Oak wilt, caused by the fungus Bretziella fagacearum, has become a serious disease threat to oaks in the eastern and central United States. While the spread of oak wilt has not been rapid, it can be found in all counties of Illinois and 18 other states. The estimated range of oak wilt runs from southern Michigan through central Pennsylvania along the Appalachian Mountains south to Georgia, westward to the Great Plains and including much of Texas, then northward into Minnesota.
All oaks are susceptible to oak wilt. However, the red oak subgenus (red, black, Hill’s, pin, and scarlet) is more susceptible to oak wilt than the white oak subgenus (white, bur, English, swamp white, and chinkapin). Trees in the red oak group usually die quite rapidly, often within weeks or months after infection. Trees in the white oak group typically develop symptoms more slowly, showing branch dieback for years before dying. In some cases, white and bur oaks recover after one year of infection.