Spread of levan-positive populations of Pseudomonas savastanoi pv. savastanoi, the causal agent of olive knot, in central Italy

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.