A bacteriocin produced by Pseudomonas syringae pv. ciccaronei, used at different purification levels and concentrations in culture and in planta, inhibited the multiplication of P. syringae subsp. savastanoi, the causal agent of olive knot disease, and affected the epiphytic survival of the pathogen on the leaves and twigs of treated olive plants. Treatments with bacteriocin from P. syringae pv. ciccaronei inhibited the formation of overgrowths on olive plants caused by P. syringae subsp. savastanoi strains PVBa229 and PVBa304 inoculated on V-shaped slits and on leaf scars at concentrations of 10(5) and 10(8) CFU ml(-1), respectively. In particular, the application of 6,000 arbitrary units (AU) of crude bacteriocin (dialyzed ammonium sulfate precipitate of culture supernatant) ml(-1) at the inoculated V-shaped slits and leaf scars resulted in the formation of knots with weight values reduced by 81 and 51%, respectively, compared to the control, depending on the strains and inoculation method used. Crude bacteriocin (6,000 AU ml(-1)) was also effective in controlling the multiplication of epiphytic populations of the pathogen. In particular, the bacterial populations recovered after 30 days were at least 350 and 20 times lower than the control populations on twigs and on leaves, respectively. These results suggest that bacteriocin from P. syringae pv. ciccaronei can be used effectively to control the survival of the causal agent of olive knot disease and to prevent its multiplication at inoculation sites.