Issue 13, August 25, 2014

Oak Wilt

Oak wilt is a devastating fungal disease that essentially plugs up the vascular system and disrupts the water and nutrient flow within oak trees. The disease progresses quickly, killing mature trees in the red oak group within one season.

Leaf symptoms vary depending on the oak species involved. Generally oaks in the red oak group (pointed leaf lobes) 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. 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.

Oak Wilt Symptoms or Northern Red Oak. Photo Credit: Nancy Pataky

Oak Wilt Symptoms or Northern Red Oak.

The white oak group (rounded leaf lobes) 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 dieback and stressed appearance.

Accurate diagnosis and confirmation of the disease is essential. There is no cure for oak wilt. Therefore, infected trees are often promptly removed. Incorrectly diagnosing the disease can result in the unnecessary removal of a healthy tree as well as expensive treatments to protect nearby trees. Always consider the possibility that other insect and disease pests can cause symptoms similar to those of oak wilt. Examples of common oak pests and problems can be found within the Plant Clinic Report on Oak Problems.

A plant diagnostic laboratory can confirm or rule out the presence of oak wilt. The University of Illinois Plant Clinic receives and tests an average of 50 samples per year for oak wilt. Of those sampled, only a small number, 10-15 trees, are confirmed to have the disease. This is good news for many homeowners and arborists in that the majority of sampled trees are not found to be infected with the disease and do not require tree removal or expensive and unnecessary treatment options. The samples not found to be infected with oak wilt either had symptoms that were caused by other pests or the test results were inconclusive due to poor sample quality (too dry, wrong portion of the tree sampled, not shipped correctly, etc.). Inconclusive results can be avoided by reviewing the tips and suggestions for oak wilt sampling provided within Issue 10 of the 2013 Home Yard and Garden Pest Newsletter.

For more information about oak wilt, consult Report on Plant Disease, no. 618, "Oak Wilt and Its Control" (Adobe PDF). (Travis Cleveland)

Travis Cleveland

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