Extensive variation of sequence within isolates of Grapevine virus B+.

Four regions covering 1247 nucleotides of the RNA genome of 20 isolates of a Vitivirus, Grapevine virus B (GVB), from three countries were analyzed. All the regions in these isolates varied in sequence as compared to the published GVB sequence. Of these, the intergenic region varied the most, with 73.2% nucleotide sequence homology, while ORF4 encoding coat protein varied the least when compared both at nucleotide sequence (80.3% homology) and at amino acid sequence levels (90.6% homology). The variations were scattered along each region length and were higher at the nucleotide level than at the amino acid level, but none resulted in a frame shift or stop codon. These results indicate that GVB may exist as a heterogeneous population, possibly resulting from mixing different strains by grafting practices or by RNA-RNA recombination in the grapevine, the only known natural host of this virus. Although it has been reported that GVB is associated with corky bark disease, no corky bark symptoms were observed in any of the GVB positive grapevine sample collected from Australia.