Researchers developing disease-resistant American elmsby Dan Gunderson, Minnesota Public Radio
Fargo, N.D.. — Minnesota could lose millions of ash trees to the emerald ash borer.
Forty-five years ago, disease struck another immensely popular tree, the American elm. Towering elms that shaded many streets were destroyed by Dutch elm disease.
The American elm is making a slow comeback.
Since 1961, millions of elms succumbed to Dutch elm disease, transforming canopy-shaded streets to barren boulevards.
Cities planted a variety of replacements, but there's no replacing the American elm, according to North Dakota State University professor Dale Herman.
"There's probably no tree that has been introduced and I don't know if one will ever be introduced that can totally replace that cathedral form of the American elm," said Herman. Herman has been developing new tree varieties for 38 years. He's searched eastern North Dakota and western Minnesota for disease resistant elm trees.
"If you find a tree in an area where many, many American elms were native originally and there's only one left in an area, it could potentially have high Dutch elm resistance," explained Herman.
In 1990, a graduate student working for Dale Herman found a lone healthy elm tree along the Wild Rice River south of Fargo. That tree is still healthy today, nearly 20 years later and the cuttings taken from the tree and planted are growing in research plots.
"All of our progeny propagated from that to date have not contracted Dutch elm disease," said Herman. "That's about as far as we can go. We feel pretty assured that it has high resistance but can not make a claim it would never, ever get it because disease strains change over time."
The new variety is called Lewis and Clark, Prairie Expedition. A few have been planted on Fargo city boulevards and Dale Herman says a nursery is developing stock so the tree could be commercially available in two years.
There's high demand for the classic American elm shape, but Dale Herman hopes the new trees aren't too popular.
"The last thing we want to do is recommend a [single] tree cultivar. What we want to recommend is a diversity of trees," said Herman. "Some researchers are saying you shouldn't plant more than 10 percent of a given genus and no more than five percent of a given species in a tree population in an urban area or city."
Several disease resistant elm trees have been developed on the east coast with names like Valley Forge and Princeton.
But University of Minnesota Associate Professor of Horticultural Science Jeff Gillman believes it's better to develop trees that are native to the harsh Midwestern climate.
He says the U of M is testing several disease resistant trees that were found in Minnesota, but it will be eight to 10 years before any of those trees are commercially available. It takes about 15 years to develop and test a new tree variety.
Gillman says there's high demand for disease- resistant American elms because people love the classic cathedral shape. But he says most cities in Minnesota are determined to not repeat past mistakes of planting blocks and blocks of the same tree variety.
"If we all demand these arching trees over our streets then to some extent they're forced to try and do that," said Gillman. "We have to trust our urban foresters to do what's best for our urban forests instead of demanding that we get a certain shape or structure over our streets which could end up biting us in the butt."
When Dutch elm disease struck 40 years ago, the Green ash became the preferred replacement on many boulevards. Now those boulevards are likely to be bare again as the emerald ash borer moves across the region.
Among the trees growing on those boulevards will be the rediscovered American elm.
- Morning Edition, 06/16/2009, 7:25 a.m.