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A deadly tree disease that could threaten the oak trees of Wisconsin’s
North Woods has struck for the first time in the heart of “cabin
country,” according to the state Department of Natural Resources.

Brian Schwingle, a DNR forest health specialist, said oak wilt, which
kills the oaks common in northern forests (red and black oaks as well
as northern pin oaks), has been found in Oneida County on lakeshore
properties near Eagle River and just three miles west of the
Chequamegon-Nicolet National Forest.

“It certainly does threaten the national forest,” Schwingle said. “But
just as much of a threat, this is the middle of cabin country. ...
People up here have lived with these trees on their properties for 30
years or more. They’re like family to them.”

Oak wilt is not new to Wisconsin. It has struck already in central and
southern parts of the state. But oaks threatened in southern areas,
such as white and bur oaks, are more likely to survive the disease.
The northern species are less likely to survive, he added.

Schwingle said the disease probably arrived in the area via firewood,
which harbored an insect pest called the sap beetles. The insects
carry the disease spores from infected wood to fresh wounds on trees,
and once infected, a tree can die in as little as two or three months.
The most common symptom is rapid browning and shedding of all leaves
within one spring and summer.

The disease can also spread from tree to tree, via entangled roots.
The roots of an oak can spread up to 70 feet, Schwingle said, and this
makes the disease especially dangerous on a lot where there are
several oak trees.

“One year, one oak may die, and the next year, the tree next to it may
die,” Schwingle said. “So if someone sees one of their oak trees die
in a single season, they should contact us immediately.”

Schwingle said trees that are stressed by dry conditions are more
susceptible, ominous because northern counties have been in a drought
that has persisted for more than five years.

While the loss of the northern oak trees can be a sentimental blow to
cabin owners, it can also have a broader economic and ecological
impact. A recent study by the U.S. Forest Service and the University
of Minnesota showed that oak wilt would likely kill 20 percent of all
oaks in Anoka County, Minn., in 20 years if no management occurred.
That study showed the cost of tree removal alone was likely to be $143
million over the 20 years.

Treatment of the disease is difficult and expensive, Schwingle said.
Options include cutting the roots of the dead tree away from the roots
of other nearby trees. If property owners suspect oak wilt, they
should immediately call a certified forester or arborist, Schwingle
added.

[Editor's note: This story has been changed to reflect two
corrections. Two-lined chestnut borers do not spread the fungus that
causes oak wilt, although they do cause damage to the trees that is
very similar to the fungus. The fungus is spread in Wisconsin mostly
by sap beetles. Also, fungicide is effective for treating oak wilt in
trees such as white or bur oaks but not red oak species such as
northern pin oaks.]