Linkage of Vfa4 in Malus x domestica and Malus floribunda with Vf resistance to the apple scab pathogen Venturia inaequalis

The Vf locus from Malus floribunda clone 821 is an important source of resistance to apple scab disease caused by Venturia inaequalis, and has been introduced into numerous cultivars of domesticated apple, Malus x domestica. Cloning of the putative Vf locus has revealed that it contains several receptor-like Vf candidate genes. In order to determine which of these genes is most closely linked to Vf resistance, primers were designed based on conserved regions in the Vf candidate genes adjacent to a variable portion of the leucine-rich repeat (LRR) domain, to yield PCR product length polymorphisms. PCR products were obtained from 31 cultivars of M. x domestica, of which 19 are known to contain Vf resistance, and from 10 selections of M. floribunda. PCR products corresponding in size to Vfa1 and Vfa2 were found in all the plants tested. However, a PCR product with 100% predicted amino acid identity to Vfa4 was found only in M. xdomestica cultivars known to have Vf scab resistance. This PCR product was also found in most, but not all, selections of M. floribunda tested, including the original source of Vf resistance, M. floribunda 821. The PCR product matching Vfa4 appears to be the most closely linked to Vf resistance and should be a valuable tool for monitoring Vf inheritance in apple.