Development of a DNA marker for Vm, a gene conferring resistance to apple scab

Primers of arbitrary sequence were screened by bulked segregant analysis to identify DNA fragments linked to Vm, a gene conferring resistance to apple scab (Venturia inaequalis). A 687-bp fragment generated by 10-base primer OPB12 gave tight linkage (no recombinants) in a population of 59 individuals from the cross 'Empire' x NY74828-12. The DNA fragment was sequenced, and two specific primers were generated to produce a STS marker for Vm. These primers were used in New Zealand to screen a second population of 180 individuals from a different cross. In this population, the marker displayed 8% recombination with Vm. A survey of Malus germplasm indicated that the marker was present in relatively few accessions, but was present in all known sources of Vm, including the cultivar Murray. However, none of the other sources of scab resistance, nor any other commercial cultivar, displayed the OPB12 fragment. These results suggest that this marker should be useful for the pyramiding of scab resistance genes in cultivated apple and, perhaps, for identifying other accessions in the Malus germplasm collection that possess Vm.