Genetic mapping of polygenic scab ( Venturia pirina ) resistance in an interspecific pear family

The need for chemical control of scab ( Venturia pirina , V. nashicola ) is the main barrier preventing sustainable pear production. As cultivars with durable resistance are not available yet commercially, the development of molecular markers for early selection is desirable to enhance the efficiency of breeding such cultivars. Interspecific pear progeny PEAR1 × PEAR2 derived from European ( Pyrus communis ) and Asian ( P. pyrifolia and P. ussuriensis ) pears was infected with three single-spore isolates of V. pirina using the droplet inoculation technique. Illumina Infinium ® HD Assay technology was employed to genotype the progeny with single nucleotide polymorphism markers for map construction. With one linkage group missing in each parent, the parental maps covered 17 linkage groups in total, 1,132.3 and 1,136.8 centimorgan for the female and male parents, respectively. Resistance mapping resulted in the identification of seven quantitative trait loci (QTLs) by Kruskal–Wallis analysis. Parent PEAR1 contributed a QTL on linkage group (LG) 17 that was effective against all three scab isolates, while PEAR2 contributed one on LG7 that was effective against two isolates. The other five QTLs, on LG2 and LG5 of PEAR2, and LG7 and LG10 of PEAR1, displayed differential interactions, with each QTL being paired with a single incompatible isolate. Additive effects of combined resistance loci displayed a higher level of resistance than single loci, and the role of nonhost resistance is discussed.