Arboriculture & Urban Forestry 43(6): November 2017 component of tolerance, but that might never com- pletely solve the DED problem. The term “resis- tance” is sometimes used when discussing DED. Resistance implies that the tree can resist infection. To date, no American elm is DED resistant. How- ever, some are quite tolerant in that aſter infection they do not continue to decline, the fungus does not spread through the entire tree, and the tree has the ability to recover (Townsend 2000). Tolerance by some individuals, and the fact that young trees are oſten less prone to DED, assured that Ameri- can elms did not disappear completely with the arrival of DED. Whether by being isolated from diseased trees and the vector, or by being more tol- erant than other trees, some elms persisted. Many, however, were reduced to sprouting stumps of no aesthetic value. Others, which would eventually succumb to DED, lived long enough to produce some seed. Aſter the initial DED outbreak, it soon became obvious that American elms were not going to become extinct, but if they would ever be a sensible choice for urban plantings, more DED-tolerant selections must be bred or isolated. Tree breeding programs require time, commit- ment, space, and long-term funding. In America, the sudden demise of elms appeared to warrant these investments. That being said, only a hand- ful of U.S. institutions attempted to tackle a very time-consuming, expensive, and complex problem. From a scientific standpoint, breed- ing probably commenced earlier than it should have, as so little was known about DED toler- ance. Yet, there was pressure to breed new elms, since the devastation quickly mobilized urban- ites, politicians, researchers, and elected officials. Several approaches to breeding better elms were possible and were initiated. One was to breed and/or select within Ulmus americana by trying to isolate DED-resistant individuals. To most, this was the preferred route because the traits of American elm were unrivaled by other elms, and also because it was the American elm that was disappearing from the American land- scape. A second approach would be interspecific hybridization within the genus Ulmus to cre- ate useful urban elms that might act as suitable substitutes for American elm. And yet another approach was to select superior individuals from within species other than U. americana. The lat- 219 ter two approaches resulted in the development of some diminutive trees that are more suitable for smaller urban spaces than American elm. Early on, not much was known about the biol- ogy of the fungus, the insect vector, or elm defense mechanisms. More informed breeding strategies could have been made with elms if a greater under- standing of DED existed at the time. With herba- ceous crops, generation times are short enough for one breeder to reach an end goal relatively quickly. When American elm breeding began, there were no genetic markers nor known gene products, the mode of DED tolerance was not clear, and the generation time required an institutional, not a personal, commitment. The assumption—and it was a correct one—was that the inheritance of DED tolerance could not be coded by a single or small number of genes (Townsend 2000). There- fore, early strategies were dependent on the prem- ise that by crossing surviving trees to each other, combining ability would result in a small sub- set of seedlings accumulating a larger number of “tolerance genes” than either of their parents. In the mid-1930s, the first American elm selection program began as a cooperative ven- ture with Cornell University and the Boyce Thompson Institute (Sinclair et al. 1974). It was extensive and long-term; 21,000 seedlings were grown out and tested. Repeated inoculations with the DED fungus occurred over a period from 1937 to 1965, with only 16 trees show- ing tolerance. Most of these were slow-growing and did not appear to transmit their DED tol- erance in subsequent crosses and/or later con- tracted elm phloem necrosis (now known as elm yellows, to be discussed later in this paper). Despite the effort, none of the selections were worthy of release to the nursery industry. This lack of success discouraged funding for breeding, especially on a plant with such a long life cycle. In the U.S., DED had marched westward by the 1950s. Reacting to widespread public concern and encouraged by state politicians, researchers at the University of Wisconsin began a research and breeding program, with student education as a focus (Guries and Smalley 1990). The pro- gram arose from a state act “to solve the Dutch elm disease problem” that was erasing landscape elms from the state. Raymond Guries, now retired ©2017 International Society of Arboriculture
November 2017
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