Photo by Dr. Tara L. Bal Oak wilt fungal mats cause the bark to split open and sap beetles crawl around on them before flying to another tree.

You might be getting into the holiday spirit and thinking about a very different type of tree this time of year, but it’s also actually the best time of the year to do a little yard work. If you want to prune your oak trees, this is also the time to do that to prevent spreading oak wilt, an invasive disease species.

Oak wilt is disease caused by a fungal pathogen (Bretziella fagacearum) that blocks water and sap nutrients moving through cells in a tree. The leaves wilt and fall off within as little as a few months of being infected and the tree eventually dies. Oak wilt can move from tree to tree through direct contact of roots that are touching belowground, or over much longer distances by sap beetles.

Sap or picnic beetles (small beetles in the family Nitidulidae) are attracted to the spores of the fungus and can carry it from one tree to another, entering fresh wounds in the trees 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 it is a recommended No-Pruning period, to prevent wounds on trees during times when the beetles are most likely flying.

During late fall and winter however, the beetles aren’t active and it’s safe to prune oak trees without risk of infection. If you have oak trees in your yard that have branches too low or just need a trim, now is the time to tackle that job. You may also want to consider scheduling any tree pruning services during the winter if the job involves oak trees.

Why are we concerned about oak wilt?

Once oak wilt is established, it will continue to spread through all of the oak trees in a forest or neighborhood. Treatments are expensive and there is no cure for individual trees once they have the disease. Oak wilt is established throughout Wisconsin and the Lower Peninsula, and in the UP it has been found in Menominee, Dickinson, and Iron Counties. Oak wilt can also be carried by people moving infected firewood from one location to another (which might attract the beetles to it in the spring), which is one of the reasons why its good to use local firewood.

Active research with Oak Wilt in the UP

In a cooperative effort between researchers at Michigan Tech and eastern Canada, multiple field sites have been monitored for sap beetle activity and weather conditions over the past 2 years. By creating small artificial wounds in trees in areas with and without oak wilt, we are able to collect the beetles out of the wounds, record the time and identify the exact species that may be potentially associated with carrying the disease. As we continue to monitor the timing between the spring weather variables, oak trees, 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 into Canada.

So far, no oak wilt disease has been confirmed in Canada, though there are plenty of oak trees! 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 you can do?

Remember to trim your oak trees in the winter! DO NOT trim or prune them in Michigan in the spring (April 15 – July 15 recommended dates). Don’t move firewood as this is also a way the fungal pathogen is moved around. In the summer, look for red oaks that lose their leaves all at once seemingly much earlier than normal autumn leaf fall. You can report suspect 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.

Dr. Tara L. Bal is an Assistant Professor in the College of Forest Resources and Environmental Science at Michigan Technological University, (tlbal@mtu.edu); Dr. Sigrid Resh is Coordinator for the Keweenaw Invasive Species Management Area (scresh@mtu.edu)