222 They Marcotrigiano: Elms Revisited are ‘Urban’ (Schreiber and Main 1976), ‘Homestead’ (Townsend and Masters 1984a), ‘Pioneer’(Townsend and Masters 1984b), ‘Path- finder’ (Spongberg 1991), ‘Dynasty’ (Santamour 1984), ‘Prospector’ (Townsend et al. 1991a), ‘Frontier’ (Townsend et al. 1991b), ‘Ohio’ (released in 1992), and ‘Patriot’ (Townsend et al. 1995a). The Wisconsin program also made interspecific elm crosses. While certain hybrids performed well in DED screening, their size, form, and grace left them far behind American elm in popularity. The end result was a few cultivars that have become more popular in Europe (Eisele 2015), where some are planted extensively because Europeans “are not fixated on the American elm type” (R. Guries, personal communication). The umbrella trade- mark name for the Wisconsin releases is Resista®, and the cultivars are ‘New Horizon’ (patented in 1994), ‘Rebona,’ (registered in Germany in 1993), and ‘Regal’ (released in 1983 and sometimes avail- able in the U.S.), all with good tolerance to DED. One of the problems with screening elms for DED-tolerance is that testing methods have never been standardized, nor have tests proved to resemble what a tree would face in its environ- ment (see, e.g., Tchernoff 1965; Takai and Kondo 1979). Inoculation methods vary from needle injection, drilling, slicing, and chiseling. In addi- tion, the amount of inoculum and position on the tree has varied. It has been shown that even if beetles are used as the testing vector, their ability to infect the tree is seasonally dependent (Takai et al. 1979). Therefore, long-term field testing is vital for evaluating the disease resistance and adaptability of new elms. New clones can be use- ful even if they are of similar disease tolerance because they may be more regionally adapted and are therefore less DED-susceptible in a less stressful environment. Organized field trials of elms in America and Europe have been ongoing for decades and have yielded much useful infor- mation. Some testing has the intention of rank- ing trees according to DED tolerance (Townsend et al. 1995b; Townsend et al. 2005), while other research takes a more holistic approach and eval- uates American and hybrid elms for a variety of diseases, insects, and general performance across the country (Townsend and Douglass 2004; Costello et al. 2005; Townsend et al. 2005; Jacobi et ©2017 International Society of Arboriculture al. 2015). The French (Pinon et al. 1998), Italians (Santini et al. 2002; Santini et al. 2007; Santini et al. 2012), and Dutch (Buiteveld et al. 2015) have bred, tested, and named new interspecific elm clones that may be viable cultivars in geographic regions of the U.S. that have similar climates. WHAT MAKES AN ELM TOLERANT OF DED? As breeding programs were initiated to find DED tolerance, researchers also began investigating the mode of action of the fungus and the response of elms to the pathogen. In highly susceptible elms, the pathogenic fungus quickly causes occlusion of (Elgersma 1973) and embolisms in (New- banks et al. 1983) xylem vessels and the infected branches wilt and die, with the disease spreading from the infection site throughout the tree. Root grafts can be a cause of tree-to-tree infection (Jacobi et al. 2013) and undoubtedly accelerate the spread of DED on urban streets planted as elm monocultures. Yet, the most common cause is transmission of the fungus by species of bark beetles (Brasier 2000; Webber 2000; Santini and Faccoli 2014). The fungus must enter the tree though a wound; feeding by the beetle provides that wound. The beetle carries with it spores from visits to previously infected elms, alive or dead. The fungus produces cell-wall-degrading enzymes (Bintz and Canevascini 1996; Przybyl et al. 2006), which result in cell-wall breakdown, allowing the fungal hyphae to spread more easily. The hyphae grow into xylem vessels, and eventu- ally the fungus sporulates in a yeast-like phase. Plants possess phenological, physical, and chemical barriers that all play a role in resistance to fungi (Li et al. 2016). It is largely accepted that resistance/tolerance of elms to Ophiostoma is associated with the ability of the tree to local- ize the pathogen (Duchesne 1993) and inhibit its growth, but how that happens has been the topic of research for decades. As with some other pathogens or wounds, the response of the tree is to attempt to compartmentalize the infected region to limit the extent of injury, which involves the formation of tyloses, cell- wall extensions that plug the xylem (Shigo and Tippett 1981). In susceptible trees, tyloses do not form quickly enough to block the prog-
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