Natural disease resistance of apples against scab

Apple scab is a major problem causing serious financial losses to apple growers in orchards, and during storage for processors. We have identified six unusual phytoalexins that seem to be part of the resistance mechanism in scab resistant apple cultivars (see Phytopathology 87:868-876, Phytochemistry 50: 231-235). As follow-up on our earlier investigation using scab-susceptible and scab-resistant cell cultures, we have analyzed phenolic compounds in leaves, stems and roots of young apple plants, in the intercellular spaces of leaves and in fruits after treatment with elicitor preparations from yeast and V. inaequalis. The major phenolic compounds found in both non-elicited or non-infected, and elicited or infected leaves were phloridzin and phloretin that accumulated in mM concentrations. The major metabolites in elicitor treated fruit sections of the scab-resistant cultivar Liberty were p-coumaric and benzoic acids. The presence of biphenyl or dibenzofuran compounds could not be detected in these tissues/compartments. We have also established cell-suspension cultures of Malus floribunda 821, an ornamental apple from which scab-resistance derives in all commercial apple cultivars. Treatment of this cell culture with elicitor preparations from yeast and V. inaequalis resulted in the production of both biphenyl and dibenzofuran compounds. These results indicate a differential response of plant organs and cell suspension cultures to elicitor treatment or pathogen invasion. We are in the process of characterizing the biochemical pathway that produces these phytoalexins. This work serves as a basis for developing novel strategies to understand the natural resistance mechanism of apples against the scab-causing organism, Venturia inaequalis, and to introduce resistance in commercially important apple cultivars.