Arboriculture & Urban Forestry 43(6): November 2017 in the trade. Although the list is extensive, it may underestimate the breadth of the elm germplasm, as some smaller institutions and growers do not list their collections on the internet and/or did not respond to requests for a list of their holdings. CONCLUDING REMARKS In 2000, it was proclaimed that even aſter a half century of conventional selection and screening, there is still no DED-resistant American elm and that this is “hardly a ringing endorsement for the status quo” (Guries and Smalley 2000). Despite this proclamation, and perhaps because of the frustra- tion at the never-ending struggle to uncover DED resistance, there appears to be a revival in the use of simple methods to reintroduce American elms. For example, researchers (Eshita et al. 2004; Hunt 2011; Slavicek and Knight 2012), nonprofits (Nature Conservancy 2016), and universities (Blanchette 2012) are promoting the idea of find- ing “lone survivors” or encouraging “natural selec- tion.” These strategies assume that mature elms, never prophylactically treated with fungicides and never contracting DED, are in fact genetically more tolerant than simply isolated or fortunate. It is hypothesized that some old elms may still be alive because decades of disease exposure resulted in a natural DED-screening process. Others are not so enthusiastic about a lone-survivor model for elm recovery. Apparently, it is not uncommon to find a single elm surviving by chance where many have died. Townsend (2000) estimates that only 1 in 100,000 elms shows any DED tolerance, as the thousands of survivors he has tested turned out to have escaped exposure and are not DED toler- ant (reported by Becker 1996).The current, lone- survivor group of cultivars is being commercially released before any scientifically sound compara- tive DED-screening data is published. The press, perhaps overzealous to pronounce the return of Elm Street, has embraced the news. There is un- founded optimism concerning lone survivors because they will “bring [the urban landscape] back to its glory of tree-lined boulevards of the beautiful American Elm” (Jensen 2013). Even if these new clones are DED tolerant, there are no American elms resistant to DED, and given climate change and urban stress, any American elm can be infected by DED and decline or die. This makes 229 the extensive use of lone-survivor clones, without the concurrent use of prophylactic fungicides, a risky proposition for those responsible for plant- ing American elms in cities or other prominent locations where tree removal cost and landscape impact are high. Suggestions to a return to street monocultures of American elm (Jensen 2013), or any species, are ill-advised. American elms that are tolerant to DED but become infected periodically may be suited for the restoration of riparian ecosys- tems rather than urban plantings. Work centered in Ohio is underway to test DED-tolerant elms in the wild in the central and northeast regions of the U.S. American elms need not disappear entirely from the streets of America. Restrained planting of tested DED-tolerant cultivars is not unreasonable, espe- cially when trees are monitored and symptomatic trees are quickly treated or removed (Veilleux et al. 2012). Yet, there is justifiable hesitance to use tol- erant rather than resistant elms in great numbers, and therefore, alternative elms are still being devel- oped and introduced. The elegance, grace, and nos- talgia of an American elm cannot be replaced by interspecific hybrids or other species that bear little resemblance to American elms. That being said, the genus Ulmus has much to offer. It is unlikely that the potential of other elm species would have been realized in the U.S. if it were not for the search for American elm substitutes to transfer disease resistance to American elm. Many of the interspe- cific hybrids are rugged urban trees deserving of a place in the American landscape. With contin- ued breeding, the phenotype of interspecific elms may improve, and introductions with architec- ture closer to American elms may be developed. Since the advent of DNA technology, much has been done to develop more accurate evolu- tionary relationships for angiosperms (APG III 2009). The taxonomy of the genus Ulmus is still being sorted out. Researchers now have genetic sequencing protocols to assist in more clearly defining phylogeny and species validity. Know- ing the relationships among species will assist breeders to develop more calculated breeding strategies. While traditional breeding is essen- tial to any elm improvement program, American chestnut and American elm genetic engineering may outpace traditional breeding methods in the race to confer to the trees complete disease ©2017 International Society of Arboriculture
November 2017
| Title Name |
Pages |
Delete |
Url |
| Empty |
Ai generated response may be inaccurate.
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success.
Downloading PDF
Generating your PDF, please wait...
This process might take longer please wait
