Viruses in plant pathogenic fungi have attracted interest because they can cause debilitation or reduce virulence (hypovirulence) of its fungal host and thereby have the potential as biological control agents. The best-known example is Cryphonectria hypovirus 1 (CHV-1), which causes hypovirulence in the chestnut blight fungus, Cryphonectria parasitica. Natural dissemination and active biological control applications have lead to a high prevalence of CHV-1 in Europe, where the pathogen is invasive since the 1930s. Multiple introductions and recombination events contributed to the evolution of the hypovirus in Europe. As most other mycoviruses, CHV-1 has no extracellular phase and fully relies on its host for survival and transmission. CHV-1 spreads by both vertical (to asexual spores) and horizontal (to other fungal individuals) transmission. Field studies have revealed that the Cryphonectria-hypovirus pathosystem is a highly dynamic but effective biocontrol system. A balanced trade-off between hypovirus virulence and fitness seems to be key for the success of CHV-1 as a biological control agent.
Hymenoscyphus fraxineus is a recent invasive pathogen that causes severe dieback of ash trees in Europe. By using an RNA sequencing approach, we detected a mitochondrial RNA virus in this fungus, named Hymenoscyphus fraxineus mitovirus 1 (HfMV1). Its genome is about 2.5 kb in length and contains one open reading frame that encodes the viral RNA-dependent RNA polymerase. The virus is transmitted at high frequency into sexual spores and reaches a high prevalence in fungal populations. We found no indication that HfMV1 induces a hypovirulent phenotype in the fungal pathogen and its potential use as a biocontrol agent is probably very limited. However, we could use the sequence diversity in the viral genome to infer the invasion history of HfMV1 and its fungal host H. fraxineus