Extreme Resistance is Epistatic to Hypersensitive Resistance to Potato Virus YO in a Solanum tubersum subsp. Andigena-Derived Potato Genotype. J. P.T. VALKONEN, Visiting Scientist, Cornell University, Ithaca, NY 14853. S. A. SLACK, Professor, Department of Plant Pathology, and R. L. PLAISTED, Professor, Department of Plant Breeding and Biometry, Cornell University, Ithaca, NY 14853; and K. N. WATANABE, Senior Cytogeneticist, International Potato Center, Apartado 5969, Lima, Peru. Plant Dis. 78:1177-1180. Accepted for publication 23 August 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-1177.

Resistance to potato virus Y (PVY°) was examined in diploid potato progeny from a cross of two diploid, interspecific hybrids, 2X(V-2)7 (extremely resistant to PVY°) and 84.194.30 (susceptible to PVY°). The pedigree of 2X(V-2)7 includes Solarium tuberosum subsp. andigena (extremely resistant to PVY ?) and 5. t. tuberosum (susceptible to PVY°). Following inoculation, 34 F1 progeny (Ry/ry Ny/ny or Ry/ry ny/ny) and 2X(V-2)7 (Ry/ry Ny/ny) remained symptomless and no virus was detected by ELISA, which is characteristic of extreme resistance controlled by the Ryadg gene. Eight progeny (ry/ry Ny/ny) developed leaf-drop, necrotic streaks, lesions, and mottle symptoms in systemically infected leaves, which are characteristic of hypersensitive resistance controlled by an Ny gene. Twenty progeny (ry/ry ny/ny) and 84.194.30 (ry/ry ny/ny) were susceptible to PVY0 and developed mottle symptoms without necrosis. These results indicated that extreme resistance and hypersensitivity to PVY° were derived from S. t. andigena and that the expression of extreme resistance is epistatic to the expression of hypersensitivity in 2X(V-2)7.