Abstract A diploid backcross population derived from a cross between Solanum tuberosum and Solanum bertha- ultii segregated for monogenic dominant hypersensitivity to Potato virus Y (PVY). We propose the symbol Ny tbr for this locus because plants carrying this gene develop necrosis after inoculation with PVY and the allele origi- nated in S. tuberosum. The gene mapped to chromosome IV between TG316 and TG208 at LOD=2.72. This loca- tion does not correspond to any other mapped resistance genes in potato. Keywords Potato virus Y · Hypersensitivity · Solanum tuberosum · Potato · Potyvirus Introduction Among plant families, the Solanaceae has been the focus of the most intensive effort to place loci controlling dis- ease resistance on genetic maps. Specifically, these ef- forts have focused on the genera containing major crop species, Solanum (potato), Lycopersicon (tomato) and Capsicum (garden pepper), creating the opportunity for a comprehensive study of the organization of disease resis- tance genes in related crop genomes (e.g., Grube et al. 2000). Towards this end, we have worked to map a num- ber of monogenically inherited diesease resistances in solanaceous species with particular focus on resistance to the largest family of plant viruses, the Potyviridae. The type member of this genus, Potato virus Y (PVY), is the most agriculturally important virus infecting the po- tato crop, causing heavy yield losses up to 80% (Hooker 1977; Beemster 1987). Resistance genes that control PVY have been identified in cultivated and wild potato species and have been used in potato breeding for many years (e.g., Cockerham 1955; Jones 1990). The “extreme resistance” gene(s) (Ry adg , and Ry sto ), i.e., resistance genes that confer complete resistance to infection at the whole plant level and that are not associated with the presence of a necrotic response to the virus (Valkonen 1997), have been mapped on chromosome XI by Brigneti et al. (1997) and Hamalainen et al. (1997), re- spectively. At present, there is no evidence as to whether Ry adg , and Ry sto represent different sources of allelic genes or distinct, linked loci. In contrast to viral resistance genes at potato loci des- ignated R for extreme resistance, potato plants that carry the gene(s) for a hypersensitive response to the virus de- velop some necrosis after inoculation. This necrosis may be limited to inoculated tissue or may spread through the plant resulting in plant death (De Bokx and Huttinga 1981; Jones 1990; Valkonen 1994; Bawden 1936). A limited necrotic response may lead to resistance to the virus, while a systemic response will cause total crop loss. In either case, this response will limit the spread of the virus to other plantings. Loci that control this type of reaction are designated N genes according to the conven- tions for potato (Valkonen et al. 1996). No gene for hy- persensitivity to PVY in potato has been reported or mapped to date although this response to PVY was noted as early as 1936 (Bawden 1936). The objective of this study was to screen parents of an existing mapping pop- ulation with PVY to determine whether a differential re- sponse could be observed and, if so, to map the genes re- sponsible for the differential phenotype. Communicated by M.A. Saghai Maroof F. Celebi-Toprak · S.A. Slack Department of Plant Pathology, Cornell University, Ithaca, NY14853, USA M.M. Jahn ( ✉ ) Department of Plant Breeding, Cornell University, Ithaca, NY14853, USA e-mail: mmk9@cornell.edu Tel.: 607-255-8147, Fax: 607-255-6683 Present address: S.A. Slack, Ohio Agricultural Research and Development Center, Ohio State University, Wooster, Columbus, OH 44691, USA Theor Appl Genet (2002) 104:669–674 © Springer-Verlag 2002 F. Celebi-Toprak · S.A. Slack · M.M. Jahn A new gene, Ny tbr , for hypersensitivity to Potato virus Y from Solanum tuberosum Maps to Chromosome IV Received: 13 April 2001 / Accepted: 20 July 2001