DNR warns Iron County landowners to avoid pruning oak trees through July 15

Iron County homeowners, cabin owners and tree-service customers risk losing healthy oaks if they cut or trim at the wrong time. The Michigan Department of Natural Resources says the safest season to prune oak trees is winter, and no oak pruning should happen from April 15 through July 15, when the fungus that causes oak wilt is most likely to spread.

Oak wilt is caused by Bretziella fagacearum, and it moves fast once it reaches a fresh wound. The disease can spread through connected root systems, by sap-feeding nitidulid beetles carrying spores from infected trees, and by moving infected firewood or unprocessed oak wood. Michigan state materials say the highest danger falls in May and June, when those beetles are especially active. Simeon Wright of the DNR’s Forest Resources Division has said the disease can kill trees fast, and red oak family species can die within weeks.

That makes identification matter for anyone handling trees around Iron County property. Black oak, northern red oak and northern pin oak are among the red oak group species most vulnerable to oak wilt. White oak, swamp white oak and bur oak are in the white oak group, which can still be infected but is generally better able to slow the disease. Red oaks have pointed leaf tips; white oak group species have rounded leaf edges. For rural landowners, cabin owners and anyone hiring a tree crew, that difference can mean whether a tree survives or is lost.

If an oak is accidentally wounded during the high-risk season, Michigan State University Extension says to seal the cut right away with tree wound paint or latex-based paint. The timing matters: beetles have been known to find fresh oak wounds within ten minutes of pruning. That advice does not apply to other tree species, where painting can slow healing instead of helping.

The warning lands as spring yard work ramps up across the western Upper Peninsula, where state foresters have documented oak wilt in Iron County and nearby Dickinson, Marquette and Menominee counties. Governor Gretchen Whitmer proclaimed May 2025 as Oak Wilt Awareness Month, citing nearly 500 million oak trees in Michigan’s rural and community forests and the economic toll the disease can bring to utilities, recreation, forestry and wood-products industries. Landowners who suspect oak wilt can report it through the DNR’s Oak Wilt Interactive Mapping Tool, the Midwest Invasive Species Information Network, the MISIN smartphone app or by email to the DNR Forest Health Division.