Tree peonies are hardy, deer-resistant shrubs that do well in garden beds and borders. A tree peony can bloom for 50 years or more, producing clusters of blossoms measuring up to 10 inches across during May and June. However, their beautiful green foliage makes the tree peony an attractive garden shrub even when not blooming. Tree peonies are generally resistant to disease, but several diseases can affect them.
Botrytis blight, caused by the fungus Botrytis cinerea, is a greyish-brown fungus that attacks the stems, leaves, buds and flowers of the tree peony. To reduce the chance of infection, it might be necessary to spray tree peonies with a fungicide as soon as the plant breaks through the ground, a second time when the plant is half-grown and a third time right before it blooms. After September, collect all the foliage of tree peonies and destroy or compost it to prevent disease, but do not cut tree peonies back unless the plant is diseased. Infected stems turn brown, dry and rot. If you spot an infected stem, cut it off and remove it immediately, do not allow it to lie on the ground near the peony.
Phytophthora blight can be more destructive than botrytis blight. However, phytophthora blight is less common. You can tell your tree peony has it if black leathery spots show up on the buds. After that happens, the stems dry up and turn brown and leathery. This disease often causes the plants to rot at the ground or at the crown. To control phytophthora blight, follow the same procedure of spraying with a fungicide and removing infected stems as you would to control botrytis blight.
Leaf blotch disease can cause leaf spots on peonies. Several different fungi can cause leaf blotch disease, which leaves infected plants with small red or reddish-brown spots on the leaf surfaces. Those spots can later get bigger and change into purplish-brown blotches on the leaves. If a peony is going to get leaf blotch disease, it usually happens right after the plant has flowered. As with botrytis and phytophthora blights, get any affected foliage away from the plant as soon as possible and destroy it to prevent it from spreading.