James A. Chatfield, Ohio State University Extension, Northeast District/Horticulture and Crop Science; Nancy A. Taylor, Plant Pathology, C. Wayne Ellett Plant and Pest Diagnostic Clinic, Ohio State University; Erik A. Draper, Ohio State University Extension, Geauga County; and Joseph F. Boggs, Ohio State University Extension, Hamilton County/Southwest District.
Oak wilt, caused by the fungus Ceratocystis fagacearum, is a lethal disease of oaks. The disease interferes with water uptake and causes a wilting syndrome which often results in death of the tree. The fungal pathogen can spread from infected trees to healthy trees by two routes.
One route occurs between neighboring trees by way of root grafts. These root grafts develop when roots from two different trees grow together and bond to one another. The tissues of the roots at these junctions become a bridge through which the oak wilt fungus can pass from one tree to the next tree.
The other route for oak wilt spread occurs above ground and involves an insect vector. There is a high risk of overland spread of oak wilt disease to healthy oak trees when the trees are wounded during the warm season months. This wounding can occur by pruning or storm damage and subsequent repair.
The greatest risk period for oak wilt spread is during the months of April, May, and June. It's during this time that picnic or sap beetles, the insects considered to be the primary vector of oak wilt disease, are most likely to be carrying spores of the fungus. These beetles are attracted to sap flows from freshly wounded trees.
Thus, it is important NOT to cause any unnecessary wounds to oaks during the high-risk period of April 15 through July 1.
A more conservative approach to preventing overland spread of oak wilt is to avoid wounding oak trees throughout the growing season (April 15 to October 1). The exception to this recommendation is the need to repair oaks after storm damage occurs. In this case, it is recommended that wounds produced during storm damage repair be treated with a latex paint to reduce the attractiveness of the wound to picnic beetles.
For more information on oak wilt disease and descriptions and pictures of symptoms, refer to OSU Extension Fact Sheet No. HYG-3306-01, Oak Wilt, for details.