PATHOGENICITY OF VENTURIA-INAEQUALIS STRAINS OF RACE-6 ON APPLE CLONES (MALUS SP)

About 40 cultivars derived from Malus floribunda 821 (Vf gene) are res istant to the five races of Venturia inaequalis previously described. Race 6, recently identified, is able to overcome the resistance of som e selections carrying the Vf gene. Thirty-seven apple clones were test ed for their susceptibility to different (races 1 and 6) strains of Ve nturia inaequalis. These clones represented a large sample of the resi stance sources used by apple breeders throughout the world (selected c lones and their ancestors). Conversely, race 6 induced symptoms on nea rly all the Vf selections, and also on Nova Easygro (Vr), on a selecti on carrying Va, on Jonsib, and on a selection derived from Malus atros anguinea 804. Race 6 did not attack susceptible cultivars such as Gran ny Smith, three selections from different polygenic sources of resista nce, Malus baccata jackii (Vbj), R 127.40.7A (Vr), and PI 172623 (Va). The pedigrees of three hybrids selected for their Vf resistance were analyzed: all the resistant race 1 parents involved in these pedigrees were susceptible to race 6, except the ancestor M. floribunda 821 and one of its Fz selected progeny (F-2 26829-2-2). The other F-2 (F-2 26 830-2) was susceptible. These results confirmed the hypothesis of a co mplex genetic background for resistance to Venturia inaequalis in wild species, which has been partly lost during the breeding process. The variability of the resistance genes introgressed in recent selections could be lower than believed, and reduced to the Vf gene in most cases . Apple breeders should increasingly focus on finding polygenic source s of resistance in order to obtain a durable resistance to scab.