Oak wilt, a deadly fungal disease affecting red oaks, was confirmed for the first time this summer in Lincoln, Sawyer, and Vilas counties.
“Confirmed trees were located north of Tomahawk in Lincoln County, south of Hayward in Sawyer County, and east of Eagle River in Vilas County,” said Brian Schwingle, DNR forest health specialist. All three locations were in yards and all three properties had oaks pruned or damaged in late spring.
Oak wilt is commonly found in the southern two-thirds of the state, but has been creeping north. Oak wilt has been confirmed in all Wisconsin counties except Ashland, Bayfield, Calumet, Door, Douglas, Forest, Iron, Kewaunee, Manitowoc, Price, Rusk, Sheboygan, Taylor, and Washburn
“The first symptoms of oak wilt are branches with wilted leaves and leaves on the ground in summer when you wouldn’t expect to see that,” said Kyoko Scanlon, a DNR forest pathologist. “These are not the brown, dry leaves of autumn. These are partially green to bronze-green and are not completely dry.”
Additional information about oak wilt and other forest health issues can be found online using the keyword “oak wilt” at dnr.wi.gov.
Oak wilt affects trees in both the red and white oak groups. Once a tree is infected with oak wilt, water and nutrients can’t move up from the root system, causing the tree’s leaves to wilt and fall. Eventually, oak wilt kills the tree.
“The red oak group, including northern red, northern pin, and black oaks, are particularly vulnerable to oak wilt. Once symptoms become visible, a tree loses most of its leaves (typically from the top downward) and dies very quickly, often within a few weeks,” said Scanlon.
“Anyone with an oak tree that is rapidly losing its leaves may want to have the tree examined for oak wilt by an International Society of Arboriculture-certified arborist or forester or send in a sample for a laboratory test,” said Don Kissinger, a DNR urban forester. “Immediate steps should be taken to protect nearby oaks if they value those trees.”
The University of Wisconsin’s Plant Disease Diagnostic Clinic can help verify the presence of oak wilt. A sample must be sent to the clinic, and a small fee is charged for the service. The clinic can be reached at (608) 262-2863, or at www.plantpath.wisc.edu/pddc (exit DNR).
Most often, oak wilt spreads from one oak to another through root grafts (connected roots 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 cut through existing root grafts prior to removal of diseased trees. Contacting an urban forestry consultant to determine the best time of year and placement of the root graft barriers is a good idea, as placement will vary depending on tree size, soil type and the distance between infected and healthy trees.
“Fungicide treatment to prevent oak wilt-caused death is an option in some cases, but repeated applications are necessary to keep an oak alive,” Schwingle said.
Oak wilt is sometimes 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 through October. Pruning or injuring the tree causes the tree to release sap, which attracts the fungus-transporting insects. If tree removal, pruning, or damage occurs to oak tree trunks or limbs between April and August, it is imperative to seal the wounds with some type of water-based (latex) paint. It does not have to be commercial tree wound paint.
Symptoms similar to oak wilt may be caused by an infestation of the two-lined chestnut borer.
“The two-lined chestnut borer is an opportunist. It will attack weakened trees, favoring red and white oaks more or less equally,” Scanlon said. The borer frequently shows up in areas where a forest tent caterpillar or gypsy moth outbreak or drought has weakened trees. The adult borer 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 often remain on the tree for a while,” 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 by watering, mulching, fertilizing and avoiding physical damage to trees is the best defense against the insect.
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 to individual property owners. But property owners with oak wilt are encouraged to contact their municipal forester or other local official to pursue a grant. Applications for the program are due by Oct. 1. If a community is interested in applying for a grant, contact the local Wisconsin DNR Urban Forestry coordinator.