The Quest for Ash Resistance to EAB: Towards a Mechanistic Understanding

Since emerald ash borer (EAB), Agrilus planipennis, was discovered in North America in 2002, it has killed many millions of ash trees in North America, and ash mortality now exceeds 99% near the epicenter of the invasion in southeast Michigan (Klooster et al. 2014). The development of EAB-resistant ash trees will be critical for restoration of ash in natural and urban forests. Goals of our collaboration are to identify, breed and screen ash germplasm for EAB resistance and silvicultural traits; and identify mechanisms of EAB resistance to facilitate breeding and screening. In Asia, EAB does not devastate its endemic hosts, which suggests that Asian ashes are inherently resistant by virtue of their coevolutionary history with EAB. In a common garden study, patterns of ash decline and mortality were largely consistent with this hypothesis. Manchurian ash had the highest rate of survival and little canopy decline. The high EAB resistance of this Manchurian ash population of seedling origin is consistent with that observed previously for the clonal Manchurian ash cultivar ‘Mancana’ (Rebek et al. 2008), which suggests that EAB resistance is a species-level trait. Fraxinus x ‘Northern Treasure’ ash, which is a Manchurian (Asian) x black ash (North American) hybrid, had similarly high survival and low canopy decline, suggesting introgression of Manchurian ash resistance genes into the hybrid. However, this pattern contrasts sharply with that observed by Rebek et al. (2008), who found ‘Northern Treasure’ ash to be highly susceptible to EAB. One potential explanation for these divergent conclusions is that there may be taxonomic confusion surrounding this cultivar that needs to be resolved. Most North American species and cultivars in the common gardens study experienced complete or nearly complete mortality, with green ash cultivars, black ash, and Oregon ash declining more rapidly than white ash cultivars. Blue ash has survived at a higher rate in the common garden than other North American species, but by 2014 had lower survival and greater canopy decline than Manchurian ash. Decline and mortality of blue ash has increased over time, suggesting that surviving trees in the plot may ultimately succumb to EAB. The European species and cultivars evaluated in the common garden also experienced high decline and mortality, including F. ornus, F. excelsior ‘Aureafolia’, and F. angustifolia subsp. oxycarpa ‘Raywood,’ which suggests that EAB has the potential to cause widespread economic and ecological impacts in Europe as it continues to spread in Russia and beyond.