If you planned on pruning oak trees this year but haven't gotten around to it yet, remember, once April arrives the risk of attracting insects that can spread the oak wilt fungus increases.
Oak wilt is a serious disease that can infect many oak species. It is caused by the fungus, Ceratocystis fagacearum. Red oaks are very susceptible to oak wilt and can die within four to six weeks of when symptoms are first observed. White and bur oaks are moderately resistant to the disease.
When a tree is infected it tries to protect itself by producing gummy material called tyloses that can clog the water conducting vessels. Tyloses prevent water from moving to the canopy and leaves begin to wilt. Leaves of infected oaks may also turn brown at the edges and fall off. The outermost ring of sapwood usually turns brown and appears streaked when the bark is peeled or as a ring when a branch is cut in cross-section. Because oak wilt often is confused with other disorders, such as anthracnose and drought stress, positive identification requires isolation of the causal fungus from the tree.
Under certain conditions compact masses of spore-producing fungal material may form on oak trees that were killed by oak wilt. These fungal masses form under the bark and, as they mature, exert pressure on the bark causing it to split open. The oak wilt fungus emits a strong fruity odor that attracts beetles. Fungal spores can attach to the beetle bodies and be carried to fresh wounds on healthy oak trees.
Healthy trees also may become infected through root grafts connecting them with diseased trees up to 50 feet away. Severing root grafts can prevent fungal spread.
Fungicide injections are available to protect healthy trees from the disease. Infected trees can be treated, but a tree with more than 20% crown loss has little chance for survival.
This article originally appeared in the March 8, 2002 issue, p. 21.