Reprint of “The victorivirus Helminthosporium victoriae virus 190S is the primary cause of disease/hypovirulence in its natural host and a heterologous host”

•Conclusive evidence that HvV190S is the primary causal agent of a transmissible disease of its natural host Helminthosporium victoriae.•Infection with HvV190S alone does not induce over-expression of the cellular multifunctional protein Hv-p68.•The host range of HvV190S is extended to include Cryphonectria parasitica strain Δdcl-2.•HvV190S-infected C. parasitica strain Δdcl-2 exhibits disease symptoms similar to those of virus-infected H. victoriae.•HvV190S infection induces anti-fungal activity in its new C. parasitica host.

A transmissible disease of the plant pathogenic fungus Helminthosporium victoriae, the causal agent of Victoria blight of oats, was reported more than 50 years ago. Diseased, but not normal, isolates, of H. victoriae contain two distinct viruses designated according to their sedimentation values as victorivirus Helminthosporium victoriae virus 190S (HvV190S) and chrysovirus Helminthosporium victoriae 145S (HvV145S). Although a viral etiology of the disease was previously proposed, conclusive evidence was lacking. Here we present unequivocal evidence based on transfecting virus-free H. victoriae protoplasts with purified virus particles showing that HvV190S is essential for disease development. Furthermore, we show an expansion of the host range of HvV190S to include Cryphonectria parasitica and we also show similarity in a subset of phenotypic traits between HvV190S-infected RNA silencing deficient mutant (Δdcl-2) of C. parasitica and a strain of H. victoriae. In virulence assays on detached American chestnut branches and Red Delicious apple fruits, HvV190S-infected C. parasitica strain Δdcl-2 was markedly less virulent than wild type and virus-free Δdcl-2 C. parasitica strains. Furthermore, the hypovirulent HvV190S-infected C. parasitica Δdcl-2 strain exhibited strong antifungal activity in dual culture with the plant pathogenic fungus Sclerotinia sclerotiorum. No such inhibitory activity was observed in comparable dual cultures with wild type and virus-free Δdcl-2 C. parasitica strains. The discovery that infection with HvV190S induced a hypovirulent phenotype in a heterologous plant pathogenic host is very significant since it might be possible to convert other economically important plant pathogenic fungi to hypovirulence using HvV190S.