Mapping of the apple scab-resistance gene Vb.

Apple scab, caused by the fungus Venturia inaequalis, is the major production constraint in temperate zones with humid springs. Normally, its control relies on frequent and regular fungicide applications. Because this control strategy has come under increasing criticism, major efforts are being directed toward the breeding of scab-resistant apple cultivars. Modern apple breeding programs include the use of molecular markers, making it possible to combine several different scab-resistance genes in 1 apple cultivar (pyramiding) and to speed up the breeding process. The apple scab-resistance gene Vb is derived from the Siberian crab apple 'Hansen's baccata #2', and is 1 of the 6 "historical" major apple scab-resistance genes (Vf, Va, Vr, Vbj, Vm, and Vb). Molecular markers have been published for all these genes, except Vr. In testcross experiments conducted in the 1960s, it was reported that Vb segregated independently from 3 other major resistance genes, including Vf. Recently, however, Vb and Vf have both been mapped on linkage group 1, a result that contrasts with the findings from former testcross experiments. In this study, simple sequence repeat (SSR) markers were used to identify the precise position of Vb in a cross of 'Golden Delicious' (vbvb) and 'Hansen's baccata #2' (Vbvb). A genome scanning approach, a fast method already used to map apple scab-resistance genes Vr2 and Vm, was used, and the Vb locus was identified on linkage group 12, between the SSR markers Hi02d05 and Hi07f01. This finding confirms the independent segregation of Vb from Vf. With the identification of SSR markers linked to Vb, another major apple scab-resistance gene has become available; breeders can use it to develop durable resistant cultivars with several different resistance genes.