A Detached Leaf Assay to Evaluate Virulence and Pathogenicity of Strains of Pseudomonas syringae pv. syringae on Pear. S. Yessad, INRA Station de Pathologie Vegetale, Centre de recherches d’Angers, rue Georges Morel, 49070 Beaucouze, France. C. Manceau, and J. Luisetti. INRA Station de Pathologie Vegetale, Centre de recherches d’Angers, rue Georges Morel, 49070 Beaucouze, France. Plant Dis. 76:370-373. Accepted for publication 9 August 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0370.

Results of the inoculation of 19 strains of Pseudomonas syringae pv. syringae on different plant material, including pear seedlings (Pyrus communis), detached leaf of pear seedlings, in vitro microcuttings, and detached leaf of in vitro microcuttings, pointed out the high susceptibility of in vitro microcuttings. Four groups of virulence were distinguished among the 19 strains. Virulent strains developed black and progressive lesions on leaves either detached or not. Weakly virulent strains induced limited necrotic spots around the inoculation point, only when a high inoculum concentration (10⁸ cfu/ml) was used. No symptoms on any plant material were observed for avirulent strains. These results were correlated with those obtained after inoculation of immature pear fruits and by inoculation of blossoms in the orchard. The detached leaf assay described herein is a very useful laboratory test to check the virulence of a high number of strains, because symptoms appear within 48 hr after inoculation.