The most common disease of peonies in this area, peony leaf blotch, is just beginning to appear now. This disease is caused by the fungus Cladosporium paeoniae and is also known as stem spot or measles. Typical symptoms include glossy purple to brown spots and blotches on the upper surfaces of the leaves. The disease may cause slight distortion of the leaves as they continue growth. Leaf symptoms are sometimes most apparent on the edges of older leaves. On the stems, symptoms appear as long, reddish-brown streaks.
Peony leaf blotch is best managed through sanitation. Diseased plant material should be removed in the fall or early spring. Proper spacing and avoiding overhead watering can also help to minimize the severity of the disease. While some fungicides are available for use against this disease, their effectiveness will be reduced if good cultural practices are not used.