The Victorivirus Helminthosporium Victoriae Virus 1905 Is The Primary Cause Of Disease/Hypovirulence In Its Natural Host And A Heterologous Host (Reprinted From Virus Res., Vol 213, Pg 238-245, 2016)

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 (Delta dcl-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 Delta dcl-2 was markedly less virulent than wild type and virus-free Delta dcl-2 C parasitica strains. Furthermore, the hypovirulent HvV190S-infected C parasitica Delta dcl-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 Delta dcl-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. (C) 2015 Elsevier B.V. All rights reserved.