By Paul Cigan, DNR plant pest and disease specialist, Paul.Cigan@wisconsin.gov or 715-416-4920

To prevent the spread of often-fatal oak wilt disease, do not prune, cut or wound oaks from April through July. Photo: Wisconsin DNR

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

The highest risk period for oak wilt introduction to a new site is in spring and early summer. Pruning and cutting oaks exposes living tree tissue beneath the bark to potential infection. The disease rapidly kills trees in the red oak group and weakens those in the white oak group.

Sap-feeding beetles spread the disease between oaks by carrying oak wilt spores from infected trees or firewood to fresh, exposed tree wounds. Healthy oaks and freshly cut oak stumps can become infected as quickly as 15 minutes after a wound is made from a pruning cut or broken branch.

DNR Forest Health specialist Paul Cigan anticipates a busy spring clean-up season and recommends using special care when performing spring clean-up work this season.

“We have seen a lot of tree damage from winter storms that brought heavy snowfall and ice accumulations, causing stems to snap and branches to break and leaving trees completely bent over in some areas of the state,” Cigan said. “While spring clean-up is key to keeping access routes clear and wildfire risk low from excess debris accumulation near structures, it can introduce oak wilt infection in oaks that are cut, pruned, or injured. So it’s important to work with care, whether in the yard or forest.”

With a relatively cool weather pattern holding around the state this year, DNR pathologist Kyoko Scanlon forecasts the possibility of a brief delay to the start of the oak wilt high-risk period this spring.

“We know that the emergence of the sap-feeding beetles that spread oak wilt depends on temperatures, so we closely monitor spring temperatures using a temperature modeling tool. Cool weather can delay beetle emergence, and based on our monitoring at this point in late March, it appears that we are on track for one of those springs,” Scanlon said.

Cigan believes the slower spring warmup provides a great opportunity for those wishing to clean up yards, trails, and forests of fallen debris before the oak wilt risk becomes too high.

“Heavily storm-damaged properties and access routes will require a bit of time to clean up, so having a couple of extra weeks before oak wilt risk significant increases could really help,” Cigan said.

Close daily monitoring of temperature accumulation at the location of interest is essential if any work that involves pruning, harvesting and injuring oaks is scheduled into April.

Although tree paint or wound dressing is not normally recommended on pruned or wounded surfaces, a light application of these products immediately after an oak is damaged or cut may be the only defense against oak wilt infection during this high-risk period. If possible, apply within 15 minutes. Wounds are not susceptible to oak wilt after 72 hours.

As of March 2023, oak wilt has been found in all Wisconsin counties except Ashland, 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 for years to come. Check with your municipality to find out if they have their own oak wilt ordinances that you should also follow.

It’s also important to note that oak wilt and other diseases move easily on or in firewood logs year-round. Keeping firewood local or purchasing Wisconsin-certified firewood is another key component of protecting trees and maintaining healthy forests.

More information is available at 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.