Spirea Diseases

Spirea is also known by the scientific name Spiraea japonica and is a member of the Rosaceae family. This evergreen produces lovely pink blooms from May to July, especially when planted in full sun. Spirea is generally free from serious disease, but occasionally suffers from a few common plant problems.

Powdery mildew is a common disease of a variety of landscape trees, flowers and shrubs. This disease affects the foliage, stems and flowers of infected trees, causing a coating of fungi to develop on leaf surfaces. As powdery mildew continues, fungal growth encompasses more and more plant tissue. Like many members of the rose family, spirea is susceptible to infection of fireblight, but damage is not usually significant. Fireblight is named for the black coloring that develops on host trees, giving them a scorched appearance. Leaf spot diseases may affect spirea but are also not especially damaging to the health and appearance of the tree. When leaf spot does occur, it is most often caused by Entomosporium maculatum, which is common in members of the rose family.

Powdery mildew causes spirea to develop a white or gray coating of fungus on its leaves, stems and flowers. This fungus looks similar to powdered sugar or talcum powder, which is where it received its name. Other symptoms include leaf curling, twisting or yellow leaves. Powdery mildew is not usually serious in spirea but can become unsightly when infection is heavy. Fireblight may cause spirea to develop water-soaked blooms, usually in the early spring. These blooms turn brown and wilt. Fireblight may cause bark to turn brown or black and cankers may appear on the trunk of infected trees. Leaf spot diseases are usually minor in spirea but are characterized by dark purple splotches on leaves that develop a gray center. On trees with a heavy infection, these splotches may enlarge until they run together and cover a large portion of the leaf surface. Leaf spot diseases are usually a cosmetic problem in spirea and not serious.

Keep your spirea healthy and stress free to avoid fungal and bacterial disease development. Water your spirea early in the morning to allow drying time before nighttime temperatures arrive. Avoid wetting the foliage of your spirea during watering, as this may increase the risk of fungal disease. For best results, use drip irrigation systems that do not soak the soil. If your spirea shows any signs of disease, remove and destroy infected branches and plant tissue to prevent the spread of disease.

Fungicides may help reduce the effects of powdery mildew, leaf spots and fireblight if symptoms are severe. Choose products containing active ingredients such as mancozeb or chlorothalonil for best results. Most fungal and bacterial diseases respond best to treatment with fungicides when symptoms are first present.