Peonies are herbaceous perennials, meaning when the plant goes dormant in the winter, the leaves fade away to reappear in the spring. The flowers are fragrant and large, reaching up to 10 inches across, and bloom in various colors. Peonies are a much-loved garden plant that, according to About Peony, can live for 100 years with proper care and attention. Peonies are susceptible to disease, but these diseases can be prevented and treated.
There are many diseases that can attack the leaves of a peony plant. Red spot or Measles of Peonies or Botrytis and Phytophthora Blight are caused by fungi. Leaf and stem spot cause discoloration of the leaves, stems and flower buds. Wild mold is another fungus that will grow on your peony leaves. Viruses such as mosaic virus and ring spot can also affect your peony leaves, causing them to discolor, curl and die.
Fungal diseases that affect the leaves of peonies are commonly caused by fungus such as Botrytis and Phytophthora Blight. These fungi can lay dormant in dried leaves and dead flowers left on the plant. In wet weather, the fungi reproduce and not only attack the leaves, they will infect the stem and buds of the peony. These diseases most commonly occur during cold weather with little sun and high moisture content. Viruses such as the mosaic virus can be spread by piercing insects such as aphids.
If a peony is infected with a disease, the leaves will become discolored and sometimes misshapen. With red spot, according to the University of Illinois, brownish-red or purple spots will appear on peony leaves in late summer. These leaves will begin to grow until entire leaves and even stems are affected. With Botrytis and Phytophthora Blight, the leaves may be covered with dark gray fuzzy spots that spread and kill the leaves and sometimes the plant. Virus infections can stunt the leaves, leaving them curled or patterned or striated.
The best way to prevent these diseases is to keep your plant healthy. Make sure your plant has good circulation, the proper fertilizer and the right amount of water and sunlight. Rake up and remove dead leaves around the plant to reduce the chance of harboring fungi. Cutting off the dead flowers where the fungus can grow will also help prevent the spread of disease.
Fungicides are not always effective when treating fungal diseases of peony leaves. If fungicides are used, spray the plant and the soil around the plant. Applications of copper sulfate are recommended. Apply the copper sulfate to the plant and the surrounding soil in the early spring when the peony shoots are about 6 inches tall. There is no treatment of viral infections of peony leaves. The plant may recover on its own. If symptoms are severe, it may need to be destroyed.