A total of 118 fluorescent pseudomonads associated with hazelnut decline, which has been occurring for many years in different areas of northern Greece and Italy, were assessed by performing a repetitive PCR analysis with enterobacterial repetitive intergenic consensus, box element, and repetive extragenic palindromic primer sets, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of whole-cell protein extracts, a carbon compound utilization analysis, and an analysis to determine the presence of the syrB gene. A subset of 53 strains was also characterized by amplified 16S ribosomal DNA restriction analysis (ARDRA) by using nine restriction endonucleases. The virulence of 40 representative strains was assessed by using serial doses. The pathogenic specificities of the strains were also verified. ARDRA carried out with HinfI revealed two main groups of strains, groups A and B, which exhibited a level of similarity of 57%. The other eight restriction endonucleases used did not separate the strains. In addition, a cluster analysis performed by the unweighted pair group method using arithmetic averages after repetitive PCR and SDS-PAGE of protein extracts also revealed the same two groups. Furthermore, the differential utilization of some carbon compounds made it possible to differentiate the groups. Virulence assessment clearly indicated that the group A strains are very virulent, whereas the group B strains proved to be mildly virulent for hazelnut. Group A included the strains isolated in northern Greece and central Italy (i.e., the province of Viterbo); these strains do not have the syrB gene, are pathogenically restricted to Corylus avellana, and belong to Pseudomonas avellanae. Group B includes the other strains obtained from hazelnut cultivated in Piedmont, Campania, Latium, Sicily, and Sardinia. They represent a distinct taxon closely related to Pseudomonas syringae pv. syringae.Pseudomonas avellanae, the causative agent of bacterial canker and decline of hazelnut (Corylus avellana L.) (5), is severely damaging the cultivated hazelnuts in northern Greece (13) and central Italy (18). In the latter area, where specialized cultivation of hazelnut occurs on more than 20,000 ha, around 1,000 ha is threatened by the bacterium and more than 40,000 trees have already died. The main field symptom is sudden wilting of the twigs, branches, and tree, especially at the end of spring and in the summer. When inoculated in early autumn through leaf scars, the pathogen can systemically reach the roots and then the other branches of a tree (18). Field surveys in the hazelnut-growing areas of Campania, Latium, Piedmont, Sardinia, and Sicily have been performed to ascertain the possible presence and relevance of symptoms resembling those of P. avellanae infection and to isolate the microorganisms responsible for the disease. Previous studies showed that the strains in northern Greece can be differentiated from those in central Italy by repetitive PCR (17). Moreover, we ascertained that in a small district in no...