Watch for Signs of Oak Wilt

Leaves infected with oak wilt drop rapidly and appear bronze-green. Photo by DNR.

Each year, a tree-killing fungal disease strikes and kills thousands of oak trees in Wisconsin’s forests, woodlots and urban areas. Oak wilt is common in southern and central Wisconsin and is becoming increasingly abundant in northern counties. It is difficult to control once the disease takes hold and prevention steps need to be taken to slow the spread.

“We are observing oak wilt in more places this year, probably due to the storms we had in the spring,” said Todd Lanigan, a forest health specialist with the Department of Natural Resources Division of Forestry. “The first symptoms of oak wilt are branches with wilted leaves dropping in summer. These are not the brown, dry leaves you see in autumn. These are partially green to bronze-green and are not completely dry.”

Oak wilt is confirmed in all Wisconsin counties except Ashland, Bayfield, Calumet, Door, Douglas, Forest, Iron, Kewaunee, Manitowoc, Sheboygan and Taylor counties.

Oaks in the red oak group, including northern red, northern pin, and black oaks, are particularly vulnerable to oak wilt.

“Oak wilt is fatal for the infected tree. Landowners need to know what to watch for to take immediate steps to protect nearby oaks,” said Don Kissinger, DNR urban forester. “Trees that die of oak wilt can still spread the disease for approximately a year after they die. It is important to know the signs and have a certified arborist or local forest health specialist help manage and contain the spread of oak wilt.”

Signs to watch for:

• Wilted leaves that drop from the top of the tree first.
• Dull green or bronze leaves that look water-soaked.
• Partially green leaves on the ground which have dropped from the tree.

The University of Wisconsin’s Plant Disease Diagnostic Clinic can help verify the presence of oak wilt. Instructions for collecting and mailing samples to the clinic are available at plantpath.wisc.edu/pddc/ (exit DNR), or by calling 608.262.2863.

Other diseases and insect infestations can mimic oak wilt. Additional information about oak wilt and other forest health issues can be found at dnr.wi.gov, keyword “forest health.”