Wisconsin DNR advises not to prune or cut oak trees from April to July to avoid wilt disease

MADISON, Wis. (WKBT) – April brings not only showers to sprout May flowers but also signals a high-risk window for the often-fatal oak wilt disease, according to the state Department of Natural Resources.
What you can’t see can kill a tree, the DNR advises, which is why it recommends not pruning or cutting oak trees from April through July.
Oak Wilt MapProperty owners who flock outdoors on warming April days to start seasonal yard maintenance and cleanup projects should keep their chainsaws and trimming tools away from the oaks.
Pruning and cutting oaks in spring and early summer leaves them vulnerable to oak wilt, which is widespread in southern Wisconsin. It rapidly kills trees in the red oak group and weakens those in the white oak group, according to the DNR.
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 attack the tree.
The DNR generally doesn’t recommend using tree paint or wound dressing on pruned or wounded surfaces, but it suggests that a light application of these products immediately — if possible within 15 minutes — may be the only defense against oak wilt infection from April through July. Wounds are not susceptible to oak wilt after 72 hours.
Sap-feeding beetles spread wilt over land by carrying the fungal spores from infected oaks to fresh wounds on healthy oaks. Underground, the beetles spread the disease from infected oaks to nearby healthy oaks through grafted, or interconnected, root systems.
“Sap-feeding beetles, like most insects, have an incredible sense of smell, which draws them to open wound surfaces to feed on sugary sap in as little as 15 minutes after a tree is wounded,” said DNR forest health specialist Paul Cigan. “This is how most new oak wilt infections start.”
Although overland infection can occur in mid-July, it is not common. However, as a matter of extreme caution, avoid wounding oaks from April through Oct. 1.
If an oak is wounded during this period, immediately and thoroughly apply pruning sealer or tree paint over the wound. Torn branches or roots should be cut clean and the cut surface painted. For additional protection, cover treated roots with soil.
Red, black and pin oaks are highly susceptible to oak wilt and can die within a few weeks after infection. White and bur oaks are much less susceptible. If infected, they can take months or years to die — or they might even recover.
Avoiding tree pruning in spring not only protects trees from disease, but also helps trees regenerate.
“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,” said Don Kissinger, a DNR urban forester. “In general, the best time to prune is in winter when trees are dormant.”
As of Jan. 31, the DNR had received reports of oak wilt in all Wisconsin counties except Ashland, Iron, Forest, Taylor, Door, Kewaunee, Calumet and Manitowoc. Several contain the highest abundance of healthy and productive oak forests in the state.
Oak wilt and other diseases move easily on or in firewood logs year-round, so keeping firewood local or buying Wisconsin-certified firewood is another critical component of protecting trees and maintaining healthy forests.
More information is available online on the DNR’s oak wilt webpage. Information about proper pruning techniques is available from community foresters in the DNR’s Proper Pruning Techniques publication.