Oak wilt continued to spread through Bayfield, Sawyer and Washburn counties in northern Wisconsin in 2019 with new detections in numerous towns.

But there was hopeful news for Douglas County.

“New detections were not made in Douglas County in 2019, but I suspect the disease is advancing slowly under the radar,” said Paul Cigan, state Department of Natural Resources forest health specialist.

Oak wilt was first detected in Barnes in Bayfield County and Dairyland in Douglas County in 2018. The fungal infection is carried by sap-feeding beetles and can have devastating effects on the oak’s vascular system, killing it within months. The fungus can spread through the root systems of the trees to infect other oak trees in the vicinity.

That’s what makes winter the ideal time to prune trees, Cigan said.

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“The best time to prune trees that lose their leaves is during winter when the trees are dormant,” he said. “Not only is it easier to see where pruning is needed when leaves are gone, but disease-carrying pests are inactive due to the cold, making pruning both more effective and less likely to invite unwanted pests.”

Although pruning in winter reduces the risk of the disease spreading through beetles, the disease can spread year-round in firewood.

“Several recent oak wilt discoveries in northern Wisconsin, including the first-ever discovery in Forest County, may have been the result of infected firewood brought from areas with oak wilt,” Cigan said. “Keep oak firewood where it is cut for one year, or until the bark is naturally loose, to prevent the spread of oak wilt.”

Yard trees and trees in urban settings should be pruned throughout their entire life to maintain a strong structure and remove deadwood. Young trees should be pruned to establish a central trunk, proper trunk taper and good branch structure and spacing. Older trees should be pruned to remove dead or hazardous limbs.

“Pruning should not remove more than 25% of the live tree crown, and the lower third of deciduous tree trunks should be free of limbs,” said Don Kissinger, DNR urban forestry coordinator.

The DNR offers more information on oak wilt, firewood and tree pruning by searching for them at dnr.wi.gov. To find certified arborists that provide pruning services, visit www.waa-isa.org/find-certified-arborist.