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Horsechestnut Leaf Blotch

June 27, 2001
Horsechestnuts are not used widely in the landscape, but we do see this disease fairly frequently. It may be known as horsechestnut leaf blotch or Guignardia leaf blotch, named for the causal fungus, Guignardia aesculi. We see it most commonly on horsechestnuts, but buckeye trees may also serve as hosts. There is some resistance available in Ohio buckeye and in bottlebrush buckeye. Otherwise, the disease is fairly common in the Midwest.

From a distance, infected trees appear to be severely scorched. On closer inspection, reddish brown leaf spots with bright yellow margins become obvious. The spots enlarge and cover most of the leaf surface; leaves then become dry and brittle and drop early. With some tree diseases, such as anthracnose, new leaves continue to emerge after infection. In the case of horsechestnut leaf blight, the disease does not occur until most of the season's foliage has emerged. For this reason, an affected tree does not seem to recover as the summer progresses.

You can distinguish this disease from environmental scorch (see issue no. 5) by the presence of tiny fruiting bodies formed by the fungus in the leaf lesions during moist weather. These structures, called pycnidia, appear black and about the size of a pinhead; they are embedded in the leaf. The leaf blight disease may affect all leaves. Sometimes the disease is worse at the base of the tree, where humidity may be slightly higher. Scorch symptoms are usually most severe on the tips of branches, especially on the side of the tree most exposed to sun and wind.

Rake and remove fallen leaves, prune to allow faster drying of foliage, and keep the tree well watered in drought to help manage this disease. Fungicides are available and protect new growth from infection, but the cost for large trees is prohibitive. Clients don't always understand that sprays protect for only one season and the disease can return the following year. Still, many fungicide options are listed in the 2001 Illinois Commercial Landscape and Turfgrass Pest Management Handbook and the Home, Yard, and Garden Pest Guide (formerly, Homeowners' Guide to Pest Management). To be effective, sprays must be initiated when buds begin to open. For new plantings, consider using a resistant species. Information about this disease may be obtained in Sinclair, Lyon, and Johnson's book, Diseases of Trees and Shrubs, as well as on the Web. Try this site (produced by Oregon State University) for pictures of the disease: http://plant-disease.orst.edu/plant_index.cfm. (Nancy Pataky)

Author: Nancy Pataky


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