Protect Your Oak Trees: Prune Before Spring to Prevent Oak Wilt Spread

Oak wilt is killing oak trees across Michigan; winter pruning can help protect them from this fatal disease.

Oak Wilt Threatens Michigan’s Oak Trees: How to Prevent It

As the brilliant reds, oranges, and yellows of Michigan’s oak trees fade into winter, experts are warning property owners to take proactive steps to protect their trees from oak wilt, a deadly fungal disease caused by Bretziella fagacearum. Oak wilt has been devastating oak populations in the Lower Peninsula and parts of the Upper Peninsula, including Dickinson, Iron, Marquette, and Menominee counties.

The best defense? Avoid pruning or wounding oaks during the high-risk infection period from April 15 to July 15, when sap-feeding nitidulid beetles are most active and can spread the disease through freshly exposed wounds. Pruning during the winter months significantly reduces the risk of infection.

Where is Oak Wilt in Michigan and How Does It Spread?

Oak wilt primarily targets red oaks, such as black oak, northern red oak, and northern pin oak, all identifiable by their pointed leaf tips. Once infected, the fungus can produce distinct gray to black fungal pads under the bark, attracting beetles with a yeasty odor. Beetles spread the fungus by feeding on sap from fresh wounds on other oaks.

Symptoms of infection include green leaves turning yellow or brown at the edges, often leading to premature leaf drop during summer. The disease can also spread from tree to tree through underground root connections, creating expanding circles of dead oaks.

Pruning Guidelines to Minimize Risk

The Michigan Department of Natural Resources (DNR) and experts from Michigan’s forestry and agriculture departments recommend winter pruning to protect trees. “Winter is considered low risk,” said Dr. Tara Bal of Michigan Technological University. However, warmer-than-usual winter days can occasionally create conditions for infection, so vigilance remains important.

Simeon Wright, forest health specialist with the DNR, highlighted that spring weather variations can shift the leaf-out dates of trees, affecting the high-risk period for oak wilt spread. When construction or landscaping activities risk damaging oaks, these projects should also be postponed to the winter months when possible.

Other Trees at Risk and Regional Spread

White oaks, distinguishable by their rounded leaves, are less susceptible but can still succumb over several years. In 2022, researchers detected oak wilt in dying Colossal chestnut trees, suggesting the disease could pose a threat to other members of the Fagaceae family.

To view verified, reported, and treated locations of oak wilt in Michigan, residents can use the interactive map at Michigan.gov/ForestHealth.

Best Practices for Oak Tree Care

• Plan pruning, trimming, or removal for winter months whenever possible.
• Avoid damaging oaks between April 15 and July 15.
• Cover fresh wounds immediately with tree wound paint or latex-based paint if damage occurs during the high-risk period.
• Do not move firewood from oak wilt-killed trees, as it may harbor and spread the fungus.

To learn more about oak wilt and management strategies, visit Michigan.gov/ForestHealth or MichiganOakWilt.org.

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