The use of metagenomics has huge potential to increase our understanding of fungal community diversity associated with Fraxinus species, the ecological functioning and interactions with the host, or specific interactions of certain fungi with other members of the fungal community (e.g. having potential antagonistic effects that are beneficial to the host). We have investigated the biological and ecological traits of the ash dieback fungus (Hymenoscyphus fraxineus) on its native host (Fraxinus mandshurica) in its native environment in the far East, including profiling the fungal community (for more information click here). We have also sequenced fungal community profiles of different species of Fraxinus from around the globe (manuscripts in preparation) which will help characterize the global diversity and distribution patterns of tree endophytes associated with different species of Fraxinus currently threatened by both Hymenoscyphus fraxineus in Europe and Emerald ash borer (Agrilus planipennis) in eastern North America (which is also infringing upon the European population.
Studies on the resistance biology of the disease have improved our understanding of the genetic basis of pathogenicity, how the fungus is able to switch from non-pathogenic to pathogenic lifestyles, and ultimately what makes Hymenoscyphus fraxineus a successful, aggressive foliar pathogen on European ash. We have conducted several studies on the genetic- and biochemical basis for pathogenicity and disease resistance (e.g. see papers in Resources). Ongoing work includes comparative studies of Asian and European Fraxinus species looking into the genetic and metabolite basis of host defense and identifying biomarkers associated with host resistance, using GWAS and LC-MS metabolomics approaches.
The work to develop a more resistant population of European ash was started by Lars-Göran Stener, tree breeder at Skogforsk, who initially assess two ash seed orchards and showed large genotypic variation in susceptibility to the disease (for more information, click here). Since 2014, several important steps toward improving the regeneration material of ash with emphasis on resistance to ash dieback were made by building an inventory database of resistant ash genotypes, establishing clonal archives for gene conservation, and testing selected trees that have been clonally propagated in long-term field trials to screen their resistance under field conditions. This work, which continues today, is a cooperative and concerted effort by Skogforsk and SLU. To read more on the projects and trials initiated, click here)
Novel approach in phenotyping have been tested using infrared spectroscopy technology to chemically fingerprint tree phenotypes on populations of European ash from six European countries. We demonstrated that by using Fourier-transform infrared spectroscopy of phenolic extracts from uninfected bark tissue, coupled with chemometrics modelling, could robustly discriminate between ash dieback-resistant and susceptible trees on the basis of its unique chemotype. The technique offers a major advancement in the application of marker-assisted technology for tree breeding (to read more about that study click here). Now we are optimizing a portable, hand-held device to demonstrate its applicability for in-field assessments.
Exploiting host resistance requires an understanding of the mechanisms that are active in protection not only against one invader, but now – in the case of common ash species in Europe, two invaders: the aggressive fungal pathogen (Hymenoscyphus fraxineus) causing ash dieback and Emerald ash borer (EAB), a buprestid beetle which is currently killing ash in Eastern Europe and is now on the fringe of invading the rest of the European continent. Ongoing work includes investigations of the susceptibility of select ‘ash dieback-resistant’ ash genotypes to EAB, volatile chemistry of different European species of Fraxinus and their effect on EAB performance and behavior, and host tree defences involved in the interaction of ash dieback and EAB.
In areas where mature trees have been identified as being remarkably tolerant, there is still scarce information on the status of the surrounding regeneration and its potential to help form the future tree layer of the stand. In a new PhD project, we are investing the status of naturally regenerated ash in stands differentially damaged by ash dieback, how this varies across different site types, and the contribution of healthy parents to offspring and potential for future restoration.
During the last decade we have identified several individuals that showed a high disease tolerance. The vitality of these trees have remained while their neighbors very often died out. In practice, it is difficult to clone the tolerant trees (making exact copies) to use them in regeneration of ash stands. It is thus vital that the progenies of tolerant trees produced from the seeds will as well be tolerant. Knowing the progenies inheritable tolerance is also vital for the progress of the breeding program to save ash.
During 2022 and 2023, in collaboration with Norway, Denmark and Lithuania, we established 12 progeny trials with plants coming from more than 100 tolerant mother trees (23 families in Sweden). The plants will undergo thorough investigations in the future and deliver new knowledge about the inheritable tolerance to the fungal pathogen Hymenoscyphus fraxineus, causing ash dieback.
