Arboriculture & Urban Forestry 43(6): November 2017 have resulted from crosses between diploid and tetraploid trees. In a rather startling discovery (Whittemore and Olsen 2011), it was reported that a survey across the natural range of wild American elms indicated that over 20% of sam- pled American elms were diploid, not tetraploid, and in some locations the diploids overlap with tetraploid populations. While tetraploids exist throughout the natural range of American elm, the diploids were most common in the Atlantic coastal plain, Cumberland Plateau, and southern Ohio. Isolated diploids were also found in cen- tral Texas, Oklahoma, and Missouri. Since early research on American elms was centered in the northeast and upper Midwest, it is almost certain that diploids were not utilized for any American elm improvement programs. Whittemore (per- sonal communication) thinks that the diploid and tetraploids may be cryptic species (i.e., species that appear morphologically identical but rarely, if ever, interbreed). Work using molecular mark- ers has shown they are quite distinct genetically. Interestingly, two DED-tolerant elms that thrived on the National Mall in Washington, D.C., since the 1930s, were found to be triploid. For decades, it was assumed that the triploids (one incorrectly marketed as ‘Washington’ and the other correctly marketed as ‘Jefferson’) were unplanned inter- specific hybrids between a tetraploid American elm and some diploid Ulmus species (Sherald et al. 1994), but a later study concluded that it was actually a triploid American elm (Pooler and Townsend 2005). When 22 cutting-propagated trees derived from the original tree were inocu- lated with the DED fungus, none developed sys- temic wilt, whereas eight of the 18 tetraploid American elms did (Sherald et al. 1994). It has not yet been investigated if this triploid could have acquired its resistance from the diploid parent. Beginning with Asian species already known to be resistant to DED (reviewed by Smalley and Guries 2000), The Morton Arboretum in Lisle, Illinois, bred Ulmus species other than Ulmus americana to develop acceptable alternatives. Their interest in interspecific hybridization and Asian spe- cies led to the arboretum’s extensive elm collection (Ware 1995). By the year 2000, The Morton Arbo- retum possessed 23 Ulmus species from China and 10 other exotic species in their collection (see Table 221 22.1 in Ware 2000). The goal, to reintroduce urban- tolerant elms to the nursery trade, was successful, with five unique introductions, some of which have had wider appeal and are more adapted to a vari- ety of climates. The releases, all currently available in the trade, are ‘Morton’ (Accolade™), ‘Morton Glossy’ (Triumph™), ‘Morton Stalwart’ (Commen- dation™), ‘Morton Plainsman’ (Vanguard™), and ‘Morton Red Tip’ (Danada Charm™). Accolade received the 2012 Tree of the Year award from the Society of Municipal Arborists (Figure 2). Although the breeder, Ware, is now deceased, some of his latter hybrids remain in evaluation, but it appears that other genera (e.g., Quercus, Carpinus, and Platanus) are a more recent focus at The Morton Arboretum (K. Bachtell, personal communication). The USDA-ARS/National Arboretum also bred and released to nurseries interspecific and Asian elm selections that are resistant to DED. While none have the size and silhouette of American elm, they are a viable option for cities and towns. Figure 2. Original specimen of The Morton Arboretum, Acco- lade™ elm (Ulmus davidiana var. japonica ‘Morton’), show- ing the general form of an American elm. Photo by Michael Marcotrigiano. ©2017 International Society of Arboriculture
November 2017
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