Time to pause pruning as oak wilt high-risk period approaches

It is again that time of year to pause operating in oak stands or pruning oak trees during the high-risk period for oak wilt, which begins in April.

The high-risk period begins April 1 in the south, April 15 in the north, and ends in both zones on July 15.

The high-risk period is determined by when the beetles that carry the fungus that causes oak wilt have emerged for the year and when they are carrying the highest number of fungal spores. When wounds are created by logging operations or other activities that expose sapwood, the beetles carry the fungus to the wounded tree, and the disease process begins.

Oak trees in the red oak group are the most susceptible to oak wilt, often dying in as little as two to four weeks after being infected. Oak in the white oak group are susceptible to a much lesser extent, but mortality can still occur, especially in bur oak trees.

Once the disease is introduced into a tree, it is transmitted to neighboring trees through interconnected root systems. Oak in the red oak group are known to commonly create functional root grafts with other red oaks, allowing the disease to rapidly expand within a stand.

Symptoms of infection start at the very top of the tree and work their way down. Leaves will begin to discolor around the margins, ranging from dingy dark green to dried, crunchy brown.

From a distance, the top of the tree may look red. The leaves will begin to fall, starting around the end of June and into July.

The best way to manage oak wilt is to prevent its introduction. Operating outside of the high-risk period is an easy way to prevent the disease from entering uninfected stands. If wounds are made to oaks unintentionally during the high-risk period, the application of a wound paint as quickly as possible will help prevent beetles from dropping the fungus off. Wounds are susceptible for 72 hours.

Once oak wilt has been identified on a property, work with your local forester or DNR Forest Health Specialist to create a management plan.

Editor’s note: Vilas County Land and Water’s 10-year plan (2025-2034) addresses oak wilt in the county. According to the plan, “Red and pin oak species are dominant species in the Northern Dry Forest cover type. Oak wilt is significantly changing community composition on public forest lands, private industrial lands, and private non-industrial lands.

“Oak are significant keystone species that provide significant pollinator habitat, bird and wildlife habitat and food resources. Unfortunately, the red oak group are more susceptible to oak wilt than the white oak group. Other communities are looking to replace red oaks with less susceptible species like burr oak to try and maintain some oak species in the forest communities.

“Education is needed to make sure the public and town/county road crews limit cutting/mowing/trimming of oak species to the time of year less likely to encourage the spread of oak wilt (August through March).

The plan also notes that other communities are investigating whether Swamp white oak or river birch could survive in Vilas County.