Oak Wilt Disease
Article #157, May 2010
By Bill Cook
With spring comes renewed concern about oak wilt, as the disease begins to be active. The disease is well-established in much of Menominee County and southern Dickinson County. Residential areas of Iron Mountain, Kingsford, Norway, and Vulcan have had trees succumb to oak wilt.
Starting at leaf-out and into August, oak wilt can be recognized by rapid wilting of leaves beginning at the top of the tree. Binoculars can help with early detection. Oaks killed last year will display spore pads that rupture the bark, usually evidenced by a slight swelling and a vertical crack.
The disease can easily be transported in logs and firewood. Harvested oak from infected stands should either be immediately turned into lumber, burned, or tightly covered by tarps. Oak wilt is one more reason why the movement of firewood should no longer be done without consideration of spreading exotic insects and diseases.
Oaks are affected by other disease organisms, too. Browning leaves concentrated in the lower portion of a tree often result from a foliar disease called anthracnose. It’s common on many tree species, especially with wet spring weather.
Recent droughts, going back to 2005, have caused enough stress to allow two-lined chestnut borers and Armillaria root rot to attack oaks, especially older oaks and those growing on marginal sites. Sometimes this can cause top-down leaf browning but trees seldom die within just a few weeks, as they do with oak wilt. Heavy gypsy moth defoliation can mask the effects of oak wilt. Dry spring weather favors gypsy moth larvae survival.
The spread of oak wilt occurs both overland and underground. Certain sap-feeding beetles can carry spores to healthy trees during the growing season. Oaks, particularly red oaks, have root systems that graft together, allowing the movement of the fungus to easily move from tree to tree.
Oak wilt can be treated but the process is expensive and can result in significant visual change. Treatment requires the elimination of both overland and underground pathways. The alternative is losing all the oaks in an area (examples abound). Treatment in residential areas is particularly troublesome and sometimes not possible.
Root grafts are broken using a deep vibratory plow. However, this doesn’t work as reliably in the boulder-filled country around Iron Mountain. Overland spread is eliminated by removing all oak trees within the infected epicenter.
MSU Extension and the Michigan DNRE are, once again, pursuing a USDA Forest Service grant to pay for the identification and trenching of oak wilt of affected sites. However, tree removal and proper disposal remain the responsibility of the landowner.
Once an oak is infected it cannot be saved.
There are rather expensive chemical treatments available for individual high value oaks that are not yet infected, but the treatment is neither guaranteed to prevent oak wilt and must be repeated over time.
The best way to deal with oak wilt is prevention. Avoid wounding oaks from April to August. Don’t prune during that time. Be careful with the lawnmower and weed-whacker. If possible, put off construction activities around oaks until the late summer.
If an oak wound does occur in the spring, this would be one of the few reasons to apply a wound sealant. In most other cases, wound sealants are not recommended. However, when the sap-feeding beetles are active, immediate application of a sealant will prevent them from reaching freshly exposed tissues.
Should your oak trees display symptoms of oak wilt or if you would like more information, contact Bill Cook at firstname.lastname@example.org or 906-786-1575. His office is at the Forest Biomass Innovation Center near Escanaba. County Extension offices and Conservation Districts also have information.
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Bill Cook is an MSU Extension forester providing educational programming for the entire Upper Peninsula. His office is located at the MSU Upper Peninsula Tree Improvement Center near Escanaba. The Center is the headquarters for three MSU Forestry properties in the U.P., with a combined area of about 8,000 acres. He can be reached at email@example.com or 906-786-1575.
by Bill Cook, Forester/Biologist, Michigan State University Extension, 6005
J Road, Escanaba, MI 49829
906-786-1575 (voice), 906-786-9370 (fax), e-mail: firstname.lastname@example.org
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