Emerging Threat Note: While this pathogen has been reported in Tennessee in the past, it is not considered to be established across the state. However, we do monitor for this pathogen and encourage residents to report potential sightings.
Oak wilt is considered the most important disease of oaks in the eastern United States. Few other diseases encountered in forestry can kill large mature trees as quickly as oak wilt. The disease is caused by a fungus (Bretziella fagacearum, formerly Ceratocystis fagacearum) that spreads through a tree’s vascular system. This fungus is carried by beetles that typically colonize fresh wounds on trees, so storm damage or pruning wounds may attract beetles, and the fungus, to susceptible oaks. In response to the fungus, the infected trees plugs up their vascular system to stop the fungus from spreading, but this also inhibits water movement, so the tree wilts and dies. Trees from the red oak group (pin oak, black oak, scarlet oak, shumard oak, northern and southern red oaks) are more susceptible to disease than white oaks, but even white oaks can be infected.
Early symptoms are often confused with drought. Leaves in the upper crown usually show symptoms first. Leaves will begin to turn greenish-grey or olive-green, and will have a wilted, limp, or water-soaked appearance. The leaves will begin to brown at the leaf tips and edges, and then progress inward. If symptoms begin in a single branch or isolated part of the crown in red oaks, they will rapidly spread through the rest of the tree within a few months. Symptoms may be halted or only spread slowly in white oaks.
Red oaks will often shed their leaves rapidly once infection starts; leaves on the ground may only be partially wilted. In cases where the tree wilts very rapidly, leaves may be retained. White oaks tend to retain wilted leaves for several months. The fungus often spreads through root grafts to neighboring trees, creating a “disease center” with recently infected and wilting trees surrounding dead trees.
The outermost rings of sapwood will almost always be discolored in the wilting branches of white oaks; discoloration may or may not be present in red oaks. Vascular discoloration can be observed by looking at the branch in cross-section or stripping away the bark to reveal the outer sapwood. Discoloration will look like dark (grayish, purplish, or black) streaking in the outermost ring of sapwood.
Under the right environmental conditions, the oak wilt fungus will produce a black or grayish fungal mat that produces spores. The mat is produced beneath the bark; enlargement of the mat causes the bark to rupture and allows insects to enter. Mats are difficult to observe, usually produced in the spring, and have a fruity aroma like bananas or bubble gum. Mats can be just a few inches wide or almost a foot across.
Oak wilt has been detected in Tennessee in the past, although no recent detections have been made. Oak wilt remains an issue for many other states in the southern U.S. as well as parts of the Midwest.
Over-land transmission by insect vectors is managed by removal of diseased trees to eliminate production of spore mats. Firewood from diseased trees should be tightly covered in plastic for at least one year or immediately burned. Avoid pruning oaks during months when spore-vectoring beetles are most active (March- July). Any wounds will make the tree more susceptible to these beetles and the oak wilt fungus they vector, so proper timing and care for pruning wounds should be taken. Root graft transmission is managed by severing root grafts between trees with a vibratory plow or trencher. Systemic fungicides are available for high-value trees and can provide protection for one to two years.
What Can You Do?
Monitor oaks for symptoms of oak wilt and report suspected oak wilt cases to the Tennessee Division of Forestry. To best care for oaks, avoid pruning oaks during the spring and summer when beetles vectoring the fungus are active. Take proper management steps when necessary to remove and properly dispose of material and prevent the spread of the fungus to neighboring oaks.