Environmental regulation of Hymenoscyphus fraxineus apothecia development

Ash dieback, incited by Hymenoscyphus fraxineus, is causing mortality of ash across Europe. The pathogen colonises hosts via ascospores that are ejected from apothecia formed on ash leaf rachises, but information is lacking on the environmental factors that affect apothecia formation. This study, undertaken in England and Wales during summer 2018 and 2019, monitored the influence of temperature, relative humidity, ground cover, and canopy openness on apothecia development. Additionally, apothecia were monitored in the laboratory by incubating infected ash rachises at 4°C, 10°C, 15°C and 20°C, and then moving to 18-21°C to assess how earlier incubation temperatures influenced subsequent apothecia formation. In the field, temperature had a positive effect on apothecia development which was amplified as relative humidity and canopy openness increased; ground cover associated with greater moisture also enhanced apothecia development. Laboratory investigations indicated a threshold temperature for apothecia formation of ~10°C, with pre-incubation temperature influencing both the rate and probability of apothecia development.