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Broad-leaved trees often host late season leaf discoloration. The cooler and wetter growing season favors the growth of common leaf fungi.

     As the summer gradually moves into the fall season, the leaves of many tree species will prematurely turn brown. A variety of leaf fungi are especially common when associated with cooler and moister conditions. Different fungi are associated with different tree species. Few of these leaf fungi present a health threat to individual trees or forests.
     Trees began physiologically preparing for the winter season in early August. They’ve been vigorously photosynthesizing for months. Leaf and flower buds are set for 2018. Flowers and fruits are mostly done for the year. Carbohydrate reserves have been translocated to root systems and other storage areas. The shorter daylight hours will drive the color change. The early leaf-changers have already begun their color season.
     It’s common to see browning leaves that present little or no health threat to trees. A number of Anthracnose species will cause the leaves of oaks, maples, and other tree species to turn brown. Areas of leaves will turn brown, usually in the lower part of the crown where the air is more humid. This can be particularly common with golf courses and manicured landscapes that utilize watering systems.
     Norway maples, quite common in residential and urban areas, contract tar spot disease at almost any time during the growing season. Small, circular, brown blotches appear and sometimes grow to engulf most of a leaf by autumn.
     Septoria cankers and other leaf diseases often cause an early brown among aspens, particularly balsam poplar. This fungus can move into twigs and stems, sometimes resulting in tree health problems.
     There are also a number of insects that can cause leaves to appear brown, such as leaf miners, skeletonizers, and a host of species that form leaf galls and similar deformations. Scale insects feeding on twigs can cause leaves to brown and curl. These insects also seldom present a health risk to trees when they occur this late in the season.
     Other wilts, blights, cankers, and other fungi that appear this time of the year usually are not a health threat. However, sometimes more serious diseases can be masked by this normal late season browning. Yet, if a tree has appeared healthy during the spring and summer, then the risk of a serious disease is much lower.
     Oak wilt, beech bark disease, Dutch elm disease, and Verticillium wilt are more serious tree diseases but usually manifest themselves earlier in the growing season. Boring insects, such as the emerald ash borer, or bark beetles can also cause browning leaves, usually from the top-down.
     Wilted foliage can be caused for non-biological reasons, too, such as root damage, drought, excessive moisture, soil compaction, and salt injury. A significant stem injury can sometimes lead to early leaf loss and either partial or whole-tree mortality.
     Treatment for late season leaf diseases is unnecessary. Fungicides are not helpful and often harmful. Homeowners can collect and dispose of diseased leaves as they fall. Pathogens often overwinter in the soil. Where too much shade exists, trees can be thinned or pruned to improve air circulation. Next spring, think twice before operating a riding lawnmower on wet soils, to avoid possible root damage.
     For the most part, late season leaf discoloration is not a reason for concern. Pathogens and insects have cycles that depend upon weather conditions, predators, and other factors in their population dynamics. Soon enough, the color season will be upon us and leaves will fall. The 2018 growing season may be completely different from this year, with another set of concerns.

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Bill Cook is an MSU Extension forester providing educational programming for the Upper Peninsula. His office is located at the MSU Forest Biomass Innovation Center near Escanaba. The Center is the headquarters for three MSU Forestry properties in the U.P., with a combined area of about 8,000 acres. He can be reached at cookwi@msu.edu or 906-786-1575.

Prepared by Bill Cook, Forester/Biologist, Michigan State University Extension, 6005 J Road, Escanaba, MI 49829
906-786-1575 (voice), 906-786-9370 (fax), e-mail: cookwi@msu.edu

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By-line should read "Bill Cook, MSU Extension" Please use the article trailer whenever possible.

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Last update of this page was 5 November, 2018