MADISON - To protect oak trees from the often-fatal oak wilt disease, state urban and forest health specialists recommend not pruning or cutting oaks from April through July.
Pruning and cutting oaks in spring and early summer leaves them vulnerable to oak wilt, which rapidly kills trees in the red oak group and weakens those in the white oak group. Any damage during this time, including broken branches caused by storms, exposes living tree tissue beneath the bark and provides an opportunity for the oak wilt fungus to infect the tree.
Sap-feeding beetles introduce the disease by carrying oak wilt spores from infected trees or firewood to fresh wounds.
"Healthy oaks can become infected in as little as 15 minutes after a wound is created," says Paul Cigan, forest health specialist for the Department of Natural Resources in Hayward.
The trees most likely to die from oak wilt infection are in the red oak group, including northern pin oak, northern red oak, red oak and black oak. The white oak group is more likely to survive infection and includes bur oak, swamp white oak, white oak and English oak.
Tree paint or wound dressing is not normally recommended on pruned or wounded surfaces, but for damaged oaks an immediate light application of these products may be the only defense against oak wilt infection from April through July.
There are other important reasons to avoid pruning in spring.
"Deciduous trees that lose their leaves in the fall are just starting to grow new buds and leaves, so the trees' food reserves are low," says Don Kissinger, DNR urban forester in Wausau. In general, the best time to prune is in winter when trees are dormant.
As of January 31, oak wilt has been found in all Wisconsin counties except Ashland, Iron, Forest, Taylor, Door, Kewaunee, Calumet and Manitowoc counties. Several of these counties contain the highest abundance of healthy and productive oak forests in the state. Taking recommended precautions will help keep them that way for years to come.
Oak wilt and other diseases move easily on or in firewood logs year-round, so keeping firewood local, or purchasing Wisconsin-certified firewood, is another important component of protecting trees and keeping forests healthy.
More information is available online at the Wisconsin DNR website, including a recently released video on oak wilt. Visit the DNR website, dnr.wi.gov, and search for "oak wilt" or "firewood." Additional information about proper pruning techniques is available from community foresters or by searching for "tree pruning [PDF]."