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Disease-Resistant Cultivars
Put American Elm on Comeback Trail
By
Terri McLean
 LEXINGTON,
Ky., (Dec. 13, 2006) – Could the American elm tree – that
once-popular urban shade tree that was ravaged by disease
decades ago – be on the verge of a comeback?
With disease-resistant cultivars of American elm now available,
odds are yes, according to John Hartman, plant pathologist with
the University of Kentucky Cooperative Extension Service.
“If people aren’t afraid to plant it,” said Hartman, who is
among a group of UK plant pathologists, entomologists and
horticulturists testing many of the new elm cultivars on the UK
campus. “Many of us remember when they all died. We don’t want
to see that happen again, so some might be worried about that.”
The source of the angst is Dutch elm disease, a fungal disease
spread by the elm bark beetle. The disease devastated whole
populations of the American elm beginning in the 1930s. In
Kentucky, where the tree is native, most were lost during the
1960s and 1970s.
“One of the things that happened with one of the original
plantings of American elms was we made the mistake of planting
too many of them; in other words, having uniform plantings of
elms throughout whole cities. When the disease ran through it,
it was pretty devastating,” Hartman said.
There has been increasing demand in recent years for American
elms with resistance to Dutch elm disease. Several new cultivars
have been introduced, including Valley Forge and New Harmony,
which have “excellent” disease resistance, and Jefferson and
Washington elms, which are National Park Service selections,
Hartman said.
Seventeen of the new elm cultivars are being tested at UK as
part of a national elm cultivar trial. Although UK’s trial is
still in the early stages, Hartman said much has already been
learned about the trees.
“Although all are supposed to be resistant to Dutch elm disease,
we already know which ones are attractive to Japanese beetles
and elm leaf miners, for example,” he said. “As the trees
mature, they will be evaluated for their form and stature and
for their fall color.”
In the meantime, Hartman said it is a good time to introduce
some of the new American elm cultivars into landscapes and along
streets, “at least on a trial basis.”
He advises using the 10-20-30 rule of tree-planting: “No more
than 10 percent of the same species, no more than 20 percent of
the same genus, and then no more than 30 percent of the same
family.”
“You don’t want to have more than 10 percent American elms in
any particular city because if a new strain of the fungus comes
along, then instead of losing all our trees, we’re only going to
lose 10 percent,” Hartman said.
The American elm is native to the eastern United States and
survives as far west as the Rocky Mountains. It is prized for
being adaptable to harsh urban settings.
“But I think the main thing is it has this very graceful, vase
shape,” Hartman said. “It doesn’t have a lot of lower branches
that interfere with what’s going on along the street, but it
does provide a nice shade by spreading out at the top.”
“American elm was the best of the urban street trees,” he added.
“Let’s hope they can make a comeback.” |
Contact: John Hartman, (859) 257-4445, ext. 80720 |
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of Agriculture, through its land-grant mission, reaches across
the commonwealth with teaching, research and extension
to enhance the lives of Kentuckians. |
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