Phenolic compounds of apple and their relationship to scab resistance

The contents of 15 different phenolic compounds in apple leaves and fruit skin from various orchards were determined by HPLC, High concentrations of flavan-3-ols were found in those orchards which were not infected by Venturia inaequalis. This observation corroborates earlier findings on different cultivars. The hypothesis that these compounds could be involved in the resistance of apple to scab was tested by inhibiting the key enzyme of the phenol biosynthesis, the phenylalanine-ammonia-lyase, in young shoots of the resistant genotype 'Sir Prize'. After inoculation of the inhibitor-treated leaves, severe symptoms of the disease occurred. The fungal infection was confirmed by histological studies. No flavonal-accumulation could be observed at the infection site which was the case in the non-inhibited but inoculated controls.