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 the oak wilt fungus and can die within 4-6 weeks. White and bur oaks are moderately resistant to the disease. Trees can be infected by the fungus through root grafts or by beetle vectors that carry spores to newly wounded trees. When a tree is infected it tree tries to protect itself by producing gummy material called tyloses which can clog the water conducting vessels. Water is prevented from moving to the canopy and leaves begin to wilt. Leaves of infected oaks can wilt, turn brown at the edges, and fall off. The outermost ring of sapwood sometimes turns brown and appears as streaks when the bark is peeled; or as a ring when the branch is cut in cross-section. Because oak wilt is often confused with other disorders, positive identification requires recovery of the causal fungus from the tree.
Avoid wounding or pruning oaks from April through July since sap attracts the beetles that carry the oak wilt fungus. Severing root grafts connecting infected and healthy trees up to 50 feet apart can be used to prevent spread. Fungicide injections are now available to protect healthy trees from the disease. Infected trees can also be treated, but a tree with more than 20% crown loss has little chance for survival.
See this bulletin about oak wilt.