Resistance and tolerance mechanisms of Fraxinus excelsior facing ash dieback

Fraxinus excelsior (common ash) in Europe has been threatened by the invasive fungal pathogen Hymenoscyphus fraxineus for almost 20 years. While a high proportion of ash trees experience severe dieback that often leads to death, around 5% seem to be resistant to or somehow tolerate the disease. Although such resistance seems to be inheritable through generations, the underlying mechanisms are still puzzling scientists. This research plan outlines a multidisciplinary project focusing on the resistance and tolerance mechanisms of F. excelsior to ash dieback caused by H. fraxineus. 

The project aims to disentangle the different resistance mechanisms acting at the foliar and shoot level using various state-of-the-art approaches, including molecular genetics, metabolomics, and microscopy. The study is structured around a common set of potentially resistant and susceptible ash genotypes. Several traits potentially involved in resistance and tolerance will be assessed. 

• In part 1 of the project, the ash dieback susceptibility of the selected genotypes will be accurately measured using controlled inoculations (leaves and stem). In the same part, the phenological timeline of the selected genotypes will be assessed, incl. bud burst and leaf abscission. 
• In part 2, the infection development will be measured in the field during the growing season to test for fungal endophytism and the potential tolerance of healthy-looking F. excelsior genotypes. 
• In part 3, the morpho-anatomical characteristics of the leaves (leaflets, rachises and petioles) of the selected genotypes will be measured using advanced microscopy assays. 
• In part 4, the systemic and induced metabolite production will be quantified and characterized for the selected F. excelsior genotypes. 
• Finally, all parameters measured will then be analyzed in relation to ash dieback susceptibility of the selected F. excelsior genotypes. 

The knowledge generated in this project will provide insights into the resistance and tolerance mechanisms of F. excelsior trees facing ash dieback, which is crucial for developing successful conservation and breeding programs. In a more general view, such knowledge will be helpful to better understand non-coevolved resistance/tolerance in pathosystems including a native host and an exotic pathogen.