Arboriculture & Urban Forestry 43(6): November 2017 Arboriculture & Urban Forestry 2017. 43(6):217–241 217 By Michael Marcotrigiano Elms Revisited Abstract. Until Dutch elm disease (DED) was accidentally introduced into the United States around 1930, the streets in many states were lined with American elms (Ulmus americana). This review highlights the aftermath of DED, and updates readers on the advances in our knowledge of the pathosystem, which consists of a tree, a fungal pathogen, and an insect vector. Conventional breeding has produced new cultivars of American elm that are more disease-tolerant, although still not resistant. Suitable DED- resistant hybrid elms have been bred using species from Europe and Asia. The discovery of diploid populations of American elm may open new opportunities in elm hybridization and genome analysis. Growing knowledge of resistance mechanisms reveals a complex interaction of anatomy, physiology, environmental factors, and tree age. The beetle’s role is largely understood but appears not to be a viable point of attack in the war on DED. The genome of the fungal pathogen has been sequenced, and gene expression studies are well under way. There is a renewed interest in understanding the evolution, genetics, and physiol- ogy of the DED pathogen. The genetic engineering of elms has been demonstrated but not with the specificity and vigor as has been reported for genetically engineered American chestnut. Elm yellows, caused by a phytoplasma, are still a deadly prob- lem for elms, although outbreaks are more regional than for DED. Germplasm resources are critical to elm improvement, and the first comprehensive survey of living elm species, hybrids, and cultivars growing in America is presented in tabular form. Key Words: American Elm; Dutch Elm Disease; Elm; Elm Bark Beetle; Elm Yellows; Germplasm Storage; Ophiostoma novo-ulmi; Transgenic Tree; Tree Breeding; Ulmus. THE DUTCH ELM DISEASE PROBLEM BEGINS There was a time when an ideal street tree domi- nated cities and towns in the United States from the East Coast to the Midwest. Thousands of streets were named for it. With its arching, graceful habit, rapid growth rate, urban tolerance, and relatively strong wood, the American elm (Ulmus americana) became one of, if not the most commonly planted tree for towns, commons, parks, campuses, and cit- ies. Saplings could be extracted from the wild and placed where they were needed. It was iconic in the American landscape and played a role in American history (Campanella 2003; Figure 1). Then, around 1930, Dutch elm disease (DED), a vascular wilt dis- ease, was accidentally introduced to the U.S.; the pathogen was likely transported on elm logs import- ed from Europe for veneer. It was first discovered in Ohio but was observed soon aſter in New York and New Jersey (May 1934). The causal organism was determined to be the fungus Ophiostoma ulmi (for- merly Ceratocystis ulmi). DED is “Dutch” not be- cause the causal organism originates from the Neth- erlands (the origin is likely Asia), but because the early researchers on the pathogen were seven Dutch scientists (Holmes 1993). The pathogen was not easy to contain because an insect vector, the European bark beetle, was in the U.S. By 1960, over 40 million elms had succumbed to the disease in the U.S. (Dunn 2000). Urban elms fared worse than woodland elms. The disease quickly caused “a major environmental catastrophe in North America” (Stipes 2000). The massive tree kill caused by DED is arguably the most significant event in the history of urban forestry, as it affected the way arborists would view the plant- ing of monocultures and how the public would view street trees and their management (Watson 2012). This review focuses on the status of elms in the U.S. with a concentration on American elms. The main goal is to bring the reader up-to-date on research and on the availability of elm germ- plasm in the U.S. Enough background informa- tion is presented to set the stage for the current state of research. Only cursory information is given ©2017 International Society of Arboriculture
November 2017
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