Tree peonies, known botanically as paeonia suffruiticosa, are actually deciduous woody shrubs that are cold hardy and do not die back in the winter as herbaceous peonies do. Native to China, tree peonies are prized for their large lush flower heads that bloom in a wide range of colors from deep magenta purple to coral and crisp white. Tree peonies are long lived but also very slow growing, which makes disease damage difficult to overcome once established.
Peony Leaf Blotch
Peony leaf blotch is a fungus that strikes tree peonies exposed to extended periods of hot, humid weather with too little fresh air circulation. It is also called peony measles or stem rot and manifests itself by making dark purple-black spots of the topside of the leaves. Spots on the underside of leaves are deep brown in color. It is more common to see the spots around the edges of the peony leaves on the perimeter of the plant. The leaves may become warped or crinkled in shape and no longer lay flat. When the fungus is on the young tender stems, it makes long brown streaks and red raised measle-like spots can appear on the top stems of the shrub. Leaf blotch necessitates cutting all affected foliage and stems down to just above the crown in the fall or early spring. Follow up by spraying fungicides and clearing any debris or old plant material from the surface of the soil.
Botrytis blight is a fungus that attacks the the peony flower buds, flower stems and leaves. It can strike throughout the growing season but seem to be particularly triggered by heavy rains, high ambient humidity and extended periods of sunless and cloudy weather. The tender young tissues will will and become soft with rot and the buds will darken and shrivel or remain closed and never open to bloom. When infected buds do open the petals will be stuck together and misshapen. A grayish brown velvety mold can coat the worst affected plant parts. Botrytis blight is highly contagious to nearby plants as the fungal spores travel on the wind. Once identified, the most damaged plants material should be removed and destroyed and fungicide for peony botrytis sprayed on all nearby peony plants.
Phytophthora blight is another type of fungal infection that affects the tree peony buds, leaves, flower stalks and the plant crown. It will turn the infected portions of the peony brownish black and will toughen the plant tissues making them rough in texture. Tender young growth is most susceptible and can darken the entire length of the stalk. The plant crown may become moldy with rot.