Oak wilt diseased leaves curl, turn brown and fall to the ground. Photo courtesy of MDNR.

EDITOR’S NOTE:  This story originally appeared in our January 2022 issue. The ideal time to prune oak trees to avoid oak wilt disease is during winter, late November or early December up to March 14. The pruning period of March 15-April 14 is still considered less prone (“low risk”) to spread oak wilt disease. April 15  through July 15 is identified as the highest risk period.

MI Backyard

By Jennifer Devine

Oak wilt has become a widespread invasive nuisance that can rapidly kill red oak trees, as well as other oak species. While white oaks can become infected, their ability to recover is far greater.

The presence of oak wilt was addressed in western Michigan counties during a 2016-2019 treatment and study period made possible through the Michigan DNR Michigan Invasive Species Grant Program. Read a synopsis of this study by Josh Shields, Ph.D., conservation district forester for Manistee, Mason and Lake counties, in our online oak-wilt story.

What is oak wilt?

Caused by the fungus Bretziella fagacearum, oak wilt is a lethal disease that infects open wounds of oak trees, most often from mid-April through mid-July but at other times as well. The fungus is carried by the primary culprit sap beetle (Nitidulidae) or other spore-carrying insects that transmit disease to healthy trees. Leaves of infected trees wilt because the tree’s water-carrying system is affected, according to a University of Wisconsin oak wilt bulletin. A single infected tree can infect others when beetles carry the fungus from tree to tree or through the trees’ shared root systems. Eventually, if left untreated, oak wilt will kill the host oak and possibly surrounding oaks. The disease can also be transmitted when infected oak trees are cut for firewood and moved.

Which oaks are affected?

Oaks in the red oak family are the most susceptible and can die within weeks. This group has pointed leaf tips and in northern Michigan includes northern pin oak, northern red oak and black oak.

Oaks with the best chance of survival belong to the white oak family. The leaf tips of this group are rounded. Michigan’s white oak family includes bur oak, white oak and swamp white oak.

How do I identify oak wilt?

The first clue is If an oak tree has browning leaves or a majority of leaves in the crown are shedding and falling to the ground.  Leaves suddenly wilt and fall from the top of the tree first. Wilted or downed leaves may be either brown or green or include both colors, according to the MDNR.

Second clue is the fermented fruit fragrance that can permeate from behind a bumped-up cracked area of bark in a tree infected the previous year. You should be able to easily peel the bark and see the 2-inch ovular gray area of infection known as a fungal mat or pressure pad.

Third clue is the presence of dying oak trees surrounding an already dead oak. This may be evidence that the host tree has already been killed by oak wilt, and the infection has spread to surrounding oaks through spore-carrying insects or via the roots.

Josh Shields, Ph.D., forester with the Manistee and Mason-Lake conservation districts, said it’s important not to confuse oak wilt with oak decline, and a Michigan State University bulletin, written by Bill Cook, can help you spot the differences, though a professional should also be consulted:  https://tinyurl.com/4s2bc78m

How can I help stop the spread of disease?

The highest risk period for oak wilt infection is April 15-July 15, with lower risk from March 15-April 14 and July 16-Oct. 31. It is recommended that you do not cut or prune oak trees on your property during these times. Cutting oak trees is also restricted on public lands in these same periods.

There is no risk of spreading the disease in winter.

“Beetles that can carry the disease from tree to tree are not very active now, and the trees are not vulnerable to infection,” stated Simeon Wright, forest health specialist with the DNR Forest Resources Division, in a December press release. “The beetles are attracted to fresh bark damage or wounds where tree limbs have been removed.”

If you MUST trim an oak tree during high-risk times, or you accidentally hit one with a weed whipper, take care to immediately use a professional pruning sealant on the wound. Sealants can be found at any hardware store. Shields said the sealant provides a physical barrier between the wound and sap-feeding beetles which can spread the infection.

After cutting an oak tree for firewood, MDNR suggests placing a plastic tarp around the logs and burying the edges underground, to keep beetles which spread the fungus from reaching the wood. Leaving the tarp in place for 6-12 months (until the bark falls off) kills the infection.

To help reduce the spread of disease, buy firewood only from the immediate area in which you are living or camping.  Do not bring firewood from home to a campsite and do not transport firewood from campground to campground.

What do I do if I suspect my oak tree is diseased?

For help with identification and reporting, email the Department of Natural Resources Forest Health Division: DNR-FRD-Forest-Health@michigan.gov or call 906-203-9466. Report infections at Michigan.gov/ForestHealth. DNR requests that you take one or more photos of the invasive species you are reporting and make note of location, date and time. This aids in verification. You may be asked to provide your name and contact information if follow-up is needed.

A laboratory can test for oak wilt. As of Dec. 1, Michigan State University’s Diagnostic Clinic has suspended tests for suspected oak wilt infections and will resume in spring 2022. Go to CANR.MSU.edu/PESTID/ or call 517-355-4536.

Other ways to report include the Midwest Invasive Species Information Network (MISIN) online reporting tool  or download the smartphone app: http://www.misin.msu.edu/tools/apps/#home

What treatment methods are most effective?

Always consult with your local conservation district forester or a contractor approved for oak wilt management. Inoculations, trenching and installing barriers, as well as other methods, are currently in use to combat the disease. Visit MichiganOakWilt.org for a list of oak wilt-qualified experts.

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Special thanks to Josh Shields, Ph.D., Forester and Wildlife Biologist, Forestry Assistance Program (FAP)-Manistee and Mason-Lake conservation districts, for his science-based review of the above article.

Jennifer Devine is a homesteader living on 20 acres. She and her husband are licensed to sell black cherry, white oak and white pine trees. They and their children live off grid and have chickens, ducks and rabbits, a large garden and a lot of woods.