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Quantitative Trait Loci (QTL) Analysis Reveals Both Broad-Spectrum and Isolate-Specific QTL for Scab Resistance in an Apple Progeny Challenged with Eight Isolates of Venturia inaequalis

First, second, and seventh authors: Institut National de la Recherche Agronomique (INRA), Unité Mixte de Recherche Génétique et Horticulture GenHort, 42 rue Georges Morel, BP 57, 49071 Beaucouzé cedex, France; third author: FAW-Swiss Federal Research Station for Fruit-Growing, Viticulture and Horticulture, 8820 Wädenswil, Switzerland; fourth author: Max-Planck-Institute for Breeding Research, Carl von Linne Weg, 10, 50829 Cologne, Germany; fifth author: Plant Research International, P.O. Box 16, Droevendaalsesteeg 1, 6700 AA Wageningen, The Netherlands; and sixth author: INRA, Unité Mixte de Recherche en Pathologie Végétale, 42 rue Georges Morel, BP 57, 49071 Beaucouzé cedex, France

The major scab resistance gene Vf, extensively used in apple breeding programs, was recently overcome by the new races 6 and 7 of the fungal pathogen Venturia inaequalis. New, more durable, scab resistance genes are needed in apple breeding programs. F₁ progeny derived from the cross between partially resistant apple cv. Discovery and apple hybrid ‘TN10-8’ were inoculated in the greenhouse with eight isolates of V. inaequalis, including isolates able to overcome Vf. One major resistance gene, Vg, and seven quantitative trait loci (QTL) were identified for resistance to these isolates. Three QTL on linkage group (LG)12, LG13, and LG15 were clearly isolate-specific. Another QTL on LG5 was detected with two isolates. Three QTL on LG1, LG2, and LG17 were identified with most isolates tested, but not with every isolate. The QTL on LG2 displayed alleles conferring different specificities. This QTL co-localized with the major scab resistance genes Vr and Vh8, whereas the QTL on LG1 colocalized with Vf. These results contribute to a better understanding of the genetic basis of the V. inaequalis-Malus × domestica interaction.