Hypovirulence detected in Brazilian isolates of Cryphonectria cubensis

A single 3 kb segment of double-stranded (dsRNA) was present in three of 30 Brazilian isolates of Cryphonectria cubensis. These dsRNA-containing isola tes showed morphological characteristics suggestive of hypovirulence and we re significantly less virulent than dsRNA-free isolates. One isolate, howev er, with morphological characteristics suggestive of hypovirulence, showed reduced virulence, but was free from dsRNA. Conversion of virulent isolates with normal morphology to a morphology associated with hypovirulence was a chieved by pairing hypovirulent and virulent isolates of the same vegetativ e compatibility group (VCG). This suggests that dsRNA can be transmitted to isolates of the same vegetative compatibility group by hyphal anastomosis. Converted isolates exhibited the same hypovirulence-associated traits as t hose of the original dsRNA-containing hypovirulent isolates. These studies suggest that a single 3 kb segment of dsRNA alters both morphology and viru lence by conferring hypovirulence on the pathogen; the first such report fo r Brazilian isolates of C. cubensis.