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Oak Wilt

June 18, 2003

Generally when your red, black, or pin oak tree has oak wilt, you know something is terribly wrong. Branches quickly progress from green to brown in a scattered pattern throughout the tree or from the top down. The tree does not recover. This disease has begun to appear in 2003 in Illinois.

Oak wilt is caused by a fungus (Ceratocystis fagacearum) that enters the water-conducting vessels of the sapwood and causes them to become plugged. Symptoms vary depending on the oak species involved. Generally, oaks in the red–black group devel-op discolored and wilted leaves at the top of the tree or at the tips of the lateral branches in late spring and early summer (now). Leaves curl slightly and turn a dull, pale green, bronze, or tan, starting at the margins. Usually by late summer, an infected tree drops all its leaves. In some years, we see red oaks progress from scorched foliage to total defoliation in 3 weeks.

The white and bur oak group generally shows symptoms on scattered branches of the crown. The disease is often confused with general dieback and decline. Leaves on infected white oaks become light brown or straw colored from the leaf tip, progressing toward the base. The leaves curl and remain attached to the branches. This tree group may die in one season but is likely to survive for years with many dead branches in the crown. Recent incidences of anthrac-nose on white oak have caused concern among many tree specialists who fear oak wilt. Anthracnose causes brown spotting scattered over the leaves and may cause slight leaf cupping.

Other problems can mimic oak wilt, including drought, construction damage, soil compaction, changes in the soil grade or water table, lightning damage, nutritional disorders, insect and animal injuries, chemical damage, cankers, and root decay. None, however, has the distinct vascular discoloration of oak wilt. To detect the discoloration, peel the bark back with a knife. Healthy sapwood is white or tan. An oak wilt suspect shows brown and white streaking of the wood. Samples without streaking do not yield the oak wilt fungus even if it is present elsewhere in the tree. Therefore, the disease can go undetected if not properly sampled. There is a slight brown streak to healthy wood as the air comes into contact with the sapwood. The distinct discoloration from oak wilt is visible as soon as the bark is peeled back and does not intensify as the wood dries. Sometimes, the discoloration is visible just under the bark; and other times, it is deeper in the wood and visible only when viewed from the end of a cut branch.

If you think your tree is infected with oak wilt, the Plant Clinic can prepare cultures from the wood to detect the fungus. Samples should be 8 to 10 inches long, about thumb thickness, alive but showing symptoms, and must contain vascular discoloration. It takes at least 7 days for the fungus to develop in the lab to the point that a positive confirmation can be made. Often, a 2-week incubation is needed. Submit samples on disposable ice packs. Temperatures in mail trucks are high enough to kill this fungus in wood samples, resulting in false negative tests.

Oak wilt is particularly threatening because an infected tree cannot be saved and because it is difficult to keep the fungus from spreading to nearby oaks. The fungus infects through fresh wounds via a beetle vector. It can also spread through root grafts between trees. An infected tree cannot be saved; but there is hope for surrounding trees, so a positive diagnosis may be important. Pruning of oaks should be done only in the dormant season if at all possible. Pruning now causes wounding that may serve as an infection site to beetles carrying the fungus. Refer to Report on Plant Disease (RPD), no. 618, for more on oak wilt--on the Extension VISTA Web site or from your local Extension office.

Author: Nancy Pataky


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