As a result of ash dieback (caused by the fungal pathogen Hymenoscyphus fraxineus) and the spread of the emerald ash borer (Agrilus planipennis) from its point of introduction near Moscow, ash trees are highly endangered as a tree species in Europe.
A loss of ash in Swiss forests as a result of these invasive species would not only reduce tree diversity but also endanger an important part of forest biodiversity. Many insect, animal and fungal species are specialized on ash trees or on the forest communities they dominate.
How can ash trees cope with these two threats? Do ash trees still have a future in our forests at all?
WSL, and in particular the Forest Health and Biotic Interactions Research Unit, is at the forefront of research to answer these questions. Our studies on ash trees and their interactions with invasive pests and pathogens help to ensure the continued existence of ash trees in Switzerland.
The search for tolerant ash trees
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.
Antagonists of ash dieback and emerald ash borer
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.
We are grateful for the financial support from the Federal Office for the Environment for the projects SURVIVASH, RESISTASH (collaboration: IAP), ViroSearch and ParaSearch (collaboration: CABI), the Swiss National Science Foundation for the project DEFENDASH (contribution no. 310030_189075/1), the SwissForestLab for BIPAGD and the EU project HOMED. We also thank WaldSchweiz and Joanna Wierig for their contribution to the videos.