Phenolic profiles in apple leaves and the efficacy of selected phenols against fire blight (Erwinia amylovora)

The content and type of phenolic compounds in apple leaves as potential markers of resistance to fire blight were analysed. The amounts of phenolic acids and flavonoids were determined before and after E. amylovora inoculation of leaves of two cultivars: ‘Enterprise’ (highly resistant) and ‘Idared’ (highly susceptible). The basic levels of phenolics in both cultivars was similar but, following the inoculation, in the resistant one faster and more distinguishable changes were observed. The difference between the cultivars was related to the content of the compounds and the rate of release of free phenols from their glucosides. Regarding age dependency, the levels of eight out of 15 phenolics was significantly higher in young leaves of ‘Idared” than in ‘Enterprise’. In the older leaves the differences were limited to four compounds. The amount of salicylic acid in ‘Idared’ was lower than in ‘Enterprise’. In ‘Idared’ accumulation of salicylate after infection was better pronounced than in ‘Enterprise’. Higher levels of naringenin glucosides, 4-hydroxbenzoic acid and gentisic acid were found in ‘Enterprise’. The activity of 13 phenolics tested in vitro against the pathogen showed that gallic acid, phloroglucinol, hydroquinone and phloretin, suppressed its growth. The aqueous solutions of gallic acid, phloroglucinol and hydroquinone also significantly limited the development of disease on pear fruitlet slices but only hydroquinone maintained its protective activity for longer time. It also showed very high efficacy in preventing disease spread on apple shoots. The study adds novel information on the contribution of specific phenolics to apple resistance to fire blight.