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Prune oaks in winter to prevent oak wilt

Emily Klawiter/KISMA Winter pruning of oak trees

You probably aren’t thinking too much about yard work this time of year, but winter is the best time to do any pruning you have planned for your oak trees. If you want to trim back your oak trees, remove any branches, or even take down a tree for firewood or hazard management, winter is the best time to do so to prevent spreading oak wilt, a disease that is an invasive species.

Oak wilt is caused by a fungal pathogen (Bretziella fagacearum) that blocks water and sap nutrients moving through a tree. The leaves will turn a bronze color, wilt and fall off within as little as a few months of being infected, and the tree will eventually die. Oak wilt can move from tree to tree through any direct contact between oak roots that are touching belowground. It can also spread much longer distances overland by sap beetles. Sap or picnic beetles (small beetles in the family Nitidulidae) are attracted to the fungus and can carry it from one tree to another by going into fresh wounds leaking sap during warmer months. These beetles are most active in the spring, so in Michigan there is a high-risk period from April 15 to July 15. This is a time when oak trees are considered most likely to become infected, so this is a recommended No-Pruning period. Preventing wounds on trees in the spring when the beetles are most likely flying is the best method to prevent oak wilt.

During late fall and winter however, the beetles aren’t flying around and it’s safe to prune your oak trees due to a low risk of infection. If you have low hanging branches that you were planning to cut next time you mowed your grass, now is actually the best time to tackle that job. Winter pruning can also make it easier to see individual branches without leaves or undergrowth in the way, and you don’t have to be worried about stepping on your landscaping. You may also want to consider contacting any tree pruning services during the winter if the job involves oak trees.

Why are we concerned about oak wilt?

Treatments for oak wilt are expensive and there is no cure to save individual trees once they have the disease. Once oak wilt is established, it will continue to spread through all oaks in a forest or neighborhood if it is not removed. Oak wilt is currently established throughout Wisconsin and the Lower Peninsula, and in Menominee, Dickinson, and Iron County in the Upper Peninsula. Oak wilt can also be moved by people moving infected firewood from one location to another (which might attract the beetles to it in the spring), which is another good reason to not move firewood.

Active research with Oak Wilt in the UP

Researchers at Michigan Tech and in Canada have been monitoring sap beetle activity and weather conditions over the past 3 years across the UP and parts of eastern Canada. By creating small artificial wounds in trees in areas without and without oak wilt, we are able to collect the beetles out of the wounds and identify the species that may be potentially associated with carrying the disease if it moves further north beyond its current range. As we continue to monitor the timing between the spring weather variables, oak tree phenology, and the sap beetles, we aim to have a better understanding of the risk of oak wilt moving into other counties in the UP or further into Canada. We also hope to provide information to confirm the best recommended dates for the no-pruning guidelines to help best prevent the disease. In May 2023, oak wilt was confirmed for the first time in Ontario, Canada. This research has been supported by the US Forest Service Forest Health Protection Program, in collaboration with the Michigan Department of Natural Resources, American Forest Management, Ontario Forest Research Institute, Manitoba Natural Resources and Northern Development, and Natural Resources Canada.

What can you do?

Trim your oak trees in winter, instead of the spring or summer seasons. DO NOT trim or prune them in Michigan between April 15 to July 15. Don’t move firewood, use local firewood. In the summer, watch for red oaks that lose their leaves all at once, much earlier than normal autumn leaf fall. You can report suspected oak wilt in Michigan through multiple webpages or apps found through the Michigan DNR Oak Wilt webpage at https://www.michigan.gov/invasives/id-report/disease/oak-wilt.

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