Inheritance studies of apple scab resistance and identification of Rvi14, a new major gene that acts together with other broad-spectrum QTL

Scab, caused by the fungal pathogen Venturia inaequalis, is the most common disease of cultivated apple (Malus xdomestica). The fungal races 6 and 7 have now overcome the major resistance gene Vf, which is widely used in apple breeding programmes. New breeding strategies to achieve durable resistance are thus necessary. The aim of this study was to determine the genetic basis of quantitative resistance of the apple cultivar 'Dulmener Rosenapfel', known to be scab resistant under different environmental conditions. An F-1 progeny derived from the cross between the susceptible cultivar 'Gala' and 'Dulmener Rosenapfel' was tested in a greenhouse with a multi-isolate inoculum of V. inaequalis. Rvil 4, a new major gene that conditions a chlorotic-type reaction, was mapped on linkage group (LG) 6 in a genomic region not known to be involved in disease resistance. A further three quantitative trait loci (QTL) for resistance were identified. One co-localized with Rvi14 on LG6, whereas the remaining two were detected on LG11 and LG17, in genomic regions already reported to carry broad-spectrum QTL in other genetic backgrounds. Since a selective genotyping approach was used to detect QTL, an expectation-maximization (EM) computation was used to estimate the corrected QTL contributions to phenotypic variation and was validated by entire progeny genotyping.