Recognition and Molecular Discrimination of Severe and Mild PVYo Variants of Potato virus Y in Potato in New Brunswick, Canada

A field isolate of Potato virus Y (PVY) was collected in New Brunswick, Canada in 2007 due to unusual symptoms observed on different potato cultivars. To unveil the PVY strain identity, tobacco and potato bioassays, PVYo and PVYN-specific antibody-based enzyme-linked immunosorbent assays, and reverse-transcription polymerase chain reaction (PCR)-based genotyping were carried out. All the assays demonstrated that the isolate, designated as PVYo-FL in this study, belonged to the PVYo strain group. Greenhouse tests with the potato cvs. FL 1533 and Jemseg confirmed the severe nature of infection by PVYo-FL. The complete genome sequences of PVYo-FL and PVYo-RB, the latter a mild PVYo isolate, were determined. BLAST analysis revealed that the two isolates shared 97 and 98% sequence identities at the nucleotide and polyprotein levels, respectively. Further BLAST analysis unveiled that PVYo-FL shared 99.7% nucleotide sequence identity with PVYo-Oz, an isolate reported in New York, United States, whereas the PVYo-RB isolate shared 99.2% sequence identity with PVYo-139, a PVYo isolate reported in New Brunswick, Canada. A phylogenetic tree of available, full-length sequences of PVY isolates demonstrated two subgroups within the PVYo branch, one clustered with PVYo-RB and the other with PVYo-FL. Group-specific sense primers for differentiation of the two subgroups were developed and evaluated. A limited survey of potato tubers collected from a field plot at the Potato Research Centre, Agriculture and Agri-Food Canada, using the newly developed PCR primers, indicated that 65.3 and 2.4% of the PVYo-positive tubers were infected with PVYo isolates belonging to the PVYo-FL and PVYo-RB subgroups, respectively. Assessment of the pathogenicity of three representative isolates from each subgroup on the potato cv. Jemseg demonstrated that severe and mild symptoms were induced by the PVYo-FL-like and PVYo-RB-like isolates, respectively.