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6/2/03 Contacts: Yard and Garden Column for the Week Beginning June 6 Anthracnose: A Sickness in SycamoresBy Paula Flynn Wet spring weather can favor fungi. Sycamore trees are one of the most striking victims of these fungi. Usually large and stately, sycamore trees appear as skeletons at this time of year, void of the normal covering of green leaves. Instead, only tufts of brown, wilted leaves are scattered throughout the branches. Diseased leaves eventually fall, leaving a blanket of brown on the ground beneath the tree. This widespread disease problem of sycamore is called anthracnose. Actually anthracnose is a general plant disease term. There are many different anthracnose diseases that occur on various plants, caused by a number of different fungi. Fungi that cause anthracnose diseases tend to thrive when spring conditions are cool and wet for an extended period of time. If these conditions occur when tender leaves are emerging from buds, anthracnose can reach epidemic proportions. Diseased leaves tend to show dark spots or blotches and fall to the ground prematurely. Anthracnose on sycamore can be especially noticeable because the fungus not only infects the new leaves but also buds, shoots, twigs, and branches. Year after year of shoot death can result in tufts of dead twigs throughout the tree. These clusters of dead twigs are referred to as "witches brooms." When the sycamore anthracnose fungus invades woody twig and branch tissue, cankers are formed. Cankers are sunken, dead areas that are often elliptical in shape. As these cankers enlarge, twigs and branches may be girdled. Tissue that is girdled or restricted cannot transport water and nutrients. The section of the twig beyond the canker eventually dies. Unfortunately for the sycamore tree, the fungus survives from year to year in specialized structures in the cankered twigs and diseased shoots. Nestled in the woody tissue, these fungal structures, called pycnidia, are protected from the dry heat of summer and the cold of winter. They lie in wait until wet spring conditions occur. The black, pepper-like pycnidia are small, but can be seen on the dead or dying twigs with the naked eye. The fungus also can survive in the diseased leaves that fall to the ground. When the cool spring rains occur, these pycnidia release thousands of
spores that are whisked away by spring rains and land on newly forming
leaves, shoots, and twigs of sycamore trees. Some of the infected leaves
wilt and turn completely brown, but others only develop dark brown spots
along the veins. As the leaf symptoms progress, larger brown blotches,
sometimes V-shaped, develop along the leaf vein tissue. Because the fungus manages to hang around from year to year, there are some cultural practices that help improve tree vigor. Most are aimed at removing as much diseased leaf and twig tissue as possible to help reduce the amount of fungus that survives to the next season. As feasible, prune out cankered branches or twigs. Unfortunately, the large size of mature sycamore trees make pruning out diseased twigs a fairly unrealistic chore. Fungicides can be used to protect trees in the spring, but this may not be economical or warranted on large trees. However, fungicide treatments might be desirable in certain situations. Protective fungicides must be first applied at the time of bud swell, with two additional applications at 10-14 day intervals. Certain fungicides can be injected into sycamore trees and provide protection from this fungus. These products are injected into small holes drilled in the base of infected trees. This is a procedure that should be done by a certified arborist who has the appropriate equipment. Finally, if you enjoy sycamore trees and are looking for a substitute with a similar appearance but less vulnerability to this common disease, you might consider the London planetree. Like sycamore, it is a large tree so it is not suited to small lots. It is rated a zone 5 tree, so it may show some twig dieback if winter weather is severe. For further information on anthracnose of sycamore and other shade trees, refer to the Iowa State University Extension publication "Anthracnose of Shade Trees" (PM 1280). This publication is available through your ISU Extension county office or through the ISU Extension Distribution Center by calling (515) 294-5247. -30- |
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