DETECTION OF VARIATION IN VIRULENCE TOWARD SUSCEPTIBLE APPLE CULTIVARS IN NATURAL-POPULATIONS OF VENTURIA-INAEQUALIS

Isolates of Venturia inaequalis taken from primary lesions on seven apple cultivars (Golden Delicious, Idared, Maigold, Glockenapfel, Boskoop, Spartan, and James Grieve) were tested as mixtures or alone in cross-infection trials on graft trees of the seven cultivars. In general, isolates produced the most lesions on the cultivar genotypes from which they were isolated. None of the isolates produced the same infection type on all cultivars. Two cultivars, Boskoop and Glockenapfel, were the most resistant to most inocula but considerably less resistant to the isolates originating from them. Maigold and Golden Delicious were infected severely by all isolates. In general, specific pathogen genotypes showed specialization toward particular host genotypes. Monoconidial cultures were identified and distinguished by random amplified polymorphic DNA (RAPD) markers. In competition experiments among isolates from different cultivars inoculated on single hosts, there was strong selection against isolates not originating from the particular test cultivar. This was true even for the most susceptible apple cultivars. These results indicate the presence of differential resistance factors in susceptible cultivars that exert selection on natural V. inaequalis populations that vary in virulence. The implications of these observations for disease control are discussed.