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.

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.”

Wisconsin communities may be eligible to participate in a cost-sharing program to help combat oak wilt. The Urban Forestry Grant Program is not available to individual property owners; however, they can work with their municipalities to take steps to protect their oaks. Grant applications are due Oct. 1. Communities interested in applying for a grant can contact the local DNR urban forestry coordinator for more information.

“It is important to know the signs and have a certified arborist or local forest health specialist help outline management options, such as applying a fungicide,” said Don Kissinger, DNR urban forester.

Signs of oak wilt include:

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.

Helpful tips:

Do not prune oak trees from April through July and seal any wounds with water-based paint to prevent insect-caused infection. Check with the municipality to see if there are local oak wilt ordinances.

Control below-ground spread by severing the root system between an infected tree and healthy trees.

Do not move diseased wood for firewood.

If storing diseased wood for personal firewood use nearby, seal the entire pile with four mil plastic.

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/ 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.”

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Dorothy Robson is editor of the Westby Times. Contact her at 608-637-5625.

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