Quantitative trait locus mapping of resistance in apple to Cydia pomonella and Lyonetia clerkella and of two selected fruit traits

Apple, Malusxdomestica, is the most important fruit grown within the temperate zonobiome. It is attacked by both fruit-damaging and leaf-damaging lepidopteran pest insects, which require regular control such as the carpophagous codling moth, Cydia pomonella, or frequent control such as the phyllophagous apple leaf miner, Lyonetia clerkella. As many environmentally friendly pest control tactics are only effective at low levels of infestation, host plant resistance is a promising future component of integrated pest management systems, but knowledge is still lacking on such genetically based approaches against lepidopteran pests. The aim of the study was to identify molecular markers linked to C. pomonella and L. clerkella resistance or susceptibility in commercial apple as well as markers linked to selected fruit traits. The number of C. pomonella-infested fruits and the number of L. clerkella mines were quantified as measures of apple resistance or susceptibility to the studied moth species. Herbivore surveys on 160 apple genotypes, representing a segregating F₁ cross of the apple cultivars 'Fiesta' and 'Discovery', were carried out during two consecutive years and at two sites in Switzerland. Broad-sense heritability was 29.9% (C. pomonella), 18.2% (L. clerkella), 21.9% (fruit number) and 16.6% (fruit diameter). A subsequent analysis identified a quantitative trait locus (QTL) associated to C. pomonella susceptibility on the Discovery linkage group 10. The closest marker to this QTL was the random amplified polymorphic marker Z19-350. No significant QTL was identified for resistance to L. clerkella. A putative QTL associated to fruit number was identified on Fiesta linkage group 12. The presented QTL associated with C. pomonella susceptibility and the putative QTL linked to fruit number may facilitate marker-assisted breeding of resistant apple cultivars with cropping traits desirable for optimal fruit production.