June To October: Oak Wilt Watch!

By Michael Hillstrom, DNR Forest Health Specialist, Fitchburg, michael.hillstrom@wisconsin.gov

An image of two red oak leaves that are part brown and part green, symptomatic of oak wilt infection.

Typical browning leaf symptoms of red oak trees infected with oak wilt. Photo: Wisconsin DNR

Now is the time to watch for oak wilt! Symptoms typically appear in southern Wisconsin in late June and in northern Wisconsin during July. Watch for browning leaves starting at the top of the canopy and progressing downward. Most leaves will fall from infected trees as they die, typically within two to four weeks.

Several management methods are available to contain oak wilt pockets, so consult your local forest health specialist for guidance on your best options.

The forest health team is also working to evaluate new control methods. Rapid response is currently being tested to more formally evaluate if a new oak wilt infection in a single tree can be stopped before it reaches the roots and spreads to nearby oaks. There is observational data that this method works, but more formal tests are desired.

Additionally, oaks can be affected by various insects, diseases and abiotic issues that may resemble oak wilt.

If you suspect oak wilt in forests, you can send photos of the symptoms you are seeing to your local forest health specialist so they can determine the likely cause.

If you have a question about yard trees, please contact the Plant Disease Diagnostic Clinic at UW-Madison for sample testing (for a fee) or a certified arborist, such as those listed on the Wisconsin Arborist Association website.

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