The colder months are the best time to trim oak trees, according to the Michigan Department of Natural Resources (DNR). During these months, oak trees are dormant and less susceptible to oak wilt, a fungal disease that kills members of the red oak family — those with pointed leaf tips — and sickens white oaks — those with rounded leaf tips.
“As trees grow, their canopies often become quite dense, and lower branches are often shaded and will die. When tree canopies are very dense, extra humidity can promote fungal diseases and other issues…so pruning can help promote vigor and health of the tree,” said Simeon Wright, a forest health specialist with the DNR Forest Resources Division.
Pruning does create fresh wounds on trees, though, and that can be problematic for oaks.
Sap beetles in the family Nitidulidae are attracted to the sap that oozes from fresh wounds after pruning. If oak wilt is present on one tree, the beetles can transfer the fungus from tree to tree as they move around, feeding at the wounds.
This problem can be avoided by pruning in fall and winter.
“By pruning oaks when they're dormant during the winter, you completely prevent any risk of oak wilt infection. The beetles that spread the infection are not going to be active during the winter months, so wounds at that time have no risk of spreading oak wilt,” Wright explained. According to the Michigan Oak Wilt Coalition, the safest window to prune oaks is from November 1 to March 14.
Oak wilt is a fungus that is now widespread in the Lower Peninsula and spreading along the Wisconsin border in the Upper Peninsula. It was first detected in Michigan in the 1950s.
In addition to being spread by sap beetles, oak wilt can be spread to other oaks via root systems of nearby infected trees, and by the movement of firewood of trees that died from infection.
“There is concern about moving firewood from trees that may have recently died from oak wilt. That is how we get oak wilt into new areas of the state,” Wright said.
Symptoms of infection vary, but most infected oaks begin to wilt from the top down. Red oaks begin to shed many leaves within weeks of being infected by the fungus. The fallen leaves vary in color from brown to pale green to dark green. These oaks often die within one to two months of infection; white oaks may exhibit thinning canopies, but may be able to recover from oak wilt infection, according to the Michigan Oak Wilt Coalition.
Wright said that oak wilt has existed in Michigan for longer than other devastating illnesses and pests such as Dutch elm disease and the emerald ash borer.
“Oak wilt is slow-moving. We still have a lot of oak trees left in Michigan, and as long as we work to try to slow and prevent spread of the disease, we should continue to have an oak resource for many years to come.”
