August 10, 2010
MADISON - The first signs of oak wilt, a tree-killing fungal disease, are now appearing in trees in counties in the southern two-thirds of Wisconsin.
"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 forest pathologist with the Wisconsin Department of Natural Resources Division of Forestry. "These are not the brown, dry leaves you see in autumn. These are partially green to bronze-green and are not completely dry."
Oak wilt affects trees in both the red oak and white oak groups. Once a tree is infected with oak wilt, water and nutrients can't move upward from the root system, and that causes the tree's leaves to wilt and fall. Eventually, oak wilt will kill the tree.
Scanlon said 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.
"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. "A person should take immediate steps 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 clinic, and a small fee is charged for the service. The clinic can be reached at (608) 262-2863 or via the Internet at [www.plantpath.wisc.edu/pddc].
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 prior to removal of diseased trees. Contacting an urban forestry consultant to determine best time of year and placement of the root graft barriers is a good idea, as placement will vary depending on tree size and the distance between infected and healthy trees along with the soil type.
"There are also fungicide treatments available, but they are most effective as a preventative, and repeated applications are necessary for success," Scanlon said.
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, avoid pruning till 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 during this summer and early fall time period, it is imperative to seal the wounds with some type of water-based (latex) paint. (It does not have to be commercially purchased 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," 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 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 is the best defense against the insect. Watering, mulching, fertilizing properly, and avoiding physical damage to trees should be practiced.
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 October 1. If a community is interested in applying for a grant, contact the Wisconsin DNR Urban Forestry Coordinator in area particular region.
FOR MORE INFORMATION, CONTACT: Kyoko Scanlon (608) 275-3275 or Don Kissinger (715) 359-5793.