Phenotypic Reaction and Genetic Analysis Using AFLP-derived SCARs for Resistance to Apple Scab

Six sequence-characterized amplified region (SCAR) markers linked to the apple scab resistance gene Vf were evaluated for their utility in marker-assisted selection (MAS) in apple breeding. Of the six SCARs used in this study, ACS-6 was located left of the Vf gene, ACS-7 and ACS-9 co-segregated with Vf, and ACS-8, ACS-4, ACS-5 were located right of the Vf gene. Three families derived from crosses between scabresistant and scab-susceptible cultivars, including Liberty · Deljub , Liberty · Delcorf , and Florina · Delcorf , previously screened for scab resistance following greenhouse inoculation with the fungal pathogen Venturia inaequalis, were genotyped and compared with phenotypic reactions to scab infection in the field. For each family, a subset progeny of 30seedlin gs (propagated onto Malling 9 rootstock and of 7 years old) was selected based on fungal sporulation according to the following scheme. Ten seedlings with no visible scab sporulation on leaves were given phenotypic scores of 0(deemed resistant); 10seedlings with moderate scab sporulation were given phenotypic scores of 1.0(deemed moderately resistant); and 10seedlin gs with heavy sporulation were given phenotypic scores of 2.0(deem ed susceptible). DNA was isolated from leaf tissue collected from all 90seedlings, parents and Malus floribunda 821, the original source of the Vf gene, and screened with all six SCARs. All six SCARs were present in the two scab-resistant parents, Liberty and Florina , and M. floribunda 821; while, the two scab-susceptible parents, Deljub and Delcorf , lacked all SCARs. All SCARs were either present or absent in varying numbers of seedlings in each progeny with phenotypic ratings of either 0 (resistant) or 1.0(moder ately resistant); while all seedlings with phenotypic ratings of 2.0(suscep tible) lacked all SCARs. The inconsistencies between phenotypic scab ratings and SCAR marker data are discussed.