Update 2007.05.29: Cornell University does not recommend Liberty Elm because:

‘Liberty’ is highly susceptibility to elm yellows and is not recommended due to variability of resistance to Dutch elm disease.
– American Elm Cultivars (PDF), Recommended Urban Trees, Cornell Urban Horticulture Institute

Their top choices for American Elm Cultivars are “Valley Forge” and “New Harmony”, both of which they rate “resistant” to Elm Yellows and Elm leaf beetle, in addition to resistance to DED.

At last night’s 2nd Annual Brooklyn Blogfest, one of the highlights for me was getting interviewed by Dope on the Slope. He had some great gardening questions for me, and I hope I did them justice.

One thing I got wrong, though, was in response to his question about the American Elm, Ulmus americana. He mentioned that Home Despot is selling American Elms resistant to Dutch Elm Disease (DED). I responded that, although there are hybrids which are resistant, they’re not fully American Elms.

I Was Wrong

The Liberty elm is not a hybrid. ERI’s [Elm Research Institute] American Liberty elm is actually a group of six genetically different cultivars. All six look like classic, old fashioned American elms. …
– About the American Liberty Elm

DED is caused by at least three fungi strains: Ophiostoma (Ceratocystis) ulmi, O. himal-ulmi, and O. novo-ulmi. At least two species of bark beetles – the native elm bark beetle, Hylurgopinus rufipes, and the European elm bark beetle, Scolytus multistriatus – serve as vectors for infection. American Elms, which used to be premier street trees across the country, were devastated by the disease since it reached U.S. shores on imported lumber in 1928. European Elms have been similarly affected.

The Liberty Elm, or American Liberty Elm, is the outcome of a decades-long research and breeding program to develop a strain of American Elm, developed only from American Elms, reliably resistant to DED. These are not inter-specific hybrids. They have been developed from survivors collected and propagated from across the country. The resulting plants are propagated vegetatively for distribution.

Genetic differences provide diversity. Having six cultivars in the series is insurance against all the elms being wiped out by any disease or problem, even one that might show up in the future. ERI mixes all six cultivars in its shipments.

During the research phase for all these new elms, they were challenged with injections of the Dutch Elm Disease (DED) fungus in controlled tests. But the American Liberty elm is now long past the experiment stages, and at this point it has been through the additional test of growing in public locations around the country for over 18 years, where it has been exposed naturally to DED fungus where it may occur in those environments.

There is no known American elm variety that can be called entirely immune to DED. The American Liberty elm is resistant to DED, and its resistance has a strong record. In the 18 years since the tree’s introduction, ERI has confirmed less than 100 cases of DED among the 250,000 elms it has sent out.

– About the American Liberty Elm

Cornell University notes other DED-resistant cultivars available, including ‘Princeton’, ‘Independence’, ‘Valley Forge’, ‘New Harmony’ and ‘Jefferson’. They also note all of these elms are susceptible to Elm Yellows and should not be used where that disease occurs.

Any of you gardeners out there have any knowledge about these trees? Do you know of any growing in your area?