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 investig ate variation in virulence in Venturia inaequalis populations towards comme rcial apple cultivars. For this purpose, primary lesions were sampled in or chards with different varietal compositions and diversities. The virulence pattern of monosporic isolates, obtained by isolation of single conidia, wa s assessed by cross inoculations of the cv. Ananas Reinette, Boskoop, Glock enapfel, Golden Delicious, Gravenstein, James Grieve, Jonathan, Mai-gold, R einette de Champagne, Spartan and Yellow Transparent. All cultivars were su sceptible to some isolates and resistant to others. No cultivar behaved the same way, which suggests the presence of differential resistance in each c ultivar and corresponding virulence in some isolates. Isolates from a monoc ulture of Golden Delicious consisted mainly of a pathotype that was virulen t to Golden Delicious but not to other cultivars. In the samples from culti var mixtures, virulence pattern variation was considerable. The results giv e further evidence of the existence of a large pool of differential and eph emeral resistances in Malus, which were overcome by the local scab populati ons during co-evolution.