Oak-killing disease spreading farther north in Minnesota

Leaves from a tree infected with oak wilt
Red oak leaves from a tree infected with oak wilt. Infected trees have wilting and bronze to reddish-brown discolored leaves that may also look water-soaked.
Wisconsin DNR

A disease that kills oak trees continues to spread in Minnesota, and its latest discovery as far north as Crow Wing County is worrying foresters and landowners across the state.  

Oak trees are some of the most abundant trees in Minnesota, and they’re valuable for wildlife, lumber, firewood and shade. But the disease called oak wilt poses a threat to these trees — some of which are stately, longtime landmarks in communities across the state.

“I would say the reaction that a lot of these landowners had, with finding out that they had oak wilt, was really a feeling of being overwhelmed,” said Shannon Wettstein, district manager at the Morrison Soil and Water Conservation District who works with landowners. 

Wettstein said landowners often build a connection to their trees and become emotional when they are forced to remove them. 

Oak wilt is not new to the state; it was discovered in 1945 and has been confirmed in 40 Minnesota counties. But the Minnesota Department of Natural Resources is now finding cases farther north than before. 

“I consider oak wilt to be really one of the top forest health issues in this part of the state. In particular, because we have so many red and pin oaks up in this area,” said Rachael Dube, a forest health specialist with the DNR in Brainerd.

Dube counted 40 oak trees around her home in Brainerd. She lives less than 10 miles from where oak wilt was recently confirmed. The disease directly impacts her and others in her community that enjoy their forests.

“I talked with people not only that have had (trees with) oak wilt, but are concerned about getting it. And it's an issue that people are starting to become really passionate about in the area,” Dube said.

She also said oak wilt could have a devastating impact on resorts, golf courses and other important tourism industries in the area.

Where it’s already spread, oak wilt has damaged oak forests in the state by increasing dead pockets and allowing invasive plants like buckthorn to flourish.

The non-native disease slowly kills the leaves of the tree and, eventually, the tree itself. It spreads naturally through root systems, sap beetles and through open tree pores. People accelerate the spread when they prune oaks and carry infected firewood across the state. 

For the last five years, the DNR’s Brian Schwingle has focused on oak wilt in Minnesota. 

“If we jump on that and control it, there's a pretty decent likelihood that we can drastically slow the expansion of oak wilt in that area,” he said.

Infected trees have wilting and bronze to reddish-brown discolored leaves toward the top that quickly move down the tree. Dark green leaves may also look water-soaked, and a dark bluish-gray discoloration may be found on the trunk after peeling back the bark.

People can help stop the spread by not pruning oaks from April to July, using locally sourced firewood and by reporting oak wilt if they find it. 

Once oak wilt has infected an area, it can be difficult for landowners to successfully treat and control the disease. Morrison County expects to use $423,000 in grant money to help landowners with the costs of dealing with oak wilt. The DNR is working on training and partnerships elsewhere.

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