings. Spindly growth and reduced vigor are visible above-ground symptoms. This disease is normally spread by a gardener during spading or when other unsterilized tools are used between plants.
According to the University of Minnesota Extension website, pruners and other tools can be cleaned by spraying them with a solution of 70 percent rubbing alcohol or one containing 10 percent bleach. Mosaic Disease is typified by concentric light and dark green rings on foliage, in addition to dwarfed growth. Septoria paeoniae is a bacterial disease with symptoms of leaf spotting and stem cankers.