Does the Vf gene maintain a residual resistance to apple scab despite its breakdown by Venturia inaequalis race 6 strains?

The scab-resistance gene Vf derived from Malus floribunda 821, has been widely used in apple breeding programmes. This major gene is located on linkage group of the cultivar 'Prima' in the genetic map constructed by Maliepaard et al. (1998). Two new races of the fungus Venturia inaequalis(is able to overcome the resistance conferred by Vs have been identified (Parisi et al., 1993; Roberts and Crute, 1994). In the present study, the progeny from the cross 'Prima' x 'Fiesta' was inoculated with a monoconidial strain of race 6. This strain had already been shown to be able to break down the resistance of Vf but not the resistance of the progenitor M. floribunda 821 itself (Parisi and Lespinasse. 1996). Fourteen and 21 days after inoculation, plants were scored for resistance/susceptibility level. A QTL analysis was performed using the software MCQTL. A strong QTL (LOD 6 to 7) was found in the region of VS on linkage group the resistance allele was associated with the presence of the Vf allele. This QTL was able to explain between 17% and 20% of the variation in disease symptoms. The confidence interval of the QTL did not allow us to determine whether this partial resistance was located on the Vf gene itself or close to Vf but excluding it.