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Three Strain Groups of Potato A Potyvirus Based on Hypersensitive Responses in Potato, Serological Properties, and Coat Protein Sequences. Jari P.T. Valkonen, Visiting Scientist, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Ulo Puurand, Researcher, Institute of Biotechnology, P.O. Box 45, FIN-00014 University of Helsinki, Finland; Steven A. Slack, Professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; and Kristiina Makinen, Researcher, and Mart Saarma, Professor, Institute of Biotechnology, P.O. Box 45, FIN-00014 University of Helsinki, Finland. Plant Dis. 79:748-753. Accepted for publication 31 March 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0748.

Isolates of potato A potyvirus (PVA) were obtained from Michigan (PVA-U), Maine (PVA-M), and Hungary (PVA-Bll; complete nucleotide sequence known). PVA-U caused systemic ne-crosis in potato (Solatium tuberosum) cv. King Edward, which carries the gene Na for hypersensitivity to PVA, whereas PVA-M caused mottle symptoms without necrosis. In other potato cultivars carrying Na, PVA-U and PVA-M caused systemic necrosis. Therefore, the hypersensitivity gene in King Edward was different from Na and was named NaKE- PVA-B11 caused no hypersensitive response and infection in potato cultivars except for cv. Pito, which was infected but showed no symptoms. All PVA isolates caused necrotic local lesions on detached leaves of A6 (Solatium demissum x S. tuberosum cv. Aquifa). Thus, PVA-U, PVA-M, and PVA-B11 represented distinct strain groups of PVA, which were named PVA-1, PVA-2, and PVA-3, respectively. Determination of the coat protein (CP) nucleotide sequence of PVA-U and PVA-M and comparison with the CP of PVA-Bll revealed 16 amino acid changes among the three PVA isolates. Six changes that affected the amino acid properties (charge and/or hydrophobicity) distinguished PVA-Bll from PVA-U and PVA-M and were located at the immunodominant N-terminus