Detection of double-stranded RNA in Macrophomina phaseolina

One hundred ten isolates of Macrophomi na phaseolina, the causal agent of c harcoal rot of many different plant species, were obtained from Mexico, Som alia, and several locations in the United States, and analyzed for the pres ence of double-stranded RNA (dsRNA). Twenty-one isolates from the United St ates and Mexico contained dsRNA elements. The sizes of the dsRNAs ranged fr om ca 0.4 kbp to 10 kbp and the number of dsRNAs present among the isolates ranged from one to ten. Attempts to detect virus particles by sucrose grad ient centrifugation were unsuccessful. Three isolates having dsRNA and thre e dsRNA-free isolates were evaluated for virulence on sorghum at two differ ent field locations and for mycelial growth in PDA and V8 agar. For both lo cations, the isolates that had dsRNA were significantly less virulent (P < 0.05) than the dsRNA-free isolates. Differences between locations were not significant (P < 0.05). The contrasts between means of isolates with and wi thout dsRNA suggest a strong association between the presence of dsRNA and a reduction in both mycelial growth and the capacity to cause disease. Thes e results suggest an association between the presence of dsRNA and hypoviru lence in the fungus.