No abstract

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.

Several species of Botryosphaeriaceae and

Pseudomonas savastanoi pv. savastanoi, the causal agent of olive knot disease, has for a long time been included in subgroup 1b of phytopathogenic, fluorescent Pseudomonas species by the LOPAT determinative tests (production of levan, oxidase, pectinolytic and arginine dihydrolase activity, tobacco hypersensitivity). Pseudomonas savastanoi pv. savastanoi differs from the Pseudomonads in subgroup 1a only in being levan-negative. However, in 1990, during a survey on the spread of olive knot in Tuscany, levanpositive isolates of P. savastanoi pv. savastanoi were isolated from knots on two olive trees in an orchard in the province of Florence (Bagno a Ripoli). Some years later, to assess the further spread of levan production in populations of P. savastanoi pv. savastanoi, the survey was extended to 39 other orchards randomly scattered across Tuscany, and levan-positive bacteria were found in approximately 38% of these orchards. Phenotypic, genotypic and pathogenic characterisation allowed these levan-positive isolates to be assigned to P. savastanoi pv. savastanoi. The data suggest that in Tuscan olive orchards, levan-positive and levan-negative subpopulations of this phytopathogenic bacterium can coexist on the same plant. On the basis of the results obtained we suggest that subgroups 1a and 1b of the LOPAT determinative scheme should be combined, and that P. savastanoi should be considered a bacterial species that can be either levan-negative or levan-positive.