Spread of an atypical population of Pseudomonas savastanoi on Myrtus sp.in Sardinia.

Virulent strains of the phytopathogenic bacterium Pseudomonas savastanoi induce parenchymatous galls (knots) at the site of bacterial infection. Knots are mostly seen on the young stems, branches and twigs of different plants belonging to Oleaceae (Olea, Jasminum, Forsythia, Phillyrea, Ligustrum), Fabaceae (Retama), Rhamnaceae (Rhamnus), Myrtaceae (Myrtus) or to Apocinaceae (Nerium, Mandevilla). On ash (Fraxinus spp.) P. savastanoi causes wart like excrescences. During surveys carried out in the spring of 2010 in six locations of north-western Sardinia (Italy) on different selections of myrtle (Myrtus communis) and also on spontaneous plants, canker-like lesions of about 1 cm long were observed on branches and twigs. From the tissues below the bark, which appeared dark brown, dried and cracked, was constantly isolated a bacterium that, on the basis of the results of biochemical tests and molecular typing methods, was identified as Pseudomonas savastanoi. Pathogenicity tests were performed with 2 isolates by placing a bacterial suspension containing 108 cfu/ml on wounds made in the bark of twigs of Myrtus communis var. tarentina. After 4 to 6 weeks the centre of the wound was surrounded by an irregular mound of new tissue. Occasionally after 2-3 months these new formed tissues had joined together over the surface of the wound. Typical “knots” were never observed. Further characterization is now in progress to better evaluate the differences that exist between the myrtle population of P. savastanoi found in Sardinia and knot inducing strains isolated from Myrtus sp. in other countries of the Mediterranean basin.