MADISON, Wis. – The Wisconsin Department of Natural Resources today announced that oak wilt, a deadly disease that affects oak trees, has been found in Manitowoc County for the first time.
DNA analysis confirmed the presence of the fungus causing oak wilt (Bretziella fagacearum) in an oak tree in the town of Mishicot. The tree was planted in the mid-1980s and is believed to be a hybrid of bur oak and swamp white oak. The property owner contacted the DNR after noticing symptoms consistent with an oak wilt infection. As of Nov. 6, 2026, this tree is the only confirmed oak wilt infection in Manitowoc County. The nearest known oak wilt infections are about 20 miles away in Brown County.
“It’s possible that oak wilt has been present in the area at low levels for many years,” said Bill McNee, a DNR forest health specialist based in Oshkosh. “Alternatively, someone could have recently introduced the disease into the local area through infected oak firewood or logs.”
Overall oak wilt impacts in Manitowoc County will likely be limited, as oak species are estimated to be less than 2% of the county’s forest trees and about 1% of urban trees. Other tree species are not affected by the disease.
Oak wilt is commonly found in the southern two-thirds of the state and is increasingly being found in northern Wisconsin. The disease has now been identified in 67 of Wisconsin’s 72 counties, but three nearby counties – Calumet, Door and Kewaunee – have never had a detection of oak wilt.
The most obvious symptoms of oak wilt are seen when the disease strikes in mid- to late summer, leaving branches with wilted leaves and leaves on the ground at a time when you wouldn’t expect to see that.
“These are not the brown, dry leaves you see in autumn,” McNee said. “These are often partially green and partially bronze.”
The fungus that causes oak wilt is easily brought to new areas on firewood and other raw oak materials from infected oaks. Beetles that feed on the sap from tree wounds spread the fungus, and the disease, from tree to tree. To prevent this:
- Avoid pruning or injuring oak trees from April through July (the high-risk period).
- If oaks are cut down, pruned or damaged between April and July, immediately seal the cuts or wounds with a water-based (latex) paint or pruning sealant.
- Keep firewood local. Leave oak firewood and uncut firewood logs where they were cut for one year, or until the bark is naturally loose.
Preventing the introduction of oak wilt to a new area is much easier than containing an infection. After a tree is infected, oak wilt can spread short distances to nearby oaks through underground root connections and longer distances through contaminated beetles that are attracted to fresh oak wounds. Preventing underground spread can be difficult and expensive, with no guarantee of success.
Additional information and staff contacts can be found on the DNR’s Oak Wilt webpage.
