Oak wilt is caused by a fungus ( Ceratocystis fagacearum) that invades and blocks water-conducting vessels. As water movement within the tree is slowed, the leaves wilt and drop off the tree.
Which trees are susceptible? Oaks in the red oak group (northern red, northern pin and others with pointed leaf edges) are most susceptible. Oaks in the white oak group (white, swamp white, burr and others with rounded leaf edges) are less susceptible.
What are the symptoms? Red oak group These trees drop their leaves rapidly (usually within a 3-week period) most often in late June and throughout July and August. Some lose a portion of their leaves in September, then rapidly lose all their leaves just after they emerge in the spring.
White oak group These trees drop their leaves on several branches several years in a row. Trees in the white oak group do not always die; they may survive an infection.
How does this disease spread?
Underground Most oak wilt moves from diseased trees to healthy trees through roots that have become interconnected (root grafts). Most root grafts form between oaks of the same species; red oak roots graft more commonly than do white oak roots, and grafts between red and white oaks are very rare.
Overland Some movement of oak wilt is over land via sap-feeding beetles. In the spring, fungal mats develop under the bark of some trees that have died from oak wilt the year before. These mats force the bark to crack open. The fungus produces a sweet odor that attracts sap-feeding beetles on the mats. The beetles then fly to healthier oaks to feed on sap flowing from fresh wounds, thus infecting healthy trees.