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Natural Reaction: Watch out for oak wilt

December 20, 2019 by Jacob Friede


Oak trees are the epitome of strength. They are thick and sturdy and solid, and from the impenetrable bark to the brawn of the branches, they appear to be built indestructible. 

Unfortunately that is not the case and these impressive trees have a weakness. They are threatened by a disease called oak wilt. 

Oak wilt is caused by a fungus that gets carried by sap-feeding beetles. The beetles are attracted to the fungus of an infected tree from which they obtain spores. They then transfer spores to fresh cuts or wounds in other oak trees when they go elsewhere looking for sap.

When the fungus sets in, it’s detrimental to the oak tree.

“The fungus will stop the movement of the sap and eventually starve the plant because you’re not getting water moving around to the different pieces of the plant,” said Mariquita Sheehan, conservation specialist with the Vilas County Land and Water Conservation Department. “And the leaves go. They start losing color right away and then will all fall off.  The tree dies really quickly usually.”

Making matters worse is that oak trees are well connected underground.

“Those few trees that have roots growing near one another, they’ll grow into one another,” Sheehan said. 

Therefore the fungus can travel from an infected tree to a healthy tree through the roots, resulting in patches of dead oak trees. 

Infected trees will rapidly drop their leaves from the upper canopy on down. The leaves, especially in July and August, will wilt and appear dull green or bronze and look water-soaked. 

Oak wilt has been around for awhile. It’s been in southern Wisconsin since 1944, but through the transfer of firewood, which can contain oak wilt spores, the disease has made its way north.

Sheehan said there are a number of sections of land in Vilas County where oak wilt has been discovered.  

Most recently, this past September, it was discovered in an oak tree in the northwest corner of the town of Conover near county highway S and Rummels Road. This discovery was made by Linda Williams, a Wisconsin Department of Natural Resources forest health specialist. 

There is no cure for oak wilt and prevention is the best method to limit its spread. Therefore it is advised landowners avoid pruning, cutting, breaking, or damaging oak trees in any way from April 15 through October because the sap-seeking beetles and the spores are most active and abundant during that time.

“The sap moves in the trees when it’s warmer and not when conditions are frozen,’’ Sheehan said. “Wait until leaf off. That’s the absolute best. While it’s cooler. So before April 15 and ideally after October.”

Sheehan said loggers and roadside brush clearing operations should at least hold off from April 15 to July 15.  

“That’s the most critical time that you’re going to have transfer,” she said.

If an oak does become gashed in anyway during the growing season the cut should be covered with any kind of paint. 

“To keep insects from getting at the inner bark,” Sheehan said. 

Red oak trees, which include the northern red, the northern pin, and black oak, are most susceptible  to oak wilt in comparison to white oaks which don’t do as much interconnecting underground.  

“It doesn’t spread as quickly through the white oak group,” Sheehan explained. 

Other than prevention, the only other way to eradicate oak wilt is an extremely expensive excavation process.

“You have to take out the diseased tree and then the neighboring trees on the expectation that they probably already have the disease in it,” Sheehan explained. “Then you have to cut underground. You have to cut that connection to the trees even further away.”  

Therefore, prevention, by keeping oaks damage free from April through October, is the ideal method for controlling oak wilt, and at the very least from April 15 through July 15.

Keeping firewood local is also an extremely important measure to take.

Oak trees have the strength to hold up more than just heavy branches.  They also support ecosystems.

Their acorns are vital food for animals like turkeys, deer, bear, and squirrels, and they are home to a variety of insects that provide nutrition to the songbirds that live in their canopies.

“Oak is a fantastic wildlife tree,” Sheehan said.

So for the good of all that love and live in the woods, take precautions with regard to oak wilt. It will help as many oak trees as possible stay healthy and strong. 

For assistance with possible oak wilt identification in Vilas County, contact Sheehan at the Vilas County Land and Water Conservation Department at 715-479-3721 or email [email protected]

Jacob Friede may be reached at [email protected] or [email protected]

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