Protect Your Trees From Oak Wilt

As April brings a high risk for the often-fatal oak wilt disease across the state, the DNR recommends not pruning or cutting oak trees from April through July. What you can’t see can kill a tree.

Oak wilt is widespread in southern Wisconsin, but it is still a new and uncommon disease in much of northern Wisconsin. As spring draws people outdoors to start seasonal yard maintenance and cleanup projects, when it comes to the health of oaks trees, keeping those chainsaws and trimming tools a safe distance will go a long way to ensure that your trees stay healthy for many more spring seasons to come.

In spring and early summer, pruning and cutting oaks leaves them vulnerable to oak wilt, which rapidly kills trees in the red oak group and weakens those in the white oak group. Any damage during this time, including broken branches caused by storms, exposes living tree tissue beneath the bark and provides an opportunity for the oak wilt fungus to attack the tree.

The DNR does not recommend using tree paint or wound dressing on pruned or wounded surfaces. However, for damaged oaks, a light application of these products immediately, if possible within 15 minutes, could be the only defense against oak wilt infection from April through July. Wounds are not susceptible to oak wilt after 72 hours.

Although overland infection can occur in mid-July, it is not common. To be very cautious, avoid wounding oaks from April through Oct. 1.

If something wounds an oak during this period, immediately and thoroughly apply pruning sealer or tree paint over the wound. Torn branches or roots should be cut clean and the cut surface painted. For additional protection, cover treated roots with soil.

Oak wilt spreads overland by sap-feeding beetles that carry the fungal spores from infected oaks to fresh wounds on healthy oaks. It travels underground from infected oaks to nearby healthy oaks through grafted, or interconnected, root systems.

“Sap-feeding beetles, like most insects, have an incredible sense of smell, which draws them to open wound surfaces to feed on sugary sap in as little as 15 minutes after a tree is wounded,” says Paul Cigan, DNR Forest Health Specialist. “This is how most new oak wilt infections start.”

Red, black and pin oaks are highly susceptible to oak wilt. Once infected, they can die within a few weeks. White and bur oaks are much less vulnerable. If infected, they can take months or years to die, or they may even recover.

Avoiding tree pruning in spring protects trees from disease and helps trees regenerate. Deciduous (shedding) trees that lose their leaves in the fall are just starting to grow new buds and leaves, so the trees’ food reserves are low. The best time to prune is in winter, when trees are dormant.

As of March 2022, oak wilt is in most Wisconsin counties except Ashland, Iron, Taylor, Door, Kewaunee, Calumet and Manitowoc. Several of these counties contain the state’s highest abundance of healthy and productive oak forests. Taking recommended precautions will help keep them that way for years to come. Check with your municipality to find out if they have any oak wilt ordinances that you should also follow.

Oak wilt and other diseases move easily on or in firewood logs year-round, so keeping firewood local or purchasing Wisconsin-certified firewood is another critical component of protecting trees and maintaining healthy forests.

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