Forestry Corner- Oak Wilt Reminder

By Carli Morgan, Forester

As the warm summer months approach, we have become busier and busier with field work at the Conservation Districts. I have the pleasure of meeting new landowners every week and touring their properties. One topic I frequently discuss with landowners is forest health. This broad subject includes insect pests, fungal diseases, invasive plant species, regeneration and many more important aspects of the forest.

This week, I’d like to focus on a forest disease that poses a great threat to one of our favorite trees, the majestic oak. The disease I’m referring to is called “Oak Wilt”. It’s actually an insect and fungus interaction that causes the damage.

Throughout the spring and summer months, beetles travel from oak to oak feeding on sap. These beetles target bark openings on the tree, like those we find from recent wounds or even pruning. Beetles that feed on infected trees simultaneously pick-up fungal spores and transport them to their next feeding site.

This is how the fungus spreads to new, isolated locations. Once the fungus is on-site, it moves through the water-conducting tissue of the oak tree and effectively blocks the system causing the wilting symptoms we see in July and August. Leaves of the infected oak will turn brown and eventually drop. This effect seems to start at the top of the tree and move downward.

Unfortunately, this process may happen quickly – oaks may succumb to the disease within months or even weeks.

Oak wilt can be especially challenging to manage. One reason is that the fungus is capable of moving to nearby oak trees via the root system. Oftentimes, oaks will share connected, living root tissue called “grafts”. As the fungus travels underground, it creates a distinct pocket of dead / dying oaks.

Above image: An example of wilting that occurs on trees infected with oak wilt. This symptom is especially visible in July and August, much earlier than normal fall leaf-drop.

How does a landowner prevent oak wilt from affecting their property? The best thing you can do is follow this rule:
Do not injure, prune, or otherwise break bark on any oak trees from April 1 through August 31.

This is the time period when beetles actively feed on oak trees. Beetles target bark openings because it is easy access to get the sugary sap they seek. Remember the above rule when working or recreating around your oak trees this summer. It is usually unintended oak injuries that transmit oak wilt.

More information on oak wilt disease is available through the Clare Conservation District.
Please visit our website at:

If you suspect oak wilt is on or surrounding your property, please contact our district forester, Carli Morgan, for assistance. There may be management options to help mitigate the disease.

Clare and Gladwin
Conservation Districts
Ph: 989-539-6401

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