Identification of Venturia inaequalis Races in Morioka, Japan and Identification of a Quantitative Trait Locus Associated with Resistance to Apple Scab in ‘Akane’ Apples

Apple scab, which is caused by Venturia inaequalis (Cooke) G. Wint., is a destructive disease that affects apples (Malus × domestica Borkh.). To develop an approach to breeding apples for scab resistance, race distribution of V. inaequalis in Morioka, Japan and genetic factors affecting partial scab resistance observed on ‘Akane’ were assessed. An inoculation test using several differential hosts suggested that race 1 of V. inaequalis was the predominant race in Morioka, Japan. To characterize the genetic profile of ‘Akane’ resistance, quantitative trait locus (QTL) analyses that employed two years of natural scab infection scores in a fungicide-free orchard in Kuroishi, Aomori, Japan were performed using an F1 population derived from the ‘Orin’ × ‘Akane’ cross. A QTL allele that explained 20% of the phenotypic variance in scab resistance was detected on chromosome 17 of ‘Akane’. In the middle region of chromosome 17 of ‘Orin’ and in very close proximity to the ‘Akane’ QTL, a weak QTL allele was also detected, which explained 5% of the phenotypic variance observed. We considered that these effects were caused by alleles of the identical QTL. Haplotyping analysis indicated that this QTL resistance allele from ‘Akane’ originated from ‘Worcester Pearmain’.