230 resistance to their respective pathogens. Before transgenic elms can be commercialized, how- ever, many technical, environmental, and ethical questions need to be answered. The debate would need to take into consideration the purpose of these GMOs in comparison to agricultural crops genetically engineered for other reasons, such as herbicide resistance (Merkle et al. 2007). Will the overall stigma of GMOs prevent the introduction of GMOs of a native species like Ulmus ameri- cana, even if the reason for their development is attack by an invasive non-native pathogen? GMOs have always been controversial. Whether convincing or not, arguments for proceed- ing with the development of GMO trees exist (Strauss et al. 2001; Adams et al. 2002; Kumar et al. 2015), as does opposition (Lang 2004). The DED problem still exists many decades after the DED first appeared. While basic research on the pathogen and elm biology is being actively pursued, many of the major play- ers in conventional breeding are retired or deceased. Only a few research groups are still actively working on the DED pathosystem. Some of the attention to DED in the U.S. is diverted to equally troubling exotics (e.g., emerald ash borer, woolly adelgid, and Asian longhorned beetle). There will always be American elms, but unless greater strides are made toward devel- oping DED resistance, most American elms will not reach impressive size without regular care. Yet, it is hoped that complacency will not take over, and popular books like The Republic of Shade (Campanella 2003), a compelling account of American elms in American history, will continue to remind us about the importance of urban elms and the role that the American elm can continue to play in American culture. No matter how long it takes, the goal to restore the American elm as a prominent tree in the American landscape seems worthy and justified. Acknowledgments. Special thanks to Susan E. Bentz (USDA-ARS National Arboretum) and Chad Gibson (University of Minnesota) for reviewing earlier draſts of this manuscript. I am indebted to Louis Bernier (University of Laval) and Susan E. Bentz for making me aware of or forwarding me relevant manuscripts, the Ameri- can Public Garden Association for distributing an elm survey to its members on my behalf, and the numerous botanical gardens and commercial businesses that responded to a questionnaire. I appre- ciate the willingness of Alan T. Whittemore (USDA-ARS National Arboretum), Raymond Guries (Emeritus Professor, University of ©2017 International Society of Arboriculture Marcotrigiano: Elms Revisited Wisconsin), Kris Bachtell (The Morton Arboretum), and William A. Powell (State University of New York) to answer email queries. The Appendix could not have been generated without the assistance of many curators and businesses that provided information on their Ulmus holdings. Special thanks to my copyeditor, Connie Parks, for her attention to detail and willingness to work on short notice. Opinions in this paper do not necessarily reflect the opinions of Smith College or the Botanic Garden of Smith College. LITERATURE CITED Acquaah, G. 2012. Polyploidy in plant breeding. pp. 452–469. In: G. Acquaah. Principles of Plant Genetics and Breeding, second edition. Wiley-Blackwell, Hoboken, New Jersey, U.S. 740 pp. Adams, J.M., G. Piovesan, S. Strauss, and S. Brown. 2002. The case for genetic engineering of native and landscape trees against in- troduced pests and diseases. Conservation Biology 16:874–879. Ager, A.A., and R. Guries. 1982. Barriers to interspecific hybridiza- tion in Ulmus americana. Euphytica 31:909–920. Aoun, M., D. Rioux, M. Simard, and L. Bernier. 2009. Fungal colo- nization and host defense reactions in Ulmus americana callus cultures inoculated with Ophiostoma novo-ulmi. Biochemistry and Cell Biology 99:42–650. Aoun, M., V. Jacobi, B. Boyle, and L. Bernier. 2010. Identification and monitoring of Ulmus americana transcripts during in vitro interactions with the Dutch elm disease pathogen Ophios- toma novo-ulmi. Physiological and Molecular Plant Pathology 74:254–266. APG III. 2009. An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III. Botanical Journal of the Linnean Society 161:105–121. Baker, W.L. 1948. Transmission by leaf hoppers of the virus caus- ing elm phloem necrosis of American elm. Science 17:307–308. Becker, H. 1996. New American elms restore stately trees. Agricultural Research. Accessed 30 December 2015. Bernier, L., M. Aoun, G.F. Bouvet, A. Comeau, J. Dufour, E.S. Naru- zawa, and K.V. Plourde. 2015. Genomics of the Dutch elm dis- ease pathosystem: Are we there yet? Journal of Biogeosciences & Forestry 8:149–157. Accessed 30 December 2015. Bettini, P.P., A. Frascella, M. Kolařík, C. Comparini, A.L. Pepori, A. Santini, F. Scala, and A. Scala. 2014. Widespread horizontal transfer of the cerato-ulmin gene between Ophiostoma novo- ulmi and Geosmithia species. Fungal Biology 118:663–674. Bey, C.F. 1990. Ulmus americana. pp. 801–807. In: R.M. Burns and B.H. Honkala (Eds.). Silvics of North America, Volume 2. Hard- woods. Agricultural Handbook 654. U.S. Department of Agri- culture. Forest Service. Washington, D.C. 877 pp. Bintz, T., and C. Canevascini. 1996. Xylanases from the Dutch elm disease pathogens Ophiostoma ulmi and Ophiostoma novo-ulmi. Physiological and Molecular Plant Pathology 49:159–175. Blanchette, R.A. 2012. Finding disease resistant elm trees in Minne- sota. Environment and Natural Resources Trust Fund (ENRTF) M.L. 2013 Work Plan. Accessed 30 December 2015. Bolyard, M.G., and M. B. Sticklen. 1993. Strategies for the produc- tion of disease-resistant elms. pp. 171–180. In: M.B. Sticklen and J.L. Sherald (Eds.). Dutch Elm Disease Research: Cellular and Molecular Approaches. Springer-Verlag, New York, New York, U.S. 344 pp.
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