Oak Wilt - A New Menace


Oak Wilt is a is a disease affecting red and white oak trees. Symptoms vary by tree species but generally consist of leaf discoloration, wilt, defoliation, and death. The fungus spreads from diseased to healthy trees overland by insect or underground through the oak's root system.


Oak trees roots are interconnected with other oak trees. Diseased oaks can infect other oak trees nearby through the root system. As a result of this interconnection, The DNR labels oak wilt infestations in 6x6 mile segments.




Oak Wilt was first seen in Wisconsin in the 1940s. Within the last decade, it has spread to new counties in northern Wisconsin, counties northwest and northeast of the Twin Cities in Minnesota and also new locations in northeastern states. It is now moving north and is widespread in Vilas county.


Natural Lakes learned about Oak Wilt from our participation in Wisconsin Headwaters Invasive Partnership (WHIP). DNR Forestry is part of WHIP and Linda Williams, Wisconsin DNR Forest Health Specialist in Woodruff provided the following information on oak wilt:


Oak wilt is nearby, do you know what to look for?

Oak wilt, which kills oak trees, is becoming increasingly abundant in northern Wisconsin. This fungal disease kills oak trees by plugging the tree's water conducting system and causing the leaves to rapidly wilt and drop. After a tree is infected and killed, the fungus will move into the root system and spread to neighboring oak trees through interconnected roots. This type of spread can be difficult to manage, so preventing introduction of oak wilt is important.

Oak wilt is introduced to new areas by sap-feeding beetles that are attracted to fresh wounds on oak trees. The beetles are also transported on firewood. The closest oak wilt locations to Natural Lakes are 6 miles away, north of County K, around Boulder Lake. One was identified in 2017 and another in 2018. Oak wilt is present in a number of areas in Vilas County and it’s important to be aware and informed. The good news is that there are things you can do to prevent introduction of the disease.

To minimize the risk of introducing oak wilt:

  • Avoid pruning or injuring oak trees from April 15 through July 15 (in southern Wisconsin, this period begins April 1). This is when the disease and sap-feeding beetles are most likely to spread.

  • If oaks are removed, pruned or damaged during this time, immediately seal the wounds with a latex paint or pruning sealer. A beetle can find an injured tree in as little as 15 minutes. Take special note after storm damage, beetles can travel in windy storm conditions.

  • Keep firewood local or let the wood age in place for at least a year or until the bark is loose. Learn more about firewood movement at https://dnr.wi.gov/topic/invasives/firewood.html

Do you already have oak wilt? What to watch for:

  • Wilted leaves that drop from the top, this may start to happen as early as July.

  • Dull green or bronze leaves that look water-soaked.

  • Partially green leaves on the ground which have dropped from the tree.

  • Most leaves will drop from the tree over a period of 6 weeks, and then the tree is dead.

  • Surrounding oaks can wilt and die in the following years, creating an expanding pocket of mortality

Oak wilt will not eliminate oaks from a site forever, but it can be alarming to have oaks dying so rapidly. Additional information about oak wilt can be found at the DNR website: https://dnr.wi.gov/topic/ForestHealth/OakWilt.html


Sample of the wilt progression:


Left: July 19, tree is starting to show symptoms of oak wilt. Notice the leaves on one upper branch have already wilted and most have dropped from the tree.


Right: August 6, the tree wilted rapidly, dropping most of its leaves and was dead by the end of the season.


We asked Linda what should be done if homeowners must remove or trim an oak tree due to construction. She said mark trees before work begins to try to avoid damaging trees on the perimeter. For those trees that have to be cut, have paint or tree sealant on hand and immediately treat the stump. Untreated stumps can transmit the wilt to healthy trees nearby.


Linda Williams is currently tracking instances of oak wilt in Vilas county. If you suspect you have a tree with oak wilt, you can call or email Linda at Linda.Williams@wisconsin.gov or call 920-360-0665. Pictures as always helpful. If it is indeed oak wilt there are no programs available to eliminate it, but Linda can provide recommendations for how to eliminate it. Either doing the work yourself or hiring a tree care company to do the work.  It requires more than just cutting down trees (you have to address the root systems to stop oak wilt), but there are certainly some landowners who are comfortable doing the work and there are several choices for control that Linda can discuss with you.  Additionally, there are a number of tree care companies that Linda knows that are well versed in treating oak wilt pockets, so there are options.

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