Fungal viruses, hypovirulence, and biological control of Sclerotinia species

Hypovirulence in fungal plant pathogens refers to the reduced ability of selected isolates within a population of a pathogen to infect, colonize, kill, and (or) reproduce on susceptible host tissues and is often associated with fungal viruses and associated double-stranded RNA elements. It has been reported to occur in numerous fungal plant pathogens, including Sclerotinia sclerotiorum, S. minor, and the disparate species S. homoeocarpa. In these fungi, hypovirulence has been associated with the presence of several fungal viruses, including one species of the genus Mitovirus, another species possibly belonging to the genus Hypovirus, and a satellite RNA. Sclerotinia spp. are primarily clonal in their life strategies, with varying degrees of diversity manifested as vegetative compatibility groups within naturally occurring populations. Vegetative compatibility groups can reduce the frequency of transmission of fungal viruses between isolates that are not compatible. Agricultural populations of S. sclerotiorum typically consist of numerous clones, although several clones often represent the majority of a population within individual fields. In contrast, populations of S. minor and S. homoeocarpa are characterized by relatively few clones and may represent more promising pathogens for hypovirulence as a biocontrol strategy. Biological control has been demonstrated through applications of hypovirulent isolates to diseased plant tissues in controlled and field environments. In S. minor, disease severity was suppressed by more than 50%, and the number of sclerotia produced on treated diseased tissues was reduced by up to 90%. These sclerotia were hypovirulent and contained double-stranded RNA characteristic of the hypovirulent isolate. In S. homoeocarpa, biocontrol efficacies of up to 90% and 80% have been achieved in controlled and field environments, respectively, and were comparable with treatment with a fungicide. Single applications of the hypovirulent isolate Sh12B, containing a strain of the species Ophiostoma mitovirus 3a (OMV3a) previously described from Ophiostoma novo-ulmi in Europe, were as effective as up to four applications of fungicide, and treatment efficacy persisted into the following year. Collectively, studies of fungal viruses and hypovirulence in Sclerotinia spp. can increase our understanding of molecular mechanisms influencing the expression of virulence in these plant pathogens and expand the potential of fungal viruses as a unique mechanism of action for biological control.