Virulence pattern of Venturia inaequalis field isolates and corresponding differential resistance in Malus x domestica

All commercially important apple cultivars are susceptible in the field to scab caused by Venturia inaequalis. The scope of this study was to investigate variation in virulence in Venturia inaequalis populations towards commercial apple cultivars. For this purpose, primary lesions were sampled in orchards with different varietal compositions and diversities. The virulence pattern of monosporic isolates, obtained by isolation of single conidia, was assessed by cross inoculations of the cv. Ananas Reinette, Boskoop, Glockenapfel, Golden Delicious, Gravenstein, James Grieve, Jonathan, Mai-gold, Reinette de Champagne, Spartan and Yellow Transparent. All cultivars were susceptible to some isolates and resistant to others. No cultivar behaved the same way, which suggests the presence of differential resistance in each cultivar and corresponding virulence in some isolates. Isolates from a monoculture of Golden Delicious consisted mainly of a pathotype that was virulent to Golden Delicious but not to other cultivars. In the samples from cultivar mixtures, virulence pattern variation was considerable. The results give further evidence of the existence of a large pool of differential and ephemeral resistances in Malus, which were overcome by the local scab populations during co-evolution.