Improved Methods to Streamline Dutch Elm Disease Resistance Screening in American Elm

ABSTRACT

American elm ( Ulmus americana L.), an important floodplain tree species in the Eastern U.S., continues to suffer widespread mortality from the non‐native pathogen Ophiostoma novo‐ulmi . Reliable and efficient methods for screening potted American elms for resistance to the pathogen are lacking, hindering efforts to restore the species. The objective of this study was to compare various screening methods for efficacy in separating known‐resistant from known‐susceptible American elm genotypes. Over the course of three experiments, we evaluated symptom development in potted American elms inoculated with O. novo‐ulmi , testing tree age (1‐, 2‐, 3‐year‐old), spore dose (2.5 K, 12.5 K, 25 K spores) and inoculation method (cambium slit vs. hanging drop). Disease progression was rated weekly for at least 7 weeks and used to calculate area under the disease progress curves (AUDPC). In a fourth experiment, we used the refined methods (1‐year‐old trees, 25 K spore dose and hanging drop inoculation method), to screen for resistance in clones and progeny from large survivor elms that were previously screened in a field‐test. Mean AUDPC values from the potted assay explained 82% of the variability in the field‐test screen, indicating strong agreement. Breeders can use the results of this early screen to identify likely susceptible genotypes to exclude from subsequent field testing.