Validation of Molecular Markers Linked to Scab Resistance Genes Derived from Different Pear Species for Pyramiding in Japanese Pear

Abstract

Pear scab, a fungal disease caused by Venturia nashicola, is the most serious disease in Asian pear production, leading to decreases in yield and fruit quality. Five major scab resistance genes (Rvn1–Rvn4 and Vnlf) and two QTLs (Rvn5 and Rvn6) identified from different cultivars have been used in pear breeding programs to produce resistant cultivars. Here, we validated the effectiveness of three simple sequence repeat (SSR) markers (LFA02a04, LFA02a09, and LFA02b20) linked to Vnlf (possibly allelic to Rvn2) by using 29 cultivars and four breeding populations. Since there was little discrepancy between the phenotype and genotype in any of the breeding populations, with the frequency of correct classification ranging from 89% to 100%, these markers will be useful for breeding programs. In a population segregating for Rvn1 and Vnlf, the ratio of resistant to susceptible individuals fitted a 3:1 model, confirming that both genes are dominant. Although symptoms with sporulation were observed on leaves of some individuals with only Rvn1 and on those with no resistance gene, no sporulation was observed on leaves of gene-pyramided individuals (Vnlf + Rvn1) or on those carrying only Vnlf. Moreover, we revealed that resistant Chinese pear (Pyrus bretschneideri Rehder) ‘Changxili’ is heterozygous for a previously unidentified host-resistance gene that behaves as a single dominant gene with the observed segregation ratios in four progeny populations. These segregation results in combination with marker data suggest that this newly discovered gene, designated Rvn7, is at a locus distinct from both Rvn1 (derived from ‘Kinchaku’) and Rvn4 (derived from ‘Hongli’). We expect these findings to contribute to pear breeding programs working to develop resistant cultivars with single and multiple resistance genes.