Characterization of Venturia inaequalis pathogenicity on leaf discs of apple trees

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Characterization of pathogenicity on whole plants is required to study host-pathogen interactions between Malus × domestica and venturia inaequalis. We studied the reliability of an in vitro test of pathogenicity on leaf discs. Three strains of V. inaequalis (races 1, 6 and an English race) were inoculated in vitro onto a range of 16 Malus sp. clones including susceptible and resistant clones. The results were compared to those previously obtained in vivo. Resistant clones contained the main major known genes, i.e. Va, Vb, Vbj, Vf, Vg, Vm and Vr. Scab severity and the sporulation of the fungus were assessed 21 days after the inoculation date. The results indicated that it was possible to reproduce incompatible and compatible situations in vitro. A null severity corresponded to the avirulence of the strain for the clone considered. The resistance given by the Vb, Vbj, Vf, Vg, Vm and Vr genes were expressed in vitro. Only the clone carrying the Va gene and inoculated with the race 6 strain presented a compatible situation which was inconsistent with the observations on the whole plant. Improving this test will facilitate studies on the pathogenicity of V. inaequalis populations in relation to resistance genes of the host expressed in vitro as well as its genetic determinism.

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