Canopy decline assessment in American elm after inoculation with different doses of Ophiostoma ulmi and O. novo-ulmi

Restoration of American elm (Ulmus americana L.) in natural and urban landscapes necessitates the development of new selections that not only exhibit Dutch elm disease (DED, caused by the fungal pathogen Ophiostoma novo-ulmi and O. ulmi) tolerance, but also an increase the genetic variability of tolerant elms. Toward this end, our program tests DED tolerance of large survivor American elms, crosses between DED-tolerant American elms, and crosses between large survivor and DED-tolerant elms. Accurate phenotyping is critical to accurately assess DED-tolerance. This study examined 1) the effect of different DED pathogen doses; 2) American elm responses to two inoculation timings; and 3) the 8-week DED-induced canopy decline response of 29 American elms selections planted at the Delaware, OH, Forestry Sciences Laboratory. Results suggest a significant dose effect in which the treatment group receiving high levels of DED inoculum exhibited significantly more DED-induced foliar symptoms relative to trees receiving low dosage rates. Furthermore, there is considerable variability in the DED-induced canopy decline ratings associated with the timing of the inoculation. Finally, we observed differences in DED-induced canopy decline between selections of large survivor trees collected around the Midwest, indicating that unique tolerance mechanisms may be present in the natural elm population.