When oak leaves started raining down on Kevin O'Brien's land in Dairyland, about 2 miles north of the county line, he knew something was wrong.

Already working with a Wisconsin Department of Natural Resources biologist and forester on a deer habitat project, O'Brien was able to connect with a DNR forest health specialist to learn what was happening.

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A few of O'Brien's oak trees were diagnosed to have oak wilt - the first confirmed case in Douglas County. The fungal infection is carried by sap-feeding beetles and can have devastating effects on the oak's vascular system, killing it within months of contracting the infection. The fungus can spread through the root systems of the trees to infect other oak trees in the vicinity.

Cases of oak wilt were also discovered in Barnes and Cable in Bayfield County for the first time this summer, according to Paul Cigan, a forest health specialist with the DNR in Hayward.

Oak wilt has been established in the southern two-thirds of Wisconsin, but had been detected as far north as Burnett, Washburn, Sawyer, Price and Vilas counties in the northern part of the state prior to this summer.

"We sometimes go years without a new detection," Cigan said. "These were back to back." He said a week after sampling the oak on O'Brien's property, it was discovered in Bayfield County.

O'Brien said he was clearing land and he believes that's when he damaged the trees' bark, which made them susceptible to oak wilt.

"They are very large oaks," O'Brien said. "I decided not to do anything with them just because they are so doggone big. I would need a pretty good size piece of equipment to get them out of there. And because of the acorns they produce for the deer, I thought I would just leave them well enough alone.

"I think that was actually their demise."

A few of the oaks, about a quarter-mile off State Highway 35, 2 miles north of the county's southern border, became infected after the bark was damaged during warm-weather months, which made the trees susceptible to fungal spores carried by beetles.

The fungus grows through the tree's water-conducting system, causing it to wilt and die. Leaves of the infected trees wilt and drop to the ground in the summer, turning brown to bronze on the outer edge while the mid-vein of the leaf remains green.

Spore-bearing fungal mats develop under the bark after the tree dies. The spores can also travel underground from root to root from an infected tree to a healthy one.

"That's a very tell-tale sign of oak wilt," Cigan said. "What I ended up doing ... was collecting a sample of the stem wood with my hatchet, and I sent it down to the Wisconsin DNR forest pathology laboratory. They did a DNA test to confirm the tree had oak wilt. Because this was a new county detection, they took the sequence of DNA and sent it to the University of Wisconsin plant diagnostic laboratory to double check because this was a higher priority detection. So we have robust laboratory results confirming that it is oak wilt."

There is really no curative treatment for oak wilt, Cigan said.

"It's considered the most deadly disease of oaks that we have in the Midwest," he said. "Once it starts to get into the xylem, the water-conducting tissues, it's virtually impossible to stop it."

Cigan said oak wilt will continue spreading; "it's just a matter of time."

For O'Brien, slowing the spread meant killing healthy trees in an effort to keep the spores from moving to other healthy trees underground. Using a technique called girdling and applying a herbicide mixed with diesel fuel, and calculating which trees would need to be killed, O'Brien worked with the DNR in an effort to stop the spread underground.

Cigan said the oak on O'Brien's property doesn't root out as far as some other types of oak and doesn't spread as quickly.

Cigan said the gold standard for disrupting the root system spread is a trencher, but the cost of the equipment and getting it to the site was prohibitive.

Treatment with the herbicide and girdling was almost immediate, and the trees started to die the same day, O'Brien said.

"They were Johnny-on-the-spot," O'Brien said of the DNR's response to the oak wilt discovered on his property. He said within a few days of confirmation, the DNR was back to provide technical assistance to address the infection.

"Kevin was an ideal landowner to work with," Cigan said. "He was eager to get on this. He went and got the chemical. It was the right chemical. And he was gung ho about working with me to do it. This is a success story for sure."

Cigan cautioned the method isn't recommended for all landowners because it's experimental, and it could create hazardous trees. He recommends contacting the DNR for free assistance to evaluate what can be done if individual landowners suspect they have oak wilt.

For managed forests, whether public or private, Cigan said there are guidelines that must be followed in counties where oak wilt has been detected, including seasonal restrictions between April and July on harvesting.

Douglas County manages 280,000 acres of forest, but its oak stands are most dense in the southeastern corner of the county adjacent to the Bayfield County.

Douglas County Board Chairman Mark Liebaert, chairman of the forestry committee, said it is most likely Douglas County's oak would be affected by the find in Bayfield County. He said simply breaking a branch on the North Country Trail could leave Douglas County managed forests vulnerable.

However, the county's forest operations manager is already taking steps to help prevent the spread of the disease, which will be reflected in next month's timber sale, Liebaert said.

"There are some management things that we can do," Liebaert said.

O'Brien said he's going to take down the diseased trees and cover the wood in poly to kill the spore mats with heat. Once the spores die, the wood can be salvaged, either for firewood or by milling.

"Eventually, they will die without being exposed to new beetles," O'Brien said. "The good thing is these are absolutely incredible trees ... I can still utilize them. It will not go to waste."

Prevention key to controlling oak wilt

To prevent oak wilt:

• Avoid cutting, pruning or injuring oak trees between April 15 and July 15, when the disease and beetles are most active.

• If oaks are removed, pruned or damaged during that period, immediately cover the wounds with a water-based latex paint or a pruning sealer. Stumps can become infected.

• Keep firewood local and leave oak firewood where it is cut for one year or until the bark is naturally loose.

More information on oak wilt and firewood can be found at dnr.wi.gov, or contact the UW Plant Disease Diagnostic Clinic at 608-262-2863.