A necrotic leaf disease of leek (Allium ampeloprasum Porrum Group) is reported in Australia for the first time. The fluorescent pseudomonad consistently associated with diseased tissue was identified as Pseudomonas syringae by LOPAT tests (+,−,−,−,+), carbon utilisation, bean and lemon inoculations and fatty acid methyl ester analysis. It was confirmed as P. syringae pv. porri by pathogenicity to leeks, bulb onions, spring onions, shallots and garlic, and by genetic analysis using 16S rDNA PCR, REP, ERIC and BOX PCR, and IS50 PCR. Comparison with reference strains of pv. porri from other countries showed similarity to known strains of pv. porri. The Australian leek strains were generally uniform in their biochemical reactions although three strains tested varied in their pathogenicity to other Allium spp. and varied from published data. All Australian strains shared the same genetic profile with strains from New Zealand, France and California. However, Japanese strains from leek and onion were distinct from the Australian strains and those from New Zealand, France and California. Data strongly support the hypothesis that the pathogen is seed-borne.