Resistance in apple to shoot infection by Venturia inaequalis

Eight apple genotypes, including cultivars and breeding selections resistant and susceptible to Venturia inaequalis on foliage, were screened for shoot infection and the development of wood pustules following inoculation of shoot tips of 1–year maiden trees in the greenhouse. Where genotypes were highly resistant in terms of foliar symptoms (cvs Prima and Gavin), no shoot infection was observed. Where genotypes were highly susceptible in terms of foliar symptoms, then shoot tissue was either resistant (cv. Shinko) or susceptible (cv. Starking). In one experiment, shoot tips were inoculated sequentially as shoots extended. No pustules developed where inoculations were made later than May. In a further experiment, two leaf internode positions were inoculated on one occasion. Pustules were only observed where inoculations were made above the youngest unrolled leaf. The results suggested that shoot tips were more susceptible during early extension growth of the shoot. Pustules were noted in abundance on petioles of susceptible cultivars, and these probably contributed to early leaf abscission.