220 but a principal player at the time, stated (personal communication), “They [the legislators] could not have foreseen the magnitude of the prob- lem from a scientific standpoint.” The program consisted of hybridizing selected DED survivors and screening over 3,000 F1 progeny (Smalley and Guries 1993). There were 530 survivors after several years of growth and inoculation with the fungus. Continued evaluation resulted in six select trees that became known as the “American Liberty Multiclone.” They were released as Ulmus ‘American Liberty’, although the “cultivar” repre- sented six different genotypes. At the time, these selections were proposed as “acceptably tolerant” for certain uses (Guries and Smalley 1990). One of the six was eventually named and patented as ‘Independence’. Releasing a multiclone seemed to be a good idea at the time as it assured that genetic diversity would persist once the trees were rein- troduced (Smalley et al. 1993). In hindsight, oth- ers criticized the release of an unmarked group of trees (some of which later proved not very DED-tolerant and difficult to distinguish from one another) because it made future research on individual clones difficult, and marketing them as a group risky. In addition, there have been reports of the ‘Liberty’ clones being killed by elm yel- lows (Sinclair 2000). Although the Liberty Tree Society of the Elm Research Institute still sells them, trees of the American Liberty Multiclone have been surpassed by more recent releases that have been evaluated as more DED-tolerant (Townsend and Douglass 2001). The Wiscon- sin elm research program is no longer active. Another significant American elm breeding program is at the National Arboretum of the Agricultural Research Service of the U.S. Depart- ment of Agriculture (USDA-ARS). The program has contributed three Ulmus americana that are adequately tolerant of DED to make them a cur- rent option for limited landscape plantings. After 20 years of research, ‘New Harmony’ and ‘Valley Forge’ were released in 1995. As with most American elms, they have a vase shape and are tolerant of air pollution and poor soil. They can be propagated by cuttings (Oakes et al. 2012). ‘Jefferson’, another American elm, was released in 2005 by the USDA in conjunction with the National Park Service (Hammond 2006). It was ©2017 International Society of Arboriculture Marcotrigiano: Elms Revisited not the result of a breeding effort but had been growing for decades on the National Mall in Washington, D.C., without contracting DED. Most recently, a few American elms found as lone survivors have been cloned and released. Southwest of Fargo, North Dakota, North Dakota State University discovered a lone survivor in a stand of American elm trees (Johnson 2014). Introduced in 2004, it was named ‘Lewis and Clark’ and marketed as Prairie Expedition®. It is cold hardy to USDA Hardiness Zone 3. In 2008, ‘St. Croix’ (Palmer 2015) and ‘UASNZ’ (found in New Orleans, Louisiana, and marketed as Creole Queen™) (Select Trees 2015) were released into the trade. The former is patented. The DED toler- ance of ‘Lewis and Clark’ and ‘St. Croix’ is based on comparisons with wild-type American elms, but their rating among the many other DED-tolerant clones has not been published. No information on the disease resistance of ‘UASNZ’ is published. A noteworthy attribute for all of the above is regional adaptability, either to cold or warm cli- mates. Perhaps inspired by these lone-survivor introductions, the University of Guelph (Guelph, Ontario, Canada) has launched a Canadian initia- tive to seek out, clone, and breed American elm survivors in Canada (Elm Recovery Project 2015). The genus Ulmus contains about 40 recog- nized species (Wiegrefe et al. 1994; A.T. Whitte- more, personal communication). The species are spread between Europe, Asia, and North America (Hollingsworth et al. 2000). With the advent of DED, one strategy was to attempt to move resis- tance from non-native species by crossing them to American elm. With the exception of Ameri- can elm, all elm species are diploid (2n = 2x = 28), whereas American elm was, until recently, reported to be an entirely tetraploid species (2n = 4x = 56) (Santamour and Ware 1997). Suc- cessful hybridization (with verification) between American elm and other elms is rare (Bey 1990). Attempts at halving the chromosome number of the polyploid American elm (Lester 1971) or doubling the chromosome number of diploid elm species (Dermen and May 1966) were made in an effort to overcome the ploidy issue, but neither tac- tic resulted in progress in American elm breeding. Two triploid American elms have been discov- ered in cultivation, which suggests that these may
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