By Richard Rees, Ph.D.
What does it look like?
Oak wilt is caused by a fungus, Ceratocystis fagacearum, which plugs up the water-conducting vessels in oak trees and prevents the leaves from receiving fluids necessary for survival. The dying leaves are characterized by discoloration and wilting. Starting at the crown of the tree and spreading downward, they transition from an off-green color to bronze and eventually to brown. Finally, the entire tree defoliates. Also, brown streaks are often recognizable along the outer growth of the tree, but are not a solid indication of oak wilt.
Host material and range
The disease has only been located in the United States and is mostly found in the upper Midwest, extending as far east as Pennsylvania, and Texas. Most species are prone to it, including red, white and Texas oak. The disease usually occurs in the late spring and early summer, especially April through July. The infestation process varies by species. Red oaks are completely defoliated within two to six weeks, while white oaks can take up to a few years to die, often weakening one branch at a time.
Killing thousands of oak trees each year in home lawns, forests and woodlots, oak wilt is a serious threat. The infestation spreads easily, in one of two ways. Above ground, it can be spread “overland” by sap-beetles or transporting firewood. Additionally, the disease can spread underground through “root grafts.” The root systems of neighboring oak trees intermingle and connect to form the grafts over time, allowing the fungus to reach healthy trees up to 50 feet away from an infected tree. A single tree with oak wilt can create a largely infested area without the proper treatment.
Infection is also intensified by the forming of spore mats in the bark of the trunk. The mats put pressure on the bark, which pushes it outward until it splits, allowing for more exposure to infestation from insects.
There is no absolute cure for oak wilt, so the best options are to prevent the spread of the fungus from infected to healthy trees. Remove infected trees before spore mats form, and avoid injury to healthy oaks so they are not as susceptible to the disease.
Properly remove dead trees by debarking, burning, burying or wrapping the tree in plastic during the summer months when the surrounding trees are most prone to infection. To physically stop the spread through the root grafts, create a barrier surrounding the tree, using a vibrating plow, which will cut off connections at five feet below the surface.
What can you do?
Help isolate pockets of oak wilt by properly removing infected trees and taking the best care of surrounding healthy trees. Avoid pruning or construction in oak-populated areas during the summer months when trees are most likely to get infected. Any injury to the healthy tree creates more opportunity for oak wilt.
Richard Rees, Ph.D., is product development manager — fungicides, Bayer Environmental Science.