Which fungus causes oak wilt disease?

Oak wilt disease is caused by the fungus Bretziella fagacearum.

This pathogen attacks oaks by entering their xylem, the water-conducting tissue beneath the bark. The fungus disrupts water transport, producing leaf discoloration, wilting, defoliation, and sometimes death. It is especially dangerous to red oaks, which can die rapidly, while many white oaks develop the disease more slowly and may survive with damage limited to individual branches.

Bretziella fagacearum was first described in the 1940s in the Upper Mississippi River Valley. The disease is concentrated in the eastern United States, central Texas, and southern Ontario. Older references may use names such as Ceratocystis fagacearum, Endoconidiophora fagacearum, or Chalara quercina, so these names can appear in forestry literature even though Bretziella fagacearum is the current accepted name.

The fungus can be carried by sap-feeding insects in the family Nitidulidae. It also spreads underground when the roots of neighboring oaks form grafts. Human activity is another major risk: moving infected firewood or transporting contaminated wood can introduce the pathogen to new areas. Oak wilt is therefore not the same disease as Dutch elm disease or chestnut blight, despite their similar devastating effects on trees.