Published: March 28, 2013 by the Central Office
Contact(s): Kyoko Scanlon (Fitchburg) 608-275-3275, Don Kissinger (Wausau) 715-359-5793 or Brian Schwingle (Merrill) 715-536-0889
MADISON - If you value your oak trees, do not prune them from April through July. Spring and early summer pruning makes oak trees vulnerable to oak wilt, a fatal fungal disease of red oaks. The risk of oak wilt moving from one tree to another is highest April through July.
In fact, homeowners should take special care and avoid wounding oaks in any way from April through July. Any action that provides an opening into the tree, such as carving initials into the tree, accidentally cutting exposed roots with a lawn mower, or attaching a birdfeeder or clothes line, could provide an opportunity for the oak wilt fungus to invade and establish itself in the tree.
Builders and developers also need to be very careful, as many oak wilt infections occur through inadvertent damage to roots, trunks, or branches during the construction process.
While using paint or a wound dressing is not normally recommended on pruning cuts or wounded surfaces on most trees, oaks are an exception from April through July. An immediate light painting of wounds on oak trees is recommended during this time to help protect against the spread of oak wilt by beetles. "Just 15 minutes could be enough time for the beetles that transmit the disease to land on a fresh wound and infect your tree," said Kyoko Scanlon, DNR's statewide forest pathologist.
While the risk of spreading oak wilt is low after July, homeowners should avoid pruning or wounding oaks until November, to be on the safe side. Check with your municipality to find out if they have their own oak wilt ordinances that you should follow as well.
Oak wilt is found in all Wisconsin counties except Ashland, Bayfield, Calumet, Door, Douglas, Forest, Iron, Kewaunee, Manitowoc, Price, Rusk, Sheboygan, Taylor and Washburn counties. The most recent oak wilt infestations were confirmed in Lincoln, Sawyer and Vilas counties in 2012.
"Pruning deciduous trees in general should be avoided in the spring, as this is the time when tree buds and leaves are growing and food reserves are low," according to Don Kissinger, a DNR urban forester.
More information about oak wilt and other forest pests is available on the Wisconsin DNR website. Visit dnr.wi.gov and search the words "forest health". Additional information about proper pruning techniques is available from your community forester, a University of Wisconsin-Extension agent (exit DNR), or DNR urban forestry coordinators.