Interaction between Pseudomonas savastanoi pv. savastanoi and Pantoea agglomerans in olive knots

Results of a survey of olive knot disease in central Italy in 2002 and 2003 showed that Pantoea agglomerans was found associated with the pathogen Pseudomonas savastanoi pv. savastanoi (Ps. savastanoi) in 70% of the olive knots examined. Pathogenicity tests in which these two bacteria were co-inoculated on the stems of 1-year-old olive plants at ratios of 1:1, 1:100 and 100:1 showed that the growth of P. agglomerans was apparently aided by the presence of an actively growing population of Ps. savastanoi. At the same time, however, a dominant population of P. agglomerans at the inoculation site tended to depress the growth of Ps. savastanoi, probably because of competition for space and nutrients between these bacteria and by means of antibiotic production by P. agglomerans. In some cases the association of P. agglomerans, which in culture was found to produce indole-3-acetic acid but not cytokinins, with Ps. savastanoi resulted in an increase in the size of knots. This boosting effect of P. agglomerans on proliferation was probably due to the release of IAA by this bacterium at the inoculation sites.