BRAINERD — Despite a cool and snowy spring, oak trees in Minnesota will soon be at risk of oak wilt infection.
Oak wilt is an invasive fungal disease that kills all of Minnesota’s oak species and according to the Minnesota Department of Natural Resources, not pruning or cutting oaks from April through July is the easiest way to prevent the spread.
Spring temperatures can be unpredictable so the exact date varies, the DNR reported, but April is the average start of the period when there is a risk of oak wilt infection on fresh cuts or wounds.
Rachael Dube, a DNR forest health specialist, said oak wilt is spread to new areas by sap-feeding beetles carrying oak wilt spores. These beetles are attracted to fresh wounds on oak trees when they are trimmed or damaged in spring and early summer.
Once beetles introduce the fungus, it may spread to neighboring oaks underground through connected roots. In recent years, the disease has expanded into northern forests in Crow Wing, Cass and Pine counties.
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“Controlling oak wilt can be very expensive and often requires removing many dying and healthy oak trees to save others on a property,” Dube said. “The good news is that by following pruning guidelines, people can reduce spread of oak wilt in their yards, woods and communities.”
Dube encouraged residents to prune and cut oaks from November through February when there is no risk of oak wilt transmission.
“As a general rule of thumb, we often say that you need several consecutive days above 60 degrees,” Dube said. “We probably haven't hit that quite yet. But depending on what the temperature does in the next few weeks, we could be at risk soon.”
If residents must prune or cut oaks before July, they can greatly reduce infection risk by immediately applying pruning paint to the fresh cut or stump.
“Our general rule of thumb is to make sure to avoid wounding your oaks from April 1 to July 15 of each year,” Dube said. “If it so happens that you accidentally wound one of your trees or you have a hazardous branch in your yard and you must prune it or take down a branch, within 15 minutes, apply a water-based paint pruning paint or shellac to that wound. That both prevents the infection from getting in and reduces the chances of those beetles being attracted to your tree.”
In addition to following pruning guidelines, Dube cautions campers, cabin owners, visitors and hunters not to move infected firewood. Moving oak firewood can spread oak wilt over long distances. Use locally sourced firewood or firewood certified by the Minnesota Department of Agriculture to prevent transporting oak wilt and other invasive species.
“It's a native opportunistic insect that attacks stressed trees,” Dube said. “The symptoms of twolined chestnut borer attack look a lot like oak wilt.”
Twolined chestnut borer attacks weakened oaks of every species in Minnesota that lives in and feeds on the inner bark and cambium, a layer of cells between the bark and the wood.
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According to the DNR website, symptoms of a twolined chestnut borer infestation are first seen around mid-July. The first branches to be attacked are usually at the top of the crown or fully exposed to sunlight. Leaves on infested branches die, turn orange, and remain on the branches for weeks or months. The persistence of dead leaves is one characteristic that distinguishes Twolined chestnut borer attack from oak wilt infection.
Dube recommends checking on oak wilt fungus spread before doing any pruning or cutting. For more details on oak wilt prevention and how best to deal with infected trees and wood, visit the DNR’s oak wilt management page .
Daily updated oak wilt risk for the state can be found on the University of Minnesota Extension’s oak wilt in Minnesota page.
TIM SPEIER, staff writer, can be reached on Twitter @timmy2thyme , call 218-855-5859 or email tim.speier@brainerddispatch.com .
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