Winter is an ideal time to perform pruning and trimming work on oak leaves. / Photo Credit: Wisconsin DNR

By Art Kabelowsky, DNR Outreach and Communications, Fitchburg
Arthur.Kabelowsky@wisconsin.gov or 608-335-0167

Winter is an ideal time to perform trimming, cutting and brush removal work on and near oak trees.

For one thing, it’s a low-risk period for the trees to be infected with oak wilt, a fungal disease spread by beetles. When a red oak is infected with oak wilt, it will die that year; the disease also stresses trees in the white oak group, often fatal with bur oaks and swamp white oaks.

Sap-feeding beetles in the Nitidulidae family are the primary insect vectors in the spread of oak wilt. They feed on infected trees and then carry the fungus to fresh open wounds on healthy oaks. / Photo Credit: Wisconsin DNR

For another, now that leaves are off the trees it’s much easier to find areas that might need pruning or other work.

But most of all, it won’t prompt Linda Williams to stop her car!

Williams is the Northeast region’s forest health specialist for the Wisconsin Department of Natural Resources (DNR). During the high-risk period for oak wilt infection (April 1 through July 15), she is always on the lookout for ways to save an oak tree – sometimes an entire stand.

And you can be sure she’ll pull her car over to make the save.

Once, Williams got a report that a work crew was pruning oak trees along a roadway in June, the heart of the danger period. She visited the site and told the crew how pruning oaks at that time could lead to oak wilt infection, which would leave them with a lot of expensive and time-consuming work cutting down the dead tree(s).

The pervasive and deadly disease has been found in 66 of Wisconsin’s 72 counties after first being documented in the state in 1944. Only six northern and eastern counties have not yet had an official detection.

The fungus is spread by beetles feeding on sap from recently damaged oak – they seek out broken branches, pruning cuts and even damaged bark. Once a single tree is infected, the disease can spread to kill adjacent oak trees through their interconnected root systems.

“After speaking to the crew, they sent one crew member back to the garage to get some (tree pruning sealer) paint,” said Williams. “(They) painted the wounds that they had made that morning, and all other pruning wounds that they created throughout the day. A positive outcome – none of the trees got oak wilt.”

This year in early July, she pulled over in the middle of a trip to speak with a county crew that was starting work to remove trees in a right-of-way. Some were dead and some were alive. The trees weren’t oaks, but oaks were nearby.

coloration and will appear water-soaked but partially green. Trees in the red oak family that are infected with oak wilt will die during late summer or early fall of the same year that they are infected. / Photo Credit: Wisconsin DNR

“If they had dropped those trees, it definitely would have broken branches in nearby oak trees, which would have put them at risk for getting oak wilt,” Williams said. “After talking to the crew, they contacted the commissioner to discuss options. They decided to delay that work until after July 15. They left and went to (work in) an area of the county with little to no oaks.”

It was a good example of Williams’ goal to remind crews all over the state that if they are cutting down trees that aren’t oaks – ash trees killed by emerald ash borer, for instance – those trees may damage oaks as they fall.

“Doing that work like this after July 15 will minimize the risk to oaks in the area,” she said. “You can protect the oak and still get your work done by doing the work now (or) before April 1.”

Also, she reminds work crews that performing mowing and cutting work on rights-of-way with boom mowers (also known as flail, rotary bar, batwing or chop mowers) during the high-risk period can lead to inadvertent but deadly damage to the bark of nearby oak trees.

A fungal mat of oak wilt on the side of a tree. / Photo Credit: Wisconsin DNR

Williams also said that just in case crews prune or accidentally wound oak trees during the high-risk period, they should carry fungicidal tree wound paint and immediately cover all exposed wounds.

Do you have oak trees on your property? Now is an ideal time to work on them.

The risk of deadly tree diseases, such as oak wilt, increases in the spring. Oak wilt is Wisconsin’s most destructive oak disease, killing thousands of healthy trees annually. It’s common in the lower two-thirds of the state and has been spreading further north each year.

Oak wilt is often spread by tiny, sap-feeding beetles that carry a fungus picked up from an infected tree. The beetles are attracted by fresh open wounds on healthy trees, such as pruning wounds, and carry the infection to the healthy tree. The pests are inactive in cold temperatures, so this time of year is best to work on oaks. Another plus is that branch damage or weak branch structure is more visible in winter.

Learn more about oak wilt in Wisconsin and what you can do about it on the DNR’s webpage.