Pear blister canker viroid: sequence variability and causal role in pear blister canker disease.

The sequences of several cDNA clones of pear blister canker viroid (PBCVd) P1914T and P47A isolates have been determined. Seven out of eight P1914T clones analysed have a constant sequence which differs at six positions from that of the P2098T isolate reported previously. The remaining P1914T clone (8) has a single nucleotide substitution. The same six changes have been also observed in most of the ten P47A clones sequenced. However, some P47A clones show additional variability in positions on both strands of the central conserved region (CCR) and in another conserved sequence at the left-terminal region. This is the first report of a change affecting the upper strand of a viroid CCR. Reasons why such a change is tolerated are discussed. Infectivity bioassays have demonstrated that PBCVd is the causal agent of PBC disease.