Inoculation of Fraxinus excelsior seedlings with bacterial strains to enhance the tolerance against ash dieback

Since ash dieback has been recognized as a major threat to Fraxinus excelsior, multiple lines of research have focused on potential management to mitigate this disease. One area of focus has been the microbiome of the trees and the application of biocontrol agents (BCAs). In this study, we aimed to find suitable BCAs that can help control Hymenoscyphus fraxineus and the resulting ash dieback. Therefore, we studied the health of ash seedlings under nursery conditions via inoculation with beneficial bacterial isolates and consortia to suppress the pathogen. By using offspring of two tolerant ash trees, we aimed to optimize this via synergistic effects. We found that Schauerella fraxinea B3P038 and Luteimonas fraxinea D4P002 lowered the symptoms of ash dieback. Using strain-specific qPCR assays, only the persistence of Aureimonas altamirensis C2P003 could be proven in the inoculated ash seedlings. Similarly, the fungal pathogen was also unable to infect the plants during the summer months. In addition, we were able to show that the inoculated bacterial strains influenced the microbiome, even without persisting on the plants. These results are encouraging, as positive effects can be taken from the greenhouse to the field and further studies should follow up on the B3P038, C2P003 and D4P002 strains. However, under field conditions, a strong variation in the effects must be considered. This strongly suggests that in addition to plant scoring, monitoring inoculation success as well as pathogen abundance is necessary.