The oak wilt fungus C. fagacearum can spread two ways, one above ground and one below ground.
Most trees can become infected by the fungus below ground. The fungus moves from an infected oak to a healthy oak via root grafts underground. The disease can spread between root systems of trees growing as far as 50 feet apart. Nearby pairs of trees may graft together in dozens of places. Most grafts occur within the same root system or between trees of the same species. But root grafts may occasionally occur between different species of oaks.
Beetles can spread oak wilt above ground. This is only way the disease is known to cross highways, rivers, and other physical barriers. Above-ground spread occurs infrequently, but is the means by which new oak wilt infection centers can get started. The oak wilt fungus is most commonly spread above ground April 1 through July 15. The fungus creates sporulating mats between the bark and the wood when it invades oaks (see photograph of mats on red oaks that wilted the previous summer). Sap-feeding beetles, primarily of the family Nitidulidae, are attracted to the sweet-smelling mats. The beetles feed on the mats and carry oak wilt spores to wounds on uninfected trees, continuing the spread of the disease.
Unseasoned oak firewood can spread oak wilt. To prevent spreading oak wilt, avoid moving infected firewood off-site until it has been well aged. Fresh firewood must be covered and sealed to the ground with 4-mil plastic sheeting during the growing season of the year following the year in which the tree dies in order to prevent beetles from reaching the wood and picking up fungal spores. Otherwise, all infected wood should be chipped, burned, debarked, or buried prior to April 1 of the year following the year the tree dies. Because they dry quickly, wood chips from infected trees may be used in the landscape.