Dutch Elm Disease Resistance in American Elm: A Conceptual Framework and Case Study from Minnesota

Abstract Background Dutch elm disease (DED) has reshaped the American elm ( Ulmus americana ) component in North American landscapes for nearly a century. Although breeding and selection programs have produced a small number of commercially available DED-resistant cultivars, uncertainty remains regarding the durability of their resistance under current pathogen virulence. Methods This study presents a conceptual framework for screening and interpreting DED resistance based on historical breeding programs and long-term field studies. The framework was applied in a case study evaluating 3 commercially available American elm cultivars (‘Valley Forge’, ‘Jefferson’, and ‘Lewis & Clark’ [Prairie Expedition ® ]) inoculated with a locally derived isolate of Ophiostoma novo-ulmi ssp. americana in Minnesota. Trees were challenged with 3 inoculum concentrations, and resistance was assessed with 2 tree ages. Results All cultivars exhibited low wilt severity one year after inoculation across all inoculum concentrations. No significant effects of cultivar, inoculum concentration, tree age, or their interactions were detected, indicating stable resistance across the tested doses and age range. Conclusions These results are consistent with long-term North American and European studies showing that DED resistance in elm is quantitative and best expressed as functional survival or having the ability to limit pathogen spread, recover from localized wilt, and maintain canopy vigor over time. The conceptual framework and experimental results presented support the continued deployment and evaluation of DED-resistant American elm cultivars for urban and landscape restoration.