CHARACTERIZATION OF DSRNA IN CHALARA-ELEGANS AND EFFECTS ON GROWTH AND VIRULENCE

Forty-three isolates of Chalara elegans (synanamorph Thielaviopsis basicola) from 10 geographic locations within the United States, Canada, New Zealand, and the Netherlands were examined for the presence of double-stranded ribonucleic acid (dsRNA). Thirty-six of the isolates contained dsRNA elements and were assigned to nine groups based on the similarity of banding patterns after agarose gel electrophoresis. Hybridization experiments revealed some homology between one dsRNA segment (2.8 kb) in eight isolates. Isometric virus particles approximately 47 nm in diameter were observed in one isolate. Representative isolates from five main dsRNA groups were compared for differences in growth rate, sporulation, mycelial dry weight production, and virulence. A significant difference (P < 0.05) was observed in growth rate and virulence between isolates that contained dsRNA and those that did not. Within the dsRNA-containing isolates, only isolates in group C (with six dsRNA segments) were significantly different from the other isolates. During this study, a spontaneous change in phenotype was observed in three isolates; dsRNA analysis indicated that the banding patterns had been altered from multiple segments to either a single 2.8-kb segment (two isolates) or to no dsRNA present (one isolate). A corresponding increase in growth rate or virulence was associated with these dsRNA changes in the two partially cured isolates. However, isolates in which the 2.8-kb fragment was either eliminated (cured) or was initially absent were the least virulent. This putative association of a 2.8-kb dsRNA fragment with enhanced virulence requires further study. The results from this research show that the presence of dsRNA in C. elegans is ubiquitous and can alter fungal physiology and virulence. A reduction in growth rate and virulence due to the presence of multiple dsRNA in some isolates of C. elegans suggests that survival and pathogenicity can be affected in this soilborne facultative parasite.