Oak Wilt

Oak wilt is widespread in southern Wisconsin but in much of northern Wisconsin it is still a new and uncommon diseas

Oak wilt kills thousands of trees each year in forests, woodlots and urban areas, or  can even attack and kill healthy trees. The disease is a particularly serious problem for species in the red oak group such as northern red, northern pin and black oaks. Once wilting symptoms are apparent on a red oak, the infected tree will lose most of its leaves and die within approximately one month. Among the white oak group, bur and swamp white oaks demonstrate moderate tolerance to the disease; living several years after symptoms first appear. White oaks experience even slower disease progression and may survive infection.

It is caused by the fungus, Bretziella fagacearum. The fungus grows through the infected tree’s water conducting system, causing the tree to wilt and die. Oak wilt is introduced to an area by sap-feeding beetles that carry oak wilt spores to fresh wounds. Spore-bearing fungal mats develop under the bark in the fall or spring following the death of the infected treeThe sap-feeding beetles are attracted to the fungal mats and can transport oak wilt spores to fresh wounds or to recently cut oak stumps. Once in an area, the disease spreads to nearby oak trees through interconnected (grafted) root systems, creating an expanding pocket of dead oak trees.

Although management to control the below-ground spread of oak wilt can be complicated and costly, active management can be successful. Controlling below-ground spread requires disrupting the movement of the fungus through root grafts from infected trees to healthy trees. Physical severing of the root system using a vibratory plow or a trencher has proven to be effective if plow lines are placed correctly. Recently, field trials have shown some promising results to contain the below-ground spread of oak wilt by using herbicides on, or by uprooting the entire root mass of, healthy oaks that border the infected pocket.