Nielsen, L.R., McKinney, L.V. and Kjær, E.D. 2017. Host Phenological Stage Potentially Affects Dieback Severity after Hymenoscyphus fraxineus Infection in Fraxinus excelsior Seedlings. Baltic Forestry 23(1): 229-232.

   Ash dieback is a serious forest health problem throughout Europe attributed to the emerging, infectious pathogen Hymenoscyphus fraxineus. Preceding studies have revealed large genetic variation among Fraxinus excelsior trees in disease susceptibility, but the underlying mode of tolerance is still unknown. Previous research has revealed a genetic correlation between susceptibility and phenology, where the more tolerant genotypes were characterized by early flushing in the spring and early senescence (leaf yellowing) in the autumn. The main objective of the present study was to explore the influence of host phenological stage on symptom development after controlled inoculation with H. fraxineus. We induced early budburst in a set of seedlings, and compared the symptom development after artificial inoculation, with a control group where flushing had not been induced prior to infection. We observed that severe dieback symptoms were more frequent in seedlings infected prior to budburst compared to seedling inoculated after budburst. We speculate that resistance mechanisms may be more effective in the ash trees while in their growing seasons, and this can contribute to the observed genetic correlation between early flushing and reduced susceptibility.

Keywords: Fraxinus excelsior, Hymenoscyphus fraxineus, Inoculation, Phenology, Susceptibility, Tolerance.