Transfection of Sclerotinia sclerotiorum with In Vitro Transcripts of a Naturally Occurring Interspecific Recombinant of Sclerotinia sclerotiorum Hypovirus 2 Significantly Reduces Virulence of the Fungus

A recombinant strain ofSclerotinia sclerotiorum hypovirus 2 (SsHV2) was identified from a North AmericanSclerotinia sclerotiorum isolate (328) from lettuce (Lactuca sativa L.) by high-throughput sequencing of total RNA. The 5′- and 3′-terminal regions of the genome were determined by rapid amplification of cDNA ends. The assembled nucleotide sequence was up to 92% identical to two recently reported SsHV2 strains but contained a deletion near its 5′ terminus of more than 1.2 kb relative to the other SsHV2 strains and an insertion of 524 nucleotides (nt) that was distantly related toValsa ceratosperma hypovirus 1. This suggests that the new isolate is a heterologous recombinant of SsHV2 with a yet-uncharacterized hypovirus. We named the new strainSclerotinia sclerotiorum hypovirus 2 Lactuca (SsHV2L) and deposited the sequence in GenBank with accession numberKF898354 .Sclerotinia sclerotiorum isolate 328 was coinfected with a strain ofSclerotinia sclerotiorum endornavirus 1 and was debilitated compared to cultures of the same isolate that had been cured of virus infection by cycloheximide treatment and hyphal tipping. To determine whether SsHV2L alone could induce hypovirulence inS. sclerotiorum , a full-length cDNA of the 14,538-nt viral genome was cloned. Transcripts corresponding to the viral RNA were synthesizedin vitro and transfected into a virus-free isolate ofS. sclerotiorum , DK3. Isolate DK3 transfected with SsHV2L was hypovirulent on soybean and lettuce and exhibited delayed maturation of sclerotia relative to virus-free DK3, completing Koch's postulates for the association of hypovirulence with SsHV2L.IMPORTANCE A cosmopolitan fungus,Sclerotinia sclerotiorum infects more than 400 plant species and causes a plant disease known as white mold that produces significant yield losses in major crops annually. Mycoviruses have been used successfully to reduce losses caused by fungal plant pathogens, but definitive relationships between hypovirus infections and hypovirulence inS. sclerotiorum were lacking. By establishing a cause-and-effect relationship betweenSclerotinia sclerotiorum hypovirus Lactuca (SsHV2L) infection and the reduction in host virulence, we showed direct evidence that hypoviruses have the potential to reduce the severity of white mold disease. In addition to intraspecific recombination, this study showed that recent interspecific recombination is an important factor shaping viral genomes. The construction of an infectious clone of SsHV2L allows future exploration of the interactions between SsHV2L andS. sclerotiorum , a widespread fungal pathogen of plants.