This study aimed to demonstrate the existence of antiviral RNA silencing mechanisms in Sclerotinia sclerotiorum by probing wild-type and RNA-silencing-deficient strains of the fungus with an RNA virus and a circular DNA virus. Key silencing-related genes, specifically dicers, were disrupted in order to dissect the RNA silencing pathway and provide useful information on fungal control. Dicers Dcl-1, Dcl-2, and both Dcl-1/Dcl-2- genes were displaced by selective marker(s). Disruption mutants were then compared for changes in phenotype, virulence, susceptibility to viral infection, and small RNA accumulation compared to the wild-type strain. Disruption of Dcl-1 or Dcl-2 resulted in no changes in phenotype compared to wild-type S. sclerotiorum; however, the double dicer mutant strain exhibited slower growth. To examine the effect of viral infection on strains containing null-mutations of Dcl-1, Dcl-2 or both genes, mutants were transfected with full-length RNA transcripts of a hypovirus SsHV2L and copies of a single-stranded DNA mycovirus- SsHADV-1 as a synthetic virus. Results indicate that the ΔDcl-1/Dcl-2 double mutant which was slow growing without virus infection exhibited much more severe debilitation following virus infection. Altered colony morphology including: reduced pigmentation, significantly slower growth, and delayed sclerotial formation. Additionally, there is an absence of virus-derived small RNAs in the virus-infected ∆Dcl-1/Dcl-2 mutant compared to the virus-infected wild-type strain which displays a high percentage of virus-derived small RNA. The findings of these studies suggest that if both dicers are silenced, invasive nucleic acids which include mycoviruses ubiquitous in nature- can greatly debilitate the virulence of fungal plant pathogens.