Madison, WI (August 10, 2007)- The first signs of oak wilt, a tree-killing fungal disease, are now appearing in infected trees. Oak wilt occurs in the southern two-thirds of Wisconsin and affects trees in both the red oak and white oak groups. Oak wilt is a fungal disease that plugs the water and nutrient-conducting channels in the tree. Once a tree is infected, water and nutrients can’t move upward from the root system, causing the tree’s leaves to wilt and fall and, eventually, killing the tree.
Oaks in the red oak group, including northern red, northern pin, and black oaks are particularly vulnerable to this disease. Once symptoms become visible, a tree loses most of its leaves and dies very quickly, often within a few weeks. “Anyone with an oak tree that is rapidly losing its leaves may want to have the tree examined for oak wilt either by an arborist or forester, or send in a sample for a laboratory test,” said Don Kissinger, a DNR urban forester. “A person should take immediate steps to protect other nearby oaks on the property if they value those trees.”
Most often, oak wilt spreads from one oak to another through root grafts between neighboring trees. Removing a diseased or dead tree may not be enough to stop oak wilt from spreading. Forest health experts recommend using a vibratory plow or trencher to sever existing root grafts to help control the spread of oak wilt. “There are also fungicide treatments available, but they are most effective as a preventative and repeated applications are necessary for success,” Scanlon said.
Wisconsin communities may be eligible to participate in a cost-sharing program to help combat oak wilt. The Urban Forestry Grant Program is not available on an individual basis, and property owners with oak wilt are encouraged to contact their town or city forester or other local official to pursue a grant. Applications for the program are due by Oct. 1. Communities interested in applying for a grant should contact their regional DNR urban forestry coordinator, who are listed on the DNR Web site.
Some instances of oak wilt are caused by insects that carry the oak wilt spores to healthy trees. To prevent oak trees from being infected with oak wilt transported by insects it is very important not to prune or wound oak trees from April through July, and to take a cautious approach to pruning from late March to October. Pruning or injuring the tree causes it to release sap, which attracts the fungus-transporting insects.
Additional information about oak wilt and other forest health issues can be found on the DNR Web site.
It’s Not Always Oak Wilt
Symptoms similar to oak wilt may also be caused by an infestation of the two-lined chestnut borer. “The two-lined chestnut borer is an opportunist,” Scanlon said. “It will attack weakened trees, favoring red and white oaks more or less equally.” The borer frequently shows up in areas where a forest tent caterpillar or gypsy moth outbreak or drought has weakened trees. The adult insect lays eggs under the bark. When the larvae emerge, they eat their way through the fluid-conducting tissues of the tree, stopping the flow of nutrients to the leaves. “The leaves turn uniformly brown, but remain on the tree,” Scanlon said. “Unfortunately, an infestation of two-lined chestnut borer and oak wilt can occur at the same time, on the same tree.”
Maintaining vigorous healthy trees is the best defense against the insect. Watering, mulching, fertilizing properly, and avoiding physical damage to trees should be practiced. The University of Wisconsin’s Plant Disease Diagnostic Clinic can help verify the presence of oak wilt. A sample must be sent to clinic and there is a small fee for the service.
University of Wisconsin’s Plant Disease Diagnostic Clinic
Wisconsin Department of Natural Resources
Community Forestry Resource Center