Peonies make bright, showy additions to the garden or landscape. Luckily, peonies have very few problems, according to the University of Nebraska-Lincoln Extension. However, when the weather turns damp or if you've placed your peonies in a less-than-ideal location, your peonies may become susceptible to problems. One problem affecting peonies is Phytophthora blight, a fungus that attacks the plant, including its roots.
A fungus called Phytophthora cactorum causes Phytophthora blight in peonies. Although first identified on a cactus, Phytophthora affects more than 200 plant species, infecting both the roots and crowns of plants. Because the fungus requires water to move, Phytophthora becomes a problem during prolonged wet conditions.
Phytophthora infections resemble other fungal problems to which peonies are susceptible, but Phytophthora primarily affects the roots and lower portions of the plant. The plant develops black or brown leathery spots on the leaves and stems. Small wounds called cankers open up on the stems, causing plants to topple over, and eventually, the entire plant rots and dies. When Phytophthora invades the roots of peonies, they appear black and rotted. Plants begin to wilt because they cannot draw water in through the damaged roots.
Once you confirm infection with Phytophthora blight, the only course of action is the prevention of future infections. Infected plants cannot be saved since the disease occurs below the soil line and out of reach of fungicides. The University of Minnesota Extension recommends removing infected plants and the surrounding soil and destroying them. Do not replant peonies or other plants susceptible to Phytophthora blight in the same location, as the fungus persists in the soil and may infect subsequent plantings. Instead, choose a new location with suitable growing conditions.
Because Phytophthora likes damp conditions, selecting a good site for your peonies is the best way to prevent the disease. Soil should be well drained without water settling around the plant roots. Make sure that there is good air circulation and full sun to aid in drying the site.
Phytophthora blight shares many symptoms with Botrytis blight, another fungal disease that affects peonies. Both diseases cause plant tissue to darken and collapse, although Botrytis blight also causes a visible mold to form on above-ground structures. Because Botrytis blight affects the stems and leaves, fungicide treatment may work, so it is important to accurately distinguish between the two diseases.