Breeding for a New Generation of Prunus Rootstocks Based on Marker-Assisted Selection: A European Initiative

Breeding Prunus rootstocks with high root-knot nematode (RKN) resistance, good adaptation to chlorosis, water-logging and drought with graft compatibility with peach and rooting ability will be crucial to achieving sustainable fruit production in the Mediterranean area. This may be possible by combining resistance or tolerance traits from almond (chlorosis and drought), plum (RKN, water-logging and rooting ability), and peach (RKN, graft compatibility). The breeding strategy for RKN resistance is based upon the selection and pyramiding of major genes conferring a high-level, wide-spectrum and heat-stable resistance. Screening among Prunus species and accessions for appropriate resistance sources was carried out during a first EU project (1993-1997). Some Myrobalan (M) plum accessions were characterized as resistant to all RKN species, even under high and continuous inoculum pressure and high temperatures. This resistance is attributed to a single major gene, Ma. By contrast, peach (P) and almond (A) sources express a near-complete or incomplete spectrum of resistance. In a second EU project (1999-2003), breeding for a new generation of interspecific rootstocks, of the type M x P, M x A and M x (P x A), based on the pyramiding of Ma with other resistance (R) genes from peach (RMia) or almond was developed for Prunus species. This paper reports on the progress of this 2nd EU project. To develop marker-assisted selection (MAS), R genes are being mapped using SSR markers from a European reference Prunus map and a BSA approach. Ma and at least one peach gene (RMia) were located on linkage groups 7 and 2, respectively, of the reference map with various SSR loci nearby. Several SCAR markers located less than 1 cM on each side of or co-segregating with Ma are available for MAS of all interspecific crosses. Development of high-resolution genetic and physical maps of the Ma gene have been performed on over 1300 segregating individuals. In peach, one SCAR marker obtained by Yamamoto and Hayashi (2002) for a RKN resistant Japanese cultivar fell less than 5 cM from the RMia gene. The agronomic evaluation is in progress with special attention to chlorosis, drought, and water-logging tolerance. Interspecific hybrids M x P, M x A and M x (P x A) have a higher level of water-logging tolerance compared to reference Amygdalus rootstocks such as GF677, GN15 and GN22.