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Prevent Oak Wilt

M. Grabowski,  UMN Extension Educator

Large oak tree killed by oak wilt
S. Katovich, USDA Forest Service
It is no longer safe to prune oak trees in Minnesota. Any pruning cuts or wounds made at this time have some risk of becoming infected with oak wilt. This risk will only increase as the weather becomes warmer. Gardeners should avoid pruning or wounding oak trees at this time. If pruning is absolutely necessary, the pruning cut should be covered with shellac or a water-based paint immediately after making the pruning cut.

Oak wilt is a fungal disease that kills thousands of oak trees each year. Red oaks can wilt and die within a few weeks of infection, whereas bur and white oaks may survive several years before succumbing to the disease.

The fungus that causes oak wilt produces sweet smelling mats of fungal spores under the bark of recently killed red oak trees. Sap feeding beetles are attracted to the sweet smell and become covered in spores when they visit the fungal mats. These beetles are also attracted to fresh pruning cuts or wounds on oak trees and carry the oak wilt spores to healthy trees that have recently been pruned or wounded.

The risk of oak wilt infection through wounds or pruning cuts is divided into three categories, safe, low risk, and high risk, based on the presence of oak wilt spore mats and the activity of sap feeding beetles. Minnesota is currently in 'Low Risk' status for oak wilt.
Spore producing mat of the oak wilt fungus.
M. Grabowski,  UMN Extension

The only safe time to prune oak trees is during the safe period. This occurs when the sap feeding beetles and the fungal pathogen are not active. In Minnesota, the safe period for pruning oaks typically occurs from November through March. Early warming this spring, however, has resulted in an early transition to the low risk period. During the low risk period, the risk of infection is considered minor but because the disease is fatal and no treatment is possible for infected red oak trees, the recommendation during the low risk period is to not prune oaks if at all possible. If pruning cuts are necessary, the surface of the cut should be covered with water-based paint or shellac immediately after the pruning cut is made.

To learn more about how to recognize the symptoms of oak wilt, submit a sample for diagnosis, prevent and manage infection read 'Oak Wilt in Minnesota'.

UPDATE: Minnesota's Oak wilt risk status has now been upgraded to HIGH RISK.

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