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Oak Wilt

May 31, 2000

As stated above, oak wilt has been confirmed in a tree in western Illinois this year. Assuming the sample is taken properly, it is not too early to sample for this disease. 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 develop 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). The leaves curl slightly and turn a dull pale green, bronze, or tan, starting at the margins. Usually by late summer, an infected tree has dropped all its leaves. We have seen red oaks progress from bronze to dead in as little as 3 weeks.

The white and bur oak group generally shows symptoms on scattered branches of the crown. Their leaves become light brown or straw colored from the leaf tip toward the base. The leaves curl and remain attached to the branches. This tree group may die in one season but is much more likely to survive for many years with a stag-headed appearance. Recent appearances of anthracnose on white oak have caused concern among many tree specialists who fear oak wilt. Anthracnose will cause brown spotting scattered over the leaves and may cause slight leaf cupping as well. It will not cause vascular discoloration as seen with oak wilt.

Other problems can mimic oak wilt, including 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 found with oak wilt. To detect the discoloration, peel the bark back with a knife. The sapwood of a healthy tree is white or tan. An oak wilt suspect tree will show brown and white streaking of the wood. Samples without streaking do not yield the oak wilt fungus even if the fungus is present elsewhere in the tree. Therefore, the disease can go undetected if the tree is 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 and detect the fungus when it is present. Samples should be 6 to 8 inches long, about thumb thickness, alive but showing symptoms, and must contain vascular discoloration. Send the sample with disposable ice packs to keep it from getting too hot in the mail. Temperatures over 90 degees are thought to kill the fungus in branch sections.

Oak wilt is particularly threatening because there is no complete control or cure once the fungus infects. The fungus infects through fresh wounds and a beetle vector, and it can spread by root grafts between trees. You cannot save the infected tree, but you may be able to save surrounding trees, so a positive diagnosis is important in many cases. Pruning of oaks should be done only in the dormant season if at all possible. Refer to RPD No. 618 for more on oak wilt. This serious disease has been found in all parts of Illinois over the last decade. We have not, however, found that it is spreading any more quickly than it has in the past, and we do not expect to see an epidemic of oak wilt.

Author: Nancy Pataky


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