GENETIC CHARACTERIZATION AND MAPPING OF THE MAJOR POWDERY MILDEW RESISTANCE GENE PLBJ FROM MALUS BACCATA JACKII

Abstract

Powdery mildew, caused by the obligate biotrophic ascomycete fungus Podosphaera leucotricha, is one of the main fungal diseases in commercial apple production. Only a few major resistance sources are currently used in apple breeding programmes to include mildew resistance. Some of them such as Pl1 from Malus robusta and Pl2 from Malus zumi have already been reported to be not fully active against the whole race spectrum of the apple powdery mildew fungus. New mildew resistance sources are needed that could extend the genetic basis for mildew resistance breeding. The mildew resistant Malus bacctata accession no. 419 (M. baccata var. jackii) was selected as crossing parent to produce an apple progeny segregating for resistance. Based on clearly resistant/susceptible individuals, an AFLP-based bulked segregant analysis (BSA) was performed. Several polymorphic AFLP markers were identified. Among them one marker co-segregated with the resistance phenotype. This AFLP fragment was cloned and a specific SCAR primer pair (called PLBJ-SCAR) was developed. By using an SSR-based genome-scan, the Plbj gene was located close to the SSRs CH02a10 and CH02c11 on the apple linkage group LG 10. For a verification of the monogenic inheritance of Plbj, as well as for testing the practical usefulness of the developed SCAR marker, a second population has been raised in 2004 from a cross between ‘Cox Orange’ and M. baccata 419. The correspondence between the mildew resistance phenotype and the PLBJ-SCAR is demonstrated also for this apple family.