Forest health experts say unseasonably warm weather across Wisconsin is raising concerns that oak wilt, a serious and almost always fatal fungal disease of red oaks, will likely appear sooner than normal and encourage landowners to stop pruning oaks from now through the end of July.

“Typically, the high risk period for oak wilt transmission is April through July. However, due to unusually warm spring that we have had this year, the risk of oak wilt is likely to already exist in some areas if daytime temperatures of 60 degrees Fahrenheit and above have lasted more than 7 consecutive days,” according to Kyoko Scanlon, Department of Natural Resources forest pathologist.

Homeowners should take special care to avoid wounding oaks from now through July. In fact, any action that might provide an opening into the tree, such as carving initials into the tree or attaching a birdfeeder or clothes line, could provide an opportunity for the oak wilt fungus to invade and establish itself in the tree. If an oak tree needs to be pruned from now through July, a homeowner should consider using wound dressing or paint on the cut surface as soon as the wound is created.

“Very small sap beetles transport fungal spores by landing on fungal mats found beneath the cracked bark of trees that died the previous year,” explained DNR Forest Health Specialist, Brian Schwingle. “The spores are then transmitted from a beetle’s body onto the fresh wound of a healthy oak tree while the beetle is feeding at the pruned or damaged site”

The common beetles that transmit oak wilt disease are not capable of boring into a tree. Oak wilt also can spread from a diseased tree to a healthy tree through a connected root system.

This ability to spread through root grafts means that even if only a single oak is wounded and subsequently infected with oak wilt, a new oak wilt “pocket” may develop in a location where oak wilt did not previously exist and will radiate to other oaks through the connected root systems. If no management steps are taken, the pocket could continue to expand year after year. Once oak wilt establishes itself in an area, control of the disease is both difficult and costly. The prevention of oak wilt is the best approach.

Builders and developers should also be very careful as many oak wilt infections and deaths have occurred through inadvertent damage to roots, trunks, or branches during the construction process.

Oak wilt is found in all Wisconsin counties except Ashland, Bayfield, Calumet, Door, Douglas, Forest, Iron, Kewaunee, Lincoln, Manitowoc, Price, Rusk, Sawyer, Sheboygan, Taylor, Vilas, and Washburn Counties. The most recent oak wilt infestation was confirmed in a small area of Oneida County in 2010.

Anyone interested in learning more about oak wilt and other forest pests as well as tree pruning should visit the Wisconsin DNR Forest Health Web pages for more information. Additional information about proper pruning techniques is available from your community forester, a University of Wisconsin-Extension agent or DNR urban forestry coordinators.