Winter in Wisconsin is an excellent time to prune oak trees. Pruning in winter offers all the benefits of pruning while minimizing the spread of tree diseases. One such disease is oak wilt, a fatal fungal tree disease that spreads through tiny sap-feeding beetles attracted to open wounds on trees.
DNR forest health specialist Paul Cigan says oak will will kill and oak within 3-5 weeks. It's spread by beetles picking up the fungal spores ending up on an open wound of an oak tree. Once infected, the tree shows rapid leaf loss. The disease is also spread when the root systems by dead oaks intertwine with live oaks.
Cigan says it's best to not prune from April through July, as that's when the beetles are the most active, so the cold weather months are the best time for pruning...
"...With leaf off, you can see much better where the pruning has to be focused on the individual tree crown. Whether that's to remove dead wood, improve the spacing of branches, reduce hazard limbs in a tree and target a good tree architecture. Having leaf off will allow you to make those decisions much more quickly and efficiently...."
Oak wilt is common in central and southern Wisconsin, but is less common in the Northwoods. Spots of it have been found in Oneida, Vilas, Lincoln, Langlade and Price counties.
More information is on the DNR website.