Oak Wilt

Is the oak tree in your backyard not looking so good these days?

The problem might be oak wilt, a fungus that invades the waterways inside the oak tree and plugs the path water takes to nourish the tree. The leaves of the tree begin to turn brown or tan and fall off from the top of the tree down as the water flow slows down.

Wisconsin, Minnesota and Michigan’s oak trees face the disease that travels both above and below ground. Red oaks are more susceptible to the disease than white oaks.

According to the Wisconsin Department of Natural Resources, root grafts between a healthy and diseased oak transmit the fungus below the ground. Red oaks graft their roots more easily than white oaks and a graft between the two species is very rare.

Above ground, sap-feeding beetles feed on the blue-green fungus that grows under the bark of oak trees that died the previous year. The beetles eat the sap and then travel to new oaks, infecting them with the disease.

Town of Dekorra supervisor Gary Leatherberry is in charge of overseeing the spread of oak wilt in Dekorra. He said that the disease has been around since he came to the area about 12 years ago.

“When you look out on Lake Wisconsin and some of the bluffs in the area, you can see the trees dead from oak wilt,” Leatherberry said. “You look at a lot of places around here, there are sick oaks. They are not the strong beautiful oaks like you are used to seeing.”

While it is hard to see the signs and symptoms of an infected tree, the DNR offers testing of the tree to see if it is indeed afflicted. A fee may be charged for the testing. The DNR website contains information on how to sample the tree and where to send it at dnr.wi.gov.

Leatherberry said just because a tree looks unhealthy, it doesn’t mean it has oak wilt, but if you can afford to take the tree out to prevent the spread of the disease you are probably better off.

Some other things the DNR recommends are avoiding pruning, cutting or wounding oak from April through July in urban areas as the wounds in the tree allow the disease spreading beetles access to the inside of the tree. They also say, if doing any tree trimming when temperatures are above 50 degrees, to use a tree wound paint to cover up any exposed areas. More directions on the use of the wound paint are available on the DNR website.

If oak wilt is confirmed in the area, the problem can be managed in a couple of ways. The best way to fight the spread of oak wilt is by creating a root graft barrier. This disconnects the shared root systems of the trees by physically cutting the roots apart using a cable plow or trencher.

Dead oaks with the bark firmly attached are a risk for spreading oak wilt. Debarking is an effective way to fight the formation of the fungus under the bark and should be done in late summer, fall or winter. The DNR says these trees must receive special treatment and a forester or forest pest specialist should know the best way to remove the affected tree.

Leatherberry also said that a mixture of tree species will help stop the spread of the disease as the root grafts typically only happen between trees of the same species. He said in his time working for the town of Dekorra, not a single oak tree has been planted in the parks for fear of spreading the disease.

“Within residential areas, be aware of what and when you are pruning,” Leatherberry said. “If you own large tracts of land, get out there and walk around to look and see what you have. If you don't burn wood yourself there are a lot of people who do, and would be happy to go in and cut it out of there for you.”

For more information, check out the DNR website at dnr.wi.gov.

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