An oak being injected with fungicide to prevent oak wilt. Photo by Joseph O'Brien, USDA Forest Service, Bugwood.org

Oak wilt, a non-native, invasive fungal disease that kills all species of oak in Minnesota, has been confirmed for the first time in Crow Wing County. To reduce further spread, the Minnesota Department of Natural Resources is reminding area residents and visitors to avoid pruning oak trees until August and not move firewood.

Now located in 40 Minnesota counties, oak wilt has slowly been spreading north. The University of Minnesota Plant Disease Clinic laboratory confirmed several infected oak trees on private land on the east side of Gull Lake, 20 miles north of the previous northernmost location of oak wilt in this region. This small oak wilt pocket will be treated this autumn, and the DNR will closely monitor the surrounding area for new oak wilt infections.

Oak trees within a 20-mile radius of any known oak wilt location are at high risk of developing the disease. With this new detection, oaks in much of Crow Wing County and southern Cass County are now threatened. Brainerd, Breezy Point, Crosby, Cross Lake, Ironton, Jenkins, Lake Shore, Nisswa, Pequot Lakes and Pine River are in the high-risk zone.

“The best thing anyone can do to prevent the spread of this deadly disease is to not prune oaks from April through July – the highest risk period for oak wilt transmission,” said Rachael Dube, Northwest Region forest health specialist. Pruning during this time, Dube said, can make trees vulnerable to sap beetles that carry oak wilt spores from infected trees to fresh cuts, promoting spread of the disease. She cautions residents to limit pruning to November through March when there is no risk of oak wilt transmission through wounds.

With this new find at the peak of summer recreation in the Brainerd Lakes Area, Dube also reminds campers, cabin owners, and visitors not to move firewood. Moving oak firewood can spread oak wilt over long distances. Use only locally sourced firewood or firewood with the Minnesota Department of Agriculture (MDA) certified seal to prevent moving oak wilt.

A good indicator of oak wilt is a carpet of fallen leaves under an oak in mid-summer, when leaves should still be on trees. Crow Wing or Cass County residents should be on the lookout for this sign and report possible oak wilt by contacting the Brainerd DNR Forestry Office at 218-203-4300. Because oak wilt and other oak health issues have similar symptoms, sharing pictures of the tree really helps with diagnoses.

For more details on oak wilt prevention and how best to deal with infected trees and wood, see the DNR’s oak wilt management webpage.