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Occurrence of Potato Tuber Necrotic Isolates of Potato virus Y in a Commercial Tobacco Field in Southern Ontario, Canada

Most strains of Potato virus Y (PVY) can infect tobacco plants (Nicotiana tabacum) and cause vein clearing followed by leaf mottling, except the PVY^N strain, which induces severe vein necrosis. Some isolates within the PVY^N strain also cause potato necrotic tuber ringspot disease, but these have not been reported from Canadian tobacco fields. PVY^NTN isolates include European (EU) and North American (NA) types that are serologically identical to PVY^N, but can be distinguished by nucleic acid-based assays and potato bioassay (1,2). Some PVY isolates, PVY^N-Wi or PVY^N:O, resulting from a recombination between RNA molecules of PVY^N and the common strain, PVY^O, are identified as PVY^O in serological assays, but induce necrosis in tobacco (2). In August of 2007, two samples of tobacco (N. tabacum, unknown cultivar) leaves showing necrotic symptoms resembling those induced by PVY^N, PVY^NTN, or PVY^N-Wi were collected from a tobacco field in southern Ontario, Canada and submitted to the Charlottetown Laboratory, Canadian Food Inspection Agency, Charlottetown, PE. Virus in both samples (PVY-204 and PVY-205) reacted with PVY^N-specific antibodies 1F5 and 4E7 (3) and induced vein necrosis in tobacco (N. tabacum cv. Samsun). A multiplex reverse transcription (RT)-PCR assay (1) for the simultaneous detection and differentiation of various PVY strains amplified two fragments (181 and 452 bp) associated with EU-PVY^NTN isolates. Restriction fragment length polymorphism (RFLP) analysis targeting the P1 and NIb gene (3) also indicated that PVY-204 and PVY-205 were EU-PVY^NTN isolates. Known isolates of PVY^O, PVY^N, and NA-PVY^NTN were used in all evaluations as references (3). Furthermore, the nucleotide sequences of the P1 and NIb genes of PVY-204 and PVY-205 determined by automated cycle sequencing (3) and subjected to phylogenetic analysis indicated that the nucleotide and deduced amino acid sequences of both isolates were 96 and 95% identical, respectively, to NA-PVY^NTN isolates reported from Canada, but 99% identical (both nucleotide and amino acid) to EU-PVY^NTN isolates from Europe and Mexico (3). Potato (cv. Yukon Gold) plants mechanically inoculated with leaf sap from tobacco (N. tabacum cv. Samsun) infected with PVY-204 and PVY-205 developed various leaf symptoms including severe local and systemic necrotic lesions, leaf wilting, and leaf death in 3 to 5 weeks postinoculation under greenhouse conditions. The infected plants recovered in 5 to 6 weeks. Potato (cv. Yukon Gold) plants inoculated with leaf sap from tobacco (N. tabacum cv. Samsun) infected with a PVY^NTN isolate (HX8) (3) and healthy tobacco leaf sap were used as positive and negative controls. The number and yield of the tubers harvested from infected plants were significantly reduced (50%), and PVY-204 and PVY-205 induced typical potato tuber necrotic ringspot disease in 52.6% of the progeny tubers with an average disease index of 0.364 (C. Kerlan and K. Charlet-Ramage, EAPR Virology Meeting Proceedings, 1998). PVY^NTN was detected by RT-PCR and RFLP in all necrotic tubers and 66.7% of the asymptomatic tubers. Some tubers (15.8%) harvested from the infected plants were negative in RT-PCR targeting either P1 protein gene or NIb gene and showed neither external nor internal necrotic symptom. To my knowledge, this is the first evidence of the occurrence of PVY^NTN isolates in field-grown tobacco plants in Canada.

References: (1) J. H. Lorenzen et al. Plant Dis. 90:935, 2006. (2) R. Singh et al. Arch Virol. 153:1, 2008. (3) H. Xu et al. Can. J. Plant Pathol. 27:125, 2005.