Pruning, cutting, or wounding oaks during April through July leaves them vulnerable to oak wilt. / Photo Credit: Paul Cigan, Wisconsin DNR

By Paul Cigan, DNR Forest Health Specialist
Paul.Cigan@wisconsin.gov or 715-416-4920

With warmer spring weather fast approaching, the Wisconsin Department of Natural Resources (DNR) recommends protecting oaks from the often-fatal disease, oak wilt, by refraining from pruning, cutting or injuring oak trees from April through July.

Oak wilt is a serious disease that kills trees in the red oak group (including pin oak, northern red oak and black oak) and weakens those in the white oak group (bur oak, swamp white oak, white oak and English oak).

It commonly occurs when insects carrying oak wilt fungal spores detect a fresh wound on a healthy oak tree and land on it. The disease is widespread in southern Wisconsin, but it is still uncommon in much of northern Wisconsin. New and often preventable cases occur each year.

Leaves infected with oak wilt appear dull green or bronze and will wilt and drop rapidly in the summer. / Photo Credit: Paul Cigan, Wisconsin DNR

“Sap-feeding beetles, like most insects, have an incredible sense of smell, which draws them to open-wound surfaces on oak trees to feed on the sugary sap in as little as 15 minutes after a wound is created,” said Paul Cigan, Wisconsin DNR forest health specialist. “This is how most new oak wilt infections start.”

As warm spring weather draws people outdoors to start seasonal yardwork, keeping chainsaws and trimming tools away from oak trees until August will go a long way toward ensuring that oaks stay healthy for many more spring seasons to come.

In general, the best time to prune is in winter, when trees are dormant and oak wilt disease-carrying insects are inactive.

As of March 2025, oak wilt has been found in all Wisconsin counties except Iron, Taylor, Door, Kewaunee, Calumet and Manitowoc. Several of these counties contain the state’s highest abundance of healthy and productive oak forests. Taking recommended precautions will help keep them that way. Check with your municipality to find out if it has its own oak wilt ordinances that should be followed.

Oak wilt and other diseases can move easily on or in firewood logs year-round, so keeping firewood local or purchasing Wisconsin-certified firewood is another critical component of protecting trees and keeping forests healthy. You can also help by reporting oaks with sudden, mid-to-late summer leaf drop to a certified professional.

More information is available on the Wisconsin DNR’s oak wilt webpage, which includes a web interface for refining the beginning of the period when pruning, wounding and harvesting oaks should be avoided.