Essential oils of Origanum compactum and Thymus vulgaris exert a protective effect against the phytopathogen Allorhizobium vitis

National audience ; Allorhizobium (Agrobacterium) vitis is a host-specific pathogenic bacterium that causes grapevine crown gall disease, affecting vine growth and production worldwide. The antibacterial activities of different aromatic plant essential oils were tested in vitro and in planta against A. vitis. Among the essential oils tested, those of Origanum compactum and Thymus vulgaris showed the most significant in vitro antibacterial activities, with a MIC of 0.156 and 0.312mg/mL, respectively. A synergistic effect of these two essential oils (1:1) was observed and confirmed by the checkerboard test. Carvacrol (61.8%) and thymol (47.8%) are, respectively, the major compounds in the essential oils of O. compactum and T. vulgaris and they have been shown to be largely responsible for the antibacterial activities of their corresponding essential oils. Results obtained in vitro were reinforced by an in planta pathogenicity test. A mixture of O. compactum and T. vulgaris essential oils (1:1), inoculated into the injured stem of a tomato plant and a grapevine at 0.312mg/mL as a preventive treatment, reduced both the number of plants developing gall symptoms and the size of the tumors.