GENETIC AND PATHOGENIC DIVERSITY OF PSEUDOMONAS SYRINGAE pv. SYRINGAE ISOLATES ASSOCIATED WITH BUD NECROSIS AND LEAF SPOT OF PEAR IN A SINGLE ORCHARD

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 determined 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 previously 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.