Virulence Pattern of Venturia inaequalis Field Isolates and Corresponding Differential Resistance in Malus x domestica

All commercially important apple cultivars are sus! ceptible in the _eld 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 di}erent 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 Trans! parent[ All cultivars were susceptible to some isolates and resistant to others[ No cultivar behaved the same waywhich suggests the presence of di}erential 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 sam! ples from cultivar mixtures\ virulence pattern variation was considerable[ The results give further evidence of the existence of a large pool of di}erential and ephemeral resistances in Malus\ which were overcome by the local scab populations during co!evolution[