Parks commissioner: Q&A about oak wilt

By Mary Jo Bertolino

Oak wilt is a fungal disease caused by Bretziella fagacearum that is carried by sap beetles into recently opened wounds to oak trees. It also is transmitted through the grafted root system from one tree to another through the crossed roots of multiple trees. In red oaks, the disease is fast, killing the tree within 4 to 8 weeks.  White oaks are less susceptible to oak wilt, killing the tree within 2 to 3 years. 

How can we protect our oaks?  

If you have an oak that is injured in a storm or winds, the immediate removal of the broken limb and sealing the open area with paint, pruning tar, polyurethane or similar item will prevent the sap beetle from transmitting the fungus to the tree. These sap beetles can infect a tree immediately, so early treatment is warranted during the spring/early summertime period.

If pruning is necessary, don’t prune from April 15to July 15.  Pruning limbs on oak trees should be sealed immediately. Tree spikes used for climbing can cause injuries to oak trees during the pruning session and should not be used. The best time to prune oaks is while the tree is dormant and before leaf budding. Sap beetles can also infect a tree before and after the April to July period and care should continue to be taken outside of these times.

How do I recognize if I have oak wilt?

If you see mass leaves dropping during a period outside of autumn, look at the leaves. The typical sign of oak wilt will appear in leaves as bronzing or browning on the outside top of the leaf with green on the inside or bottom, but the leaves can also be green. The disease will start at one branch of the tree and will work its way down until it kills the entire tree. If you notice the leaves on the top of your tree are missing, fast action may prevent an oak wilt spread that can kill all your surrounding oak trees.

A recently killed oak tree typically has a “fungal pad” under the bark of the tree trunk. The fungus will cause the bark to split. Pulling away the surrounding bark will reveal the underlying diseased wood. Oak wilt diagnosis can also be confirmed through Michigan State University Plant & Pest Diagnostics at a cost of around $45.  https://www.canr.msu.edu/pestid/submit-samples/oak-wilt.

What should be done if I find oak wilt in my trees?

Immediate action is necessary to stop the spread of oak wilt to your remaining oak trees and your neighbors’ oaks. The first call should be to an arborist. If caught early, the disease can be arrested by injections of propiconazole. These treatments are expensive and might have to be repeated.

An infected tree is best dealt with by halting the infection from spreading by trenching around the tree to cut the roots from underground spreading, since the disease can be transmitted through the grafted root system of multiple oak trees. The diameter of the infected tree will dictate how far from the tree you will need to trench. The age and size of the tree will also determine the depth of the trench. Large old oak trees can have very deep roots and the ones toward the surface can be rather deep. A 5-foot-deep trench may not be enough for a 200-year-old oak tree.

Once the trenching has been done, the trees inside of the circle will have to be girdled to kill the trees. The thought is to prevent the live trees from being infected and having to trench out from those trees too. If you have a forest of oak trees, this may seem radical, knowing that you will have to kill all the oaks within a certain area.

There are less dramatic treatments that are part of scientific research that is outside of the DNR’s recommendations. David Roberts uses glyphosate treatments off label with excellent results and if you prefer to go that route, you must contact him and to be considered for participation in his research. Roberts can be contacted through his website, https://plantdoctorllc.com/ .

What do I do with the oak trees that have been girdled? 

These trees should be left to die and not removed for at least a year. If you must remove the tree, it can be harvested for lumber, but the bark must be removed or covered with a tarp to prevent spread of the fungus. The tree stumps should be treated with fungicide and/or herbicide if they are infected, painted if they are not yet infected, or ground into mulch.

What happens to the area where I have had to kill the oaks?

If left to nature, other plants will fill in the area where the oaks no longer exist.  The area may be planted with oak wilt resistant trees or cared for in other ways by the homeowner.  One of the concerns with open disturbed areas is the multitude of invasive plants that will take advantage of the tree loss. The cottonwood tree is also an opportunistic tree that you might find has found a spot on your property.  Planting a diverse number of trees in the area will help to protect your trees from predatory insects and diseases.  

A few things to consider with oak trees:

Do not transport firewood out of an infected area. “Fruiting mats” may form on firewood and can attract sap beetles.

Prune oaks only during the dormant period and before leaf budding. Outside of the dormant season, seal areas of pruning immediately with a pruning sealer or paint, preferably latex paint.

Plant diverse trees to help with tree health and avoid loss of multiple trees due to pests and pathogens.

More information will be provided this winter by Barry Conservation District Officer Ben Savoie through the Laketown indoor winter programs. If you are interested in attending this program in January or February, you may contact parks@laketowntwp.org to be placed on a reminder list. The date has not yet been set.

Mary Jo Bertolino is a member of the Laketown Township Parks and Recreation Commission. She can be reached at parks@laketowntwp.org.