Systemic spread of Pseudomonas savastanoi pv. savastanoi in olive explants

The manner in which the bacterium Pseudomonas savastanoi pv. savastanoi (Pss), the causal agent of knot disease, infects olive plants is erratic and has not been fully documented. To investigate the process of Pss invasion, olive explants were inoculated in vitro and examined visually and by light microscopy at 2-weekly intervals for 10 weeks. In all host genotypes tested, interaction with the pathogen resulted in: (i) a progressive collapse of the stem, originating at the inoculation site at the apex of the explant, and proceeding downwards; and (ii), localized outgrowths on the stem located at various distances from the inoculation site. Histological analysis revealed that the anatomy of the outgrowths closely resembled that of knots formed in vivo; they showed that Ps. savastanoi also diffused within the olive explants through the xylem vessels, and that the olive host reacted to pathogen invasion, possibly by producing substances of polysaccharidic and/or phenolic nature.