Oak wilt disease

The season for oak wilt is about to begin. A bit of knowledge might prevent a lot of heartache.

With spring comes renewed concern about oak wilt, as the disease begins to be active. The exotic disease is well-established across much of Michigan and Wisconsin and has been in the region since at least the mid-1940s.

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 often display spore pads that rupture the bark, usually evidenced by a slight swelling and a vertical crack.

The best way to deal with oak wilt is prevention. Avoid wounding oaks from April to August. Do not 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.

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 is 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. Bulletin E3169 from Michigan State University Extension describes the disease and treatment.

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 of 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 does not work reliably in boulder-filled soils or where bedrock is near the soil surface. Overland spread is eliminated by removing all trees in the red oak group within the infected epicenter.

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 oak wilt fungus requires live tissue to survive. So, once an oak stand has been killed, no fungus remains on-site. Re-planting oaks may be a possibility, although there will likely be a risk of oak wilt several decades down the road. 

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