Oak wilt is a vascular disease of oak caused by a nonnative fungus. It is one of the most serious diseases of oak in the eastern United States, killing thousands of trees each year. Where site conditions favor disease, the fungus can spread from tree to tree, killing most if not all oaks in the stand. As the disease spreads across the landscape, the loss of oaks can impact native plant and animal communities, property values, and forest industries.
The disease is well established in the southern half of Minnesota and is spreading into northern Minnesota. The disease is of greatest concern in stands dominated by oak growing on sands in areas exposed to frequent wounding. In these areas, infected wounds start new infection pockets. In sandy soil, abundant root grafts promote rapid below-ground spread so infection pockets expand rapidly and coalesce. Where root grafting is limited (e.g., on heavier soils, on steep slopes, or in diverse stands), oak wilt tends to be less damaging. In Minnesota, disease occurrence is highest in the 10-county metropolitan area and parts of Rochester.
Although disease survey records suggest it may have been present since 1912, oak wilt was unknown until it was first described in Wisconsin in 1944. During the early years of its existence, oak wilt was known primarily in the Midwest. It is now common in the central Appalachians and in Texas. Oak wilt has not been found west of the Rocky Mountains or outside the United States. Except for Texas and a few locations in the Carolinas, it is largely absent from the Gulf and Atlantic coastal states. Where it came from or how it got here is unknown. But its narrow range relative to its host species, the lack of host resistance, and the lack of genetic diversity among fungal isolates points to a nonnative source.