Variabilità of Pseudomonas savastanoi population isolated from myrtle.

On the base of the pathogenic ability of strains isolated from different hosts, the populations of Pseudomonas savastanoi are subdivided in six pathovars: savastanoi, nerii, fraxini, retacarpa, glycinea and phaseolicola. The typical symptom caused by pvs savastanoi, nerii and retacarpa is an outgrowth of plant cell tissue called knot, to whose differentiation concur auxins and cytokinins synthesized by the bacterium; the ash strains P. s. pv fraxini induce wart-like excrescences; on the other hand the strains from bean and soybean P. s. pvs glycinea and phaseolicola cause halo blight and blight on their respective hosts. Regarding phenotypic characters a great heterogeneity has been reported among the pvs savastanoi, nerii, fraxini and retacarpa as well as between strains belonging to the same pv. The presence of P. savastanoi on Myrtus communis has been reported only recently and a comparative analysis of the variability that exists within bacterial populations associated to this host has never been carried out. In this study we have characterized a set of strains originally isolated from myrtle in 3 different countries of the Mediterranean basin by means of a polyphasic approach including multilocus sequence analysis, substrate metabolisation profiles, phytohormone production and cross inoculation tests. While MLSA showed a low degree of variability, we observed a great heterogeneity both on phenotypic and phytopathogenic characters, suggesting the existence of a very complex population structure of P. savastanoi associated to this host.