Pathogenicity of Venturia inaequalis Strains of Race 6 on Apple Clones (Malus sp.). L. Parisi, Station de Pathologie Vegetale et Phytobacteriologie; INRA, 42, rue Georges Morel, BP 57, 49071 Beaucouze Cedex, France. Y. Lespinasse, Station d' Amelioration des Especes Fruitieres et Ornementales, INRA, 42, rue Georges Morel, BP 57, 49071 Beaucouze Cedex, France. Plant Dis. 80: ll 79-1183. Accepted for publication 8 July 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1179.

About 40 cultivars derived from Malus floribunda 821 (Vf gene) are resistant to the five races of Venturia inaequalis previously described. Race 6, recently identified, is able to overcome the resistance of some selections carrying the Vf gene. Thirty-seven apple clones were tested for their susceptibility to different (races 1 and 6) strains of Venturia inaequalis. These clones represented a large sample of the resistance sources used by apple breeders throughout the world (selected clones and their ancestors). Conversely, race 6 induced symptoms on nearly all the Vf selections, and also on Nova Easygro (Vr), on a selection carrying Va, on Jonsib, and on a selection derived from Malus atrosanguinea 804. Race 6 did not attack susceptible cultivars such as Granny Smith, three selections from different polygenic sources of resistance, Malus baccata jackii (Vbj), R 127.40.7A (Vr), and Pl 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 F2 selected progeny (F2 26829-2-2). The other F2 (F2 26830-2) was susceptible. These results confirmed the hypothesis of a complex 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 sources of resistance in order to obtain a durable resistance to scab.