It's that time of year again when we begin to see oak wilt affecting our trees. Symptoms usually begin in late spring or early summer and include wilting, necrosis, marginal browning of leaves, and clusters of dead leaves and branches. Oak wilt is caused by the fungus, Ceratocystis fagacearum and infects many oak species including red, pin, white, and bur. Red and pin oaks are more susceptible to oak wilt than white and bur oaks, and can die within 4-6 weeks. White oak species typically develop symptoms more slowly and can die between 1-20 years after infection. This disease is spread by beetle vectors or root grafts, which transmit fungal spores from infected trees to healthy ones. The oak wilt fungus invades the water conducting tissues of the tree. As a defense mechanism, the plant attempts to stop the pathogen by creating a blockage with tyloses. Tyloses are outgrowths from cells adjacent to the xylem vessels. Once in the xylem, fungal spores are reproduced and travel throughout the vessel system. Spores usually become embedded within the xylem vessels where they produce mycelium or long, filamentous cells, which can spread between xylem vessels. This growth chemically signals the plant that infection has occurred and the production of tyloses begins. Tyloses block the xylem thereby reducing and preventing water movement throughout the tree. The reduction of water movement causes foliage to wilt and eventually causes tree death. The discoloring from dead cells and the blockage produced in the xylem creates the appearance of dark rings when the branch is cut in a cross section. If the bark is peeled back streaks can be seen. Because oak wilt is often confused with other disorders, positive identification requires recovery of the causal fungus from the tree by culturing pieces of wood. The oak wilt pathogen has a unique disease cycle because it can be spread by both beetles and root grafts. Sap beetles travel from infected to healthy trees, depositing oak wilt spores through wounds in the tree. In addition, when trees are in close proximity to each other their roots graft together. In this manner, the fungus can spread from tree to tree through the water conducting vessels in the roots. Avoid wounding or pruning oaks during the growing season 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. For more information regarding oak wilt symptoms and control, contact the ISU Plant and Insect Diagnostic Clinic at 515-294-0581 or see ISU Extension publication SUL15, available online or at your local county ISU Extension office.
Clusters of necrotic and dead leaves on a red oak.
Marginal browning of red oak leaves.