QTL analysis of the genetic architecture determining resistance to fire blight in an apple progeny

Fire blight, caused by the bacterial pathogen Erwinia amylovora, is one of the most destructive diseases of apple (Malus x domestica). In order to analyse the genetic determinism of resistance to fire blight in apple, a quantitative trait analysis (QTL) approach was used. A F, progeny of 164 individuals derived from a cross between the apple cultivars 'Prima' and 'Fiesta' was inoculated in greenhouse conditions. Seven copies per genotype were used. The length of the necrosis observed on shoots was scored 7 and 14 days after inoculation. The MapQTL software was used for QTL analyses, using two previously built maps of the parents, and the symptoms scored on shoots. Digenic interactions between all pairwise combinations of genetic markers were tested using a two-way ANOVA model with the SAS software. QTL were detected at the same locations both 7 and 14 days after inoculation. Two weak effect QTL deriving from 'Prima' were detected on linkage groups (LG) 3 and LG16. One strong effect QTL deriving from 'Fiesta' was detected on LG7 that explained 46.6% of the phenotypic variation observed in the progeny. Two additional significant (P < 10(-4)) digenic interactions were identified in the progeny between markers on LG9 and LG1, and markers on LG9 and LG5. The existence of a strong effect QTL in 'Fiesta' is of particular interest for breeding purposes.