A Comparison of the Growth and Asexual Reproduction by Cryphonectria parasitica Isolates Infected with Hypoviruses CHV3-County Line, CHV1-Euro7, and CHV1-Ep713

The impact of chestnut blight, caused by the fungal pathogen Cryphonectria parasitica, has diminished in Europe due to a natural biological control caused by hypovirus infection. Hypovirulence-mediated biological control has been far less successful in North America meriting further evaluation of field isolates that have the ability to produce non-lethal cankers, generate hypovirulent inoculum, and exhibit a greater ecological fitness in forest systems. In this study, Cryphonectria hypoviruses (CHV) CHV3-County Line, CHV1-Euro7, and CHV1-Ep713 were evaluated in five different isolates of C. parasitica. One hundred and eighty cankers representing each treatment combination were initiated on American chestnut sprouts in the Monongahela National Forest, West Virginia, USA. The size of cankers, the persistence of hypovirulent (HV) isolates, stroma production, and hypovirus transmission to conidia were assessed four and 12 months after canker expansion. CHV3-County Line infected isolates produced significantly smaller cankers than the isolates infected with either CHV1-type. With regard to CHV1-Euro7 isolates, the fungal genome appeared to contribute to the differences in canker size. After four months, HV isolates harboring either CHV1-type (30%) were retrieved at a significantly higher rate than isolates containing CHV3-County Line (14%). After 12 months, the HV recovery was similar among the three hypoviruses indicating smaller cankers will maintain their HV status after one year. Very few stroma were produced after one year in the field from HV isolates. In vitro, CHV3-County Line (49%) had a significantly lower rate of hypovirus transmission to conidia when compared to CHV1-Euro7 (87%) and CHV1-Ep713 (80%). Significant differences existed among the five different isolates indicating HV transmission is dependent on the fungal genome. This research provided additional evidence that each hypovirus interacts with its host differently and certain isolate/hypovirus combinations have better biological control potential than others.

Fusarivirus accessory helicases present an evolutionary link for viruses infecting plants and fungi

A significant number of mycoviruses have been identified that are related to plant viruses,but their evolutionary relationships are largely unexplored.A fusarivirus,Rhizoctonia solani fusarivirus 4(RsFV4),was identified in phytopathogenic fungus Rhizoctonia solani(R.solani)strain XY74 co-infected by an alphaendornavirus.RsFV4 had a genome of 10,833 nt(excluding the poly-A tail),and consisted of four non-overlapping open reading frames(ORFs).ORF1 encodes an 825 aa protein containing a conserved helicase domain(Hel1).ORF3 encodes 1550 aa protein with two conserved domains,namely an RNA-dependent RNA polymerase(RdRp)and another helicase(Hel2).The ORF2 and ORF4 likely encode two hypothetical proteins(520 and 542 aa)with unknown functions.The phylogenetic analysis based on Hel2 and RdRp suggest that RsFV4 was positioned within the fusarivirus group,but formed an independent branch with three previously reported fusariviruses of R.solani.Notably,the Hel1 and its relatives were phylogenetically closer to helicases of potyviruses and hypoviruses than fusariviruses,suggesting fusarivirus Hel1 formed an evolutionary link between these three virus groups.This finding provides evidence of the occurrence of a horizontal gene transfer or recombination event between mycoviruses and plant viruses or between mycoviruses.Our findings are likely to enhance the understanding of virus evolution and diversity.