Tri-County News

Oak wilt – Now high risk


By Katie Drewitz, U of M Extension

Oak wilt is one of the deadliest diseases for oak trees in Minnesota. As of the end of March we are currently at high risk for the spread of oak wilt. Oak trees should not be pruned right now, and construction near oak trees should also be avoided.

Oak wilt is a fungal disease (Ceratocystis fagacearum) that can be spread by insect. Sap beetles, Carpophilus sayi and Colopterus truncates, are the insects that spread the disease from an infected tree to an uninfected tree. They are attracted to trees that have been killed by oak wilt as well as to trees that have fresh, wood-penetrating wounds, such as limb pruning. These insects are active from early spring through summer. To avoid attracting them to your healthy oak tree, you should only prune oaks from November through early March. If pruning cannot be avoided outside of that timeframe, or if a branch breaks off, the wound should immediately be treated with a water-based paint, wound sealer, or shellac.

The other way that oak wilt spreads is through root grafting below ground. This happens when a tree is infected and the fungal disease spreads through the interconnected root systems of nearby oak trees. This root grafting spread has been seen to spread to trees that are up to 50 feet from the original infected tree. If you are concerned about below-ground spread of oak wilt, you may consider hiring a professional to assist with the root cutting.

If you are concerned that your tree has oak wilt, you can submit photos and information to your local Extension educator. You can also submit samples to the University of Minnesota Plant Disease Clinic for testing. Submission information can be found at pdc.umn.edu/submit-sample. There are fees associated with the lab testing.

Remember that the easiest way to prevent the spread of oak wilt is to avoid pruning or removing oak trees during the high-risk times. Oak wilt is currently found in most of the eastern half of Minnesota. 

The known range in Minnesota where oak wilt is threatening is shown in red. Map updated May 2019. Source: UMN Extension.

For more information about this any other horticulturally related topic, please reach out to your local Extension educator. Residents in Stearns, Benton, and Morrison Counties can call (320) 255-6169 ext. 1 or email wins0115@umn.edu.

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