Ralstonia solanacearum requires a Type III (T3) secretion system for bacterial wilt pathogenesis, but the biological functions of individual effectors remain unknown. During tomato wilt, R. solanacearum expresses popS, which encodes an AvrE-family T3 effector. popS homologs were present in all 17 sequenced R. solanacearum strains, and the phylogeny of popS parallels that of the R. solanacearum species complex, suggesting that PopS is an ancient effector needed for association with plants. We determined that popS is required for full virulence on multiple Solanum crop hosts (susceptible potato and susceptible and quantitatively resistant tomato), but not for virulence on a related epidemiologically relevant weed, S. dulcamara. The popS mutant was also significantly delayed in tomato stem colonization following direct inoculation through cut petioles. AvrE-type effectors in other plant pathogenic bacteria suppress plant defenses triggered by the plant signaling molecule salicylic acid (SA). The popS mutant induced higher expression of SAresponsive tomato PR genes than its wild-type parent. Further, pretreating plant roots with SA exacerbated the popS virulence defect. Finally, PopS was dispensable for bacterial colonization of SA-deficient NahG transgenic tomato plants. These results indicate that this conserved T3 effector suppresses SAmediated defenses in tomato roots and stems, which are the natural infection courts of this soilborne vascular pathogen. (texte intégral)