SUMMARY

Ninety fluorescent pseudomonads obtained from pear cultivars Coscia and Tosca showing symptoms of bacterial blast were assessed by biochemical tests and pathogenicity on lemon fruits, and pear and lilac leaves. Presence of the syrB gene and ice-nucleation activity was also determined. In addition, genetic variability was assessed using repetitive sequence PCR with the BOX A1 primer. The patterns were analyzed with UPGMA and Dice’s coefficients. Eighty-nine isolates were deter-mined as putative Pseudomonas syringae pv. syringae. Four genomic patterns, A, B, C and D, were obtained by BOX-PCR analysis. Patterns A and C were identical to those shown by other P. s. pv. syringae strains previ-ously isolated from pear in other areas. Pattern B was associated solely with leaf spot symptoms observed on cv Coscia. The syrB gene was present in 38.8 % of the isolates, whereas ice-nucleation activity was observed in 77.7%. Isolates possessing the syrB gene were the most aggressive in pathogenicity tests. Different populations of P. s. pv. syringae appear to be involved in inducing blast symptoms on pear in central Italy. Key words: bacterial blast, syrB gene, ice-nucleation activity, repetitive sequence PCR. Pseudomonas syringae pv. syringae van Hall has been repeatedly found associated with blast of pear (Pyrus communis L.) causing economic losses in many areas of the world. The main symptoms are bud and blossom blast, necrosis of immature fruits, leaf and fruit spots, and cankers on branches and trunk. Studies have clari-fied the relationships between the susceptibility of pear flower buds to frost, presence of ice-nucleation active bacteria (Proebsting and Gross, 1988), and the condi-tions of temperature, moisture and stage of bloom de-velopment favourable for inducing blossom blas