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Oak Wilt Appears

June 16, 2004

If you have an oak tree that shows severe leaf scorch near the top of the tree, branch death that seems to spread each year, or sudden browning and death, you may be dealing with oak wilt. This fungal disease kills oaks. Red and black oaks die quickly, usually in one season. Oaks in the white oak group may decline over many years. No oak species is immune. The Plant Clinic isolated the oak wilt fungus from two oaks in north-central Illinois last week--the first confirmed cases for 2004. This is a couple of weeks earlier than in the past 2 years. These cases should serve as a reminder to watch for oak wilt symptoms so that you recognize it when you have the real thing. Quick action may save nearby oaks.

If you have oak wilt in your area, do not prune oaks now. Pruning when trees are actively growing results in sap flow, attracting the beetles that may carry the fungal pathogen to your tree. Both the beetles and the fungus are now active. If oak wilt is present in your area, try to leave pruning of oaks at least until after midsummer. The dormant season would be an even better time for this task.

Oak wilt 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. In some years, we have seen red oaks progress from scorched foliage to total defoliation in as little as 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 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 stagheaded appearance. Anthracnose may produce some look-alike symptoms. Anthracnose causes brown spotting scattered over the leaves and may cause slight leaf cupping as well. A tree infected with only anthracnose produces healthy new leaves as temperatures turn warm. If oak wilt infects a section of a tree, the new leaves do not appear healthy.

Look for vascular discoloration to help diagnose oak wilt. If you think your tree is infected, 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 about 7 to 10 days for the fungus to develop in the lab to the point where a positive confirmation can be made. The processing time can not be shortened. Oak samples for oak wilt testing should be sent on disposable ice packs to prevent killing the fungus (in mail trucks) with high temperatures before it can be isolated in the lab.

Oak wilt is particularly threatening because there is no complete control or cure once the fungus infects. The fungus infects through fresh wounds by a beetle vector and can spread by root grafts between trees. The infected tree cannot be saved, but you may be able to save nearby trees, so a positive diagnosis is important in many cases. Refer to Report on Plant Disease, no. 618. You can obtain this report on the Web (http://www.ag.uiuc.edu/%7Evista/horticul.htm) or at local Extension offices.

Author: Nancy Pataky


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