From the Michigan DNR: Avoid oak wilt: Don't prune trees during risk period To keep oak trees healthy, don’t prune them from mid-April through the summer. According to the Michigan DNR, that’s a key infection time for oak wilt, a serious disease that can weaken white oaks and kill red oak trees within weeks. “We’re asking residents to take extra precaution with their oak trees as spring gears up,” said DNR Forest Resources Chief Deb Begalle. “Waiting to prune until fall will go a long way in preventing the spread of this disease.” Oak wilt, caused by a fungus, has been reported throughout the Midwest, including Michigan. Trees in the red oak group (including black, northern red and northern pin oaks) have leaves with pointed tips and are most susceptible. Trees in the white oak group (including white and swamp white oak) have rounded leaf edges and are less susceptible. Affected trees suddenly will begin to wilt from the top down, rapidly dropping leaves, which can be green, brown or a combination of both colors. Oak wilt is spread above ground mainly by sap-feeding beetles that carry disease spores from infected trees, or wood cut from infected trees, to fresh wounds on healthy trees. The infection also spreads below ground, through root grafts. If you must prune or remove oaks during the April 15-July 15 risk period, or have a tree that gets damaged, immediately cover all wounds with tree-wound paint or latex-based paint. Don’t move firewood, especially if it comes from oak wilt-killed trees. If you suspect firewood is tainted by oak wilt, cover it with a plastic tarp all the way to the ground, leaving no openings. This keeps beetles from moving spores from the firewood to healthy trees. Once the firewood has dried at least a year and/or all the bark loosens, the disease can no longer be spread.
I have always heard to do your pruning only in the months that have an R. This would almost line up, no pruning in May, June, July or August.
first ive heard of it myself. Sadly gypsy moth caterpillars have killed off a lot of red and white oaks around here.