MULTIPLE FIELD AND GLASSHOUSE ASSESSMENTS INCREASE THE RELIABILITY OFLINKAGE MAPPING OF THE VF SOURCE OF SCAB RESISTANCE IN APPLE

Apple scab, caused by the fungus Venturia inaequalis (Cke.) Wint., is an important disease in commercial apple production. A mapping populat ion of 155 individuals, derived from a cross between the apple varieti es 'Prima' (resistant) x 'Fiesta' (susceptible), was scored for respon se to the disease in replicated held and glasshouse trials throughout Europe. Twenty data sets were selected and cluster analysis was used t o form a consensus score for the population fitting a 1:1 segregation ratio of resistance:susceptibility. The progeny were scored with molec ular markers. A detailed map covering 54 cM of the 'Prima' linkage gro up containing the Vf gene for scab resistance was constructed using 24 molecular markers linked to the resistance gene. One isoenzyme marker (Pgm-1), six RFLP markers and 17 RAPD markers formed a linkage group with the consensus measure of resistance to scab. Four marker bridges were established with the corresponding 'Fiesta' linkage group with ad ditional markers (one isozyme, one RFLP, three RAPD and one AFLP). A l ow chi-square value indicated a good fit of the marker ordering, which was in close agreement with previously reported linkage positions for some of the markers and Vf. Differences were observed in the ability of different scoring methods to resolve susceptible and resistant clas ses. The results obtained for the consensus classification of resistan ce to scab for the population may suggest the presence of virulent ino cula at some sites, which could overcome the Vf gene for resistance. T he consequences of relying on individual scoring occasions for studyin g Vf scab resistance are discussed in the context of linkage analysis, conventional breeding selection, and marker-assisted selection.