286 Crop Breeding and Applied Biotechnology 9: 286-292, 2009 CM Andrade et al. Crop Breeding and Applied Biotechnology 9: 286-292, 2009 Brazilian Society of Plant Breeding. Printed in Brazil Received 12 March 2009 Accepted 04 August 2009 Potato clones with multiple copies of the Ry adg allele conferring resistance to Potato virus Y Cristiana Moura Andrade 1 , César Augusto Brasil Pereira Pinto 1* , Silvia Regina Rodrigues de Paula Ribeirod 1 , Leandro Santos Peixouto 1 , and Ximena Maira de Souza Vilela 1 ABSTRACT - Potato (Solanum tuberosum L.) breeding for PVY resistance is of utmost importance, in view of the great damage this disease can cause. The establishment of triplex (RyRyRyry) or quadruplex (RyRyRyRy) clones as parents facilitate breeding work because their progenies have a high proportion of clones with the resistant allele. The purpose of this study was to select clones with three to four copies of the Ry adg allele from a population derived from duplex (RyRyryry) crosses and to evaluate the agricultural traits. Testcrosses with a nulliplex genotype (ryryryry) were performed to identify the genotypes for the Ry adg allele, using the SCAR RYSC3 marker. Triplex and Quadruplex clones were identified by the marker, but a larger number of progenies of the test-cross must be tested for the confirmation of their genotypes. The agricultural performance of the clones with the Ry adg allele was better than of the control cultivars. Key words: Solanum tuberosum L., virus, resistance, PVY. 1 Departamento de Biologia, Universidade Federal de Lavras (UFLA), C.P. 37, 37200-000, Lavras, MG, Brazil. *E-mail: cesarbrasil@ufla.br INTRODUCTION The fact that potato is vegetatively propagated is favorable for the spread of diseases, mainly of the viral, which lead to crop degeneration (Secor and Rivera-Varas 2004, Daniels et al. 2002). The economically most damaging diseases are potato leafroll virus (PLRV), potato virus Y (PVY) and potato virus X (PVX). Currently PVY has caused most damage and surpassed PLRV in numbers, with losses of up to 80% (Valkonen 2007, Daniels and Pereira 2004). PVY isolates have been included in different groups of strains: PVY O , PVY C and PVY N and there can be further variations within these groups, e.g., in PVY N , designated PVY NTN and PVY N W. The PVY strains can be differentiated according to the symptoms in tobacco ( Nicotiana tabacum) and potato plants. The PVY O (common strain) causes mosaic symptoms on tobacco leaves and weak to severe mosaic on different potato cultivars, and induces defoliation in Physalis floridana. The necrotic strain (PVY N ) causes necrosis in the veins of tobacco plants, variable mosaic symptoms in potato and does not induce defoliation in Physalis. Isolates of the PVY C strain however cause systemic necrosis, a symptom known as “stipple streak” in potato cultivars carrying the Nc gene (De Bokx and Huttinga 1981). The different strains cannot be distinguished in the field, but by laboratory techniques as the serological method DAS-ELISA, using monoclonal antibodies, or the RT- PCR technique, or by use of indicator plants, a differentiation is perfectly possible (Balme-Sinibaldi et al. 2006, Glais et al. 2002, Blanco-Urgoiti et al. 1998).