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Penn State, State College Elms Withstand Lethal Tree Disease

on May 12, 2010 8:16 AM

Just about three years have passed since the elm-yellows tree disease appeared in the Centre Region, threatening to kill off the area's last iconic elms within a decade.

But the worst-case scenarios feared by local arborists have not materialized -- not so far at least, Penn State and State College tree experts said.

The disease, carried by tiny insects known as leafhoppers, claimed 47 elm trees on the University Park campus in 2008 but only three in 2009, Penn State spokesman Paul Ruskin said. He said two more elms are dead this spring and will be felled after commencement this weekend.

That will leave about 200 elm trees on campus, down from a high of about 400 a decade ago, Ruskin said. Age and another scourge -- Dutch elm disease -- also have taken a toll, but Penn State still counts one of the largest elm-tree stands among U.S. college campuses.

In the borough, arborist Alan Sam anticipates that elm yellows may kill 10 to 15 elm trees this year. Last year, State College lost 36 elms to the disease, including eight street trees.

Sam said fewer than 60 street elms -- that is, elm trees on public property -- remain in the borough, down from about 160 in the late 1980s. An exact tally of elms on private property is not maintained.

"We're going to ask people to remove (diseased) trees" from private property, Sam said, "but we're not going to force the issue."

In prior years, a borough ordinance required homeowners to remove diseased trees if "they threatened public or private resources," Sam said. He said that had proven an effective way to slow down Dutch elm disease, a fungal issue that has contributed to the local elm decline at least since the '80s.

But a more bacteria-like organism causes elm yellows, and there's no proof that removing an already-infected tree will necessarily keep elm yellows from spreading, Sam said.

Still, quick removal of diseased trees helps to reduce safety hazards and contain expenses, he said.

Researchers have yet to develop a surefire way to stop elm yellows, though the insecticide sprays used to prevent Dutch elm disease seem to control the leafhoppers that spread elm yellows, too, arborists said. Penn State researchers are developing and testing more targeted chemical methods, as well.

This year, they will examine leafhoppers themselves, trying to understand better how they relay elm yellows from tree to tree. The insects, carried on the wind, are no more than several millimeters long.

"Elm yellows is in the valley; it's going to stay here," said Gary Moorman, a professor of plant pathology at Penn State. "But I think the trees that are going to die are mostly the wild ones and the ones in town that aren't sprayed. ... We don't think it's going to be a precipitous thing."

Sam speculated that cold wintertime weather also may help limit elm yellows, perhaps killing off more leafhoppers.

Meanwhile, the borough and the university are replacing the fallen elms, mostly with other species of trees. Penn State may plant more elms in landmark areas -- near Old Main, for instance -- if researchers can identify more reliable ways to protect them from elm yellows, Moorman said.

"I think elm yellows will turn out like Dutch elm disease," he said. "We might lose a tree here or there every year (on campus). Right now, we're keeping our fingers crossed that we're not going to lose a lot of trees."

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