# Characterization of a Novel Emaravirus Affecting Ash Species (_Fraximus_ spp.) in Europe
## 4 Discussion
Although more extensive sequence data are required from future studies to confirm this observation, these first results are in line with the theory that RNA virus populations are generally genetically stable despite having a high potential for genetic diversity [60]. Biological properties of viruses such as host type and mode of transmission are thought to play an important role in shaping genetic diversity, with purifying selection pressure constraining the variability of proteins required for interactions between virus, host and vector [54; 60; 61]. High sequence identities between emaravirus variants at different locations or between different hosts have been documented in several studies. An identity of over 97.1% for EMARaV was reported by [PERSON] et al. [58] in different, closely-related hosts. With respect to geographical location, [PERSON] et al. [62], [PERSON] et al. (2013) [61] and [PERSON] et al. [54] all reported a high level of sequence identity of EMARaV at locations across Northern Europe and Russia. [PERSON] et al. [62] for example, found 95.3-100% nucleotide identities in the RNA3 coding region (944 nt) of 36 EMARaV variants across Northern Europe. Interestingly, the authors also identified a small number of EMARaV variants with notably lower sequence identities, an observation also documented in rose rosette virus (RRV) [53]. The evolutionary significance of comparably few divergent variants in a virus population remains to be investigated. Although other studies have demonstrated genetically distinct emaravirus populations, highly similar variants were also found at geographically distant locations [63; 64]. Although three distinct FMV populations were identified, based on the analysis of partial RNA3 fragments (873 nt) of 53 virus variants worldwide, highly similar virus variants at geographically distant regions were also found [64]. It was suggested that the trading of plants, leading to the long-distance migration of particular virus variants, may give rise to such observations [64]. The detection of ASaV at a tree nursery in our study indicates that the trading of ash trees could play an important role in determining genetic variability, but also in aiding the spread of the virus in general. The increasing use of HTS technologies is revolutionizing the diagnosis and characterization of plant pathogens, particularly viruses [12; 13]. Despite the new opportunities created by HTS for detecting novel viruses, an in-depth biological characterization remains essential for evaluating the significance and impact of these new species. This has led to a bottle neck in the evaluation of new species, and therefore a new standardized framework is required to solve this problem [65]. In our study, we report the discovery of a novel emaravirus in diseased _Frazinus_ spp. with the proposed name ash shoestring-associated virus (ASaV) and provide the first important information to genome structure, phylogenetic placement and genetic variability. The establishment of a reliable RT-PCR-based detection method achieved in this study and its refinement in subsequent investigations will not only help unravel the epidemiology of ASaV, but will also aid the development of potential management strategies in the future, such as the testing and certification of plant material, ensuring healthy, virus-free ash for planting. A focus on virus infection will become particularly important in the selection of healthy trees for urban and forest plantings, with tolerance to the changing climatic conditions due to global warming. We have shown that ASaV-infected ash trees are present in several European countries. They have been found in rural stands and forests such as the Swiss mountainous regions near Basel and Crans Montana but also have been propagated in tree nurseries and planted as street trees in urban sites in Germany and Sweden.