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Wednesday, June 4, 2008 03:39 pm

That wilting feeling

Why so many shade trees have dropped their leaves

Untitled Document Drive through town, and you might get confused about the seasons. Under some trees, it looks like fall. Other trees seem to be just getting started on spring. Most sycamores, for example, have lost their mature leaves and are now producing new ones. The cause of this odd phenomenon is anthracnose, a group of related fungal leaf and stem diseases that thrive with cool spring temperatures and frequent rains. Spring 2008, unfortunately, has been perfect for spreading fungal spores. Different fungi produce anthracnose on different host plants. The fungi cause tan, brown, or black lesions on the leaves, stems, flowers, and fruits of various plants. Infected leaves are marked with blemishes ranging from tiny dead spots to large circular or irregular dead blotches. Symptoms of anthracnose vary, depending on the tree species. On sycamore and maple trees, infected areas often develop along the leaf veins and midribs and expand outward to the leaf edge. This causes the leaf to become distorted. Heavily infected leaves often curl and drop prematurely, littering the ground. Most infections occur in the two weeks following bud break. Anthracnose can affect most shade trees, but the ones most likely to be hit in Illinois are ash, dogwood, elm, maple, oak, sycamore, and walnut. In the early spring, newly emerged leaf tissue in sycamores infected with cankers will suddenly wilt and turn brown. The fungus grows from leaf tissue down the leaf petiole and into stem tissue. (Do not confuse the natural fuzziness of a sycamore leaf with this infection.) The fungus spores will overwinter within the cankers. In the spring, the spores can reinfect the tree and spread the disease to other trees. Cankers in branches can girdle and kill the branch. “Twig blight” refers to the death of 1-year-old twigs. Repeated annual bud or twig dieback stimulates the development of many short twigs at the base of the dead twigs given the tree a “witches broom” appearance. Although anthracnose is a common and unsightly shade-tree disease, it is rarely fatal. However, repeated infections can weaken a tree. Healthy, vigorous trees should quickly recover. One exception is dogwood anthracnose, an aggressive disease that can cause permanent damage and death in dogwoods. So what can be done for infected trees? The first step is to properly identify the problem. A fact sheet on anthracnose diseases of shade trees can be found on the University of Illinois Extension Web site at www.aces.uiuc.edu/vista/abstracts/a621.html. The Ohio State University Extension also has a fact sheet with good pictures at ohioline.osu.edu/hyg-fact/3000/3048.html.
Fungicides are generally not recommended except for dogwood anthracnose. They must be applied before damage appears, and the whole tree must be treated. This is usually cost-prohibitive.
The fungus overwinters within fallen leaves and twigs, so good sanitation is important in minimizing the severity of the disease in the following year. Rake up and discard fallen leaves and twigs and, to prevent reinfection, remove or bury all diseased plant parts. Maintain good tree health. Water trees during periods of drought: Supply 1 to 2 inches of water weekly during dry periods, making sure to apply water slowly and deeply. Mulch the root zone of the tree with a 2- to 4-inch-deep layer organic mulch, such as wood chips. Fertilize trees in the late fall to maintain vigor. Trees that contract severe anthracnose infection and defoliate early may weaken. Defoliation depletes the energy reserve of the tree and increases its susceptibility to other pests and diseases. Yearly infections can reduce growth and may predispose the tree to other stresses.
Make sure trees have proper spacing for good air circulation, and thin out excessive twig and branch growth. Sycamore anthracnose can overwinter in cankers on stems within the tree canopy. If possible, prune out infected twigs and branch cancers. In addition to these measures, consider planting resistant tree varieties in your landscape.

Jennifer Fishburn is a horticulture educator with the University of Illinois Extension Sangamon-Menard Unit. Contact her at fishburn@uiuc.edu.
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