218 where recent reviews on specific components of DED are published. While elms in America are a main focus of this review, it should be noted that even before the U.S. epidemic, other elm species in Europe and the United Kingdom were under siege by DED (Heybroek 1993a; Mittempergher and Santini 2004; Santini et al. 2008; Tomlinson and Potter 2010; Santini et al. 2012). In fact, DED was a European problem at least 20 years before it reached North America (Brasier 2000). Recently, the causal fungus for DED has been reported in Japan (Masuya et al. 2010). Research in Europe is active with regard to DED as it relates to tree physi- ology and breeding (Santini et al. 2008; Santini et al. 2012). In Canada, the DED fungus has received a great deal of attention (e.g., Bernier et al. 2015). Marcotrigiano: Elms Revisited REACTION TO THE DED OUTBREAK IN THE UNITED STATES Even though the American chestnut was decimat- ed by a pathogenic fungus only a few years before, the reaction to the appearance of DED was slow at first and its virulence was highly underestimated both in Europe (Tomlinson and Potter 2010) and in the U.S. (Campanella 2003). In time, the impact of DED was one of the few plant-related issues that rallied the public and politicians to find a solu- tion. Although mired in federal bureaucracy, the U.S. Department of Agriculture’s Bureau of Ento- mology, the Works Progress Administration, the Civilian Conservation Corps, and the American Forestry Association were all enlisted to solve the problem (Campanella 2003). But the disease out- paced the funding, and in time, fueled by weather disasters that downed trees and increased vector habitat, the fight against DED became a los- ing battle. Then, another event, World War II, began, and with it higher priorities. It became evident that there was no chance to stop DED from changing the American landscape forever. CONVENTIONAL BREEDING FOR ELM IMPROVEMENT By the time the DED catastrophe took hold in the U.S., breeding programs and basic research were moving forward in the Netherlands, Italy, Spain, and to a lesser extent in Russia (Hey- broek 1993b; Mittempergher and Santini 2004), using species endemic to their regions but also plagued by DED. Until the rapid spread of DED in the U.S., there was little urgency to develop new elms by trying to hybridize with other elm species. Although unusual selections arose, largely via bud sports or odd seedlings, Ameri- can elm was considered a model urban tree that needed no improvement. The most current list of registered cultivar names was published over 20 years ago (Santamour and Bentz 1995). Many of these clones no longer exist, likely eliminated because they showed little tolerance to DED. Breeders realized that the DED problem is Figure 1. A photograph of the American elm (Ulmus americana) under which unsubstantiated claims state that George Washington first took command of the American army. It was made into a widely circulated postcard (circa 1909). This is but one example of the impor- tance of American elms to American popular culture. complex, a pathosystem composed of three com- ponents: a tree, a fungus, and a fungal vector. Intangibles, like environmental stress, are also involved in disease susceptibility. Therefore, breed- ing elms for a specific trait might enhance one ©2017 International Society of Arboriculture
November 2017
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