Investigations of resistance in American elm (Ulmus americana) to Dutch elm disease (caused by Ophiostoma novo-ulmi)

Abstract

Ophiostoma ulmi and Ophiostoma novo-ulmi, the causal agents of Dutch elm disease (DED), have caused catastrophic losses throughout North America, with millions of elms lost over the past eight decades. Although most genotypes of American elm are highly susceptible to DED, there are some genotypes which show increased resistance. Gaining a better understanding of what makes certain trees resistant has critical implications for developing disease management strategies and screening genotypes for resistance. For these investigations five Ulmus americana cultivars and two wild type U. americana populations were studied. When examining trees for host responses, cultivars exhibited differences in their abilities to compartmentalize infection. There appeared to be several strategies of compartmentalization that cultivars used to survive infection. Barrier zones were found in many of the trees examined, however, their presence did not ensure the trees’ survival. Some cultivars created multiple barrier zones within the same year, helping maintain functional xylem after the earlier barrier zone was breached or circumvented. The only cultivar to consistently display an effective wall 3 reaction zone, was ‘Valley Forge’ in 2016. Anatomical xylem characteristics associated with resistance were also examined. Trees with the lowest disease severity ratings, generally had smaller vessel diameters and higher vessel densities compared with those with the highest disease severity ratings. The potential use and limitations of anatomical xylem characteristics as a preliminary screening method for DED resistance are discussed. Although host factors influence disease resistance, other factors are likely playing a role. The effect of timing of inoculation on disease severity ratings was investigated using wild type U. americana trees. Trees inoculated late in the season (August 4) had lower mean disease severity ratings at all time-points compared with those inoculated early in the season (May 26). Timing of inoculation had a significant effect on disease severity ratings, however, in the year following inoculation there were no significant differences between the groups. In summary, findings from these investigations indicate resistance to DED in American elm is highly complex, involving a wide variety of factors. Implications for management, selection, and future research are discussed.