Preliminary genetic mapping of fire blight and European canker resistances in two apple breeding families

Breeding for durable disease resistance has long been a major target in the apple breeding program at Plant and Food Research. Historically, pre-breeding has focused on the introgression of major effect quantitative trait loci (QTLs) sourced from wild apples, requiring multiple generations of backcrossing. In recent years, this approach has been complemented by the identification of minor effect resistances carried by advanced breeding germplasm, including commercial apple cultivars that can be employed immediately as parents in new cultivar breeding. To obtain data for QTL mapping, we phenotyped seedling populations AA0648 and AA0701 for fire blight resistance by cut-leaf inoculation in the glasshouse, and the AA0701 family for European canker resistance by rasp wound inoculation in the field. Based on best linear unbiased prediction (BLUP) analysis, a single fire blight resistance QTL was identified in both parents of family AA0648, mapping to linkage groups LG4 of LSQUOPremA129RSQUO and LG15 of the selection parent. The combined QTLs exhibited an additive effect for a stronger, and possibly more durable resistance in this family. A third putative QTL was indicated on LG8 of A045R14T055 in family AA0701. Interestingly, no QTL was identified in the AA0701 parent 'Scired', which is moderately resistant to fire blight, whereas there was a near-significant LG4 QTL in its progeny, 'PremA129', suggesting that its other (grand)parents, all regarded as susceptible, contributed to its resistance. In family AA0701, European canker resistance QTLs were identified on LG8 of 'Scired' and LG16 of A045R14T055. These QTLs will complement those reported previously and assist the development of marker assisted selection for new apple cultivars with more durable as well as stronger resistances.