In order to find ash individuals as tolerant as possible to ash dieback, we searched for healthy ash trees throughout Switzerland and evaluated them. In winter 2019, we harvested shoots of such ash trees in the field and subsequently grafted them onto root stocks to obtain a large number of replicates of the same genotype for further experiments.

In 2020 infection trials with the fungal pathogen causing ash dieback were carried out with leaf stalks and stems of ash. From these trials, we found five highly tolerant ash genotypes. These promising ash trees originate from the cantons of Graubünden, St. Gallen, Schwyz and Thurgau. They are now tested in further trials in the maximum-security greenhouse of WSL’s Plant Protection Laboratory against several strains of the fungal pathogen H. fraxineus from Switzerland and Japan. These further tests are necessary to confirm the results and to determine if the ash trees previously identified as tolerant are also tolerant to new fungal strains potential introduced from the area of origin.

All 20 of the ash trees selected in the field (10 healthy ones with less than 25 % crown defoliation and 10 diseased ones with more than 50 % crown defoliation) were also examined for their tolerance to the emerald ash borer. Some of the ash trees differed greatly with respect to beetle tolerance. The two most beetle-susceptible ash trees (from the Tuggen and Quarten sites) were also highly susceptible to ash dieback. Conversely, fungus-tolerant ash trees from the same sites also showed apparent resistance against the beetle. Although not yet fully evaluated, these initial results give reason for hope. They indicate that certain ash trees may have increased tolerance to both ash dieback and the emerald ash borer at the same time.

As part of an EU project, we are investigating the presence of fungal viruses in Japanese isolates of H. fraxineus kept in the maximum-security laboratory at WSL. The probability of finding suitable fungal viruses against ash dieback is highest in the area of origin of the pathogen (i.e., in eastern Asia). Using so-called metagenomic analyses, we were able to detect five different RNA viruses, namely a botybirnavirus, a partitivirus, an endornavirus, an unknown RNA virus and a mitovirus. These viruses are currently being characterized in more detail in WSL’s Plant Protection Laboratory.

In addition, we plan to evaluate various beetle-parasitizing wasps for their suitability for biological control of the emerald ash borer. Both behavioral studies and environmental impact assessments are planned. Our goal is to identify suitable parasitoid wasp species that could be used in the future to control emerald ash borer in Swiss forests.