Lenticel infection in Fraxinus excelsior shoots in the context of ash dieback

Abstract

Common ash (Faxinus excelsior L.) in Europe is declining on a continental scale due to the action of Hymenoscyphus fraxineus, an invasive forest pathogen that causes ash dieback disease leading to the collapse and eventual death of ash trees through shoot infection in the crown and through stem collar infection. This study confirms for the first time lenticels as entry points for pathogens to enter shoot bark. Results show the impact of lenticel infection at a very early stage of invasion by H. fraxineus in a F. excelsior provenance trial and its correlation values with other factors such as shoot dieback, canker-like lesions and bud burst. No significant provenance effects were observed for incidence of shoot dieback, lenticel necrosis or canker-like lesions on shoots, but provenance effects were significant for bud burst phenology. The strongest correlation was observed between lenticel necrosis and canker-like lesions on the lenticels of shoots. Boheremia spp. were most frequently isolated from necrotic ash lenticels and confirmed by ITS sequencing, but also species of Diaporthe, Epicoccum, Aspergillus, Neonectria, Didymella and Hymenoscyphus fraxineus. Finally, lenticel density was similar in sets of ash genotypes that were characterized as having a high and low susceptibility to ash dieback.