Apple (Malus x domestica Borkh. cv. Golden Delicious) trees were inoculated with fungal pathogens, corresponding either to an incompatible strain of Venturia inaequalis or to non-host pathogens (Venturia pyrina, Alternaria brassicicola) in order to characterize the regulation of PR-10 genes in these different situations in relationship to symptom development. Macro- and microscopic observations of the plant-fungus interactions revealed typical symptoms of resistance with V. inaequalis and symptoms of hypersensitivity (HR) on around 5 % of leaves with V. pyrina. No HR was microscopically observed with A. brassicicola. In the non-host situations, the PR-10 expression in leaves at the transcriptional level was not (A. brassicicola) or very slightly (V. pyrina) activated. This strongly suggests that PR-10 is not a component of the non-host resistance in the interactions studied here. By contrast specific up-regulation of PR-10 was evident after inoculation with V. inaequalis