Phenotypic and genetic diversity among Pseudomonas syringae pv. phaseolicola

Abstract

The relationships among a worldwide collection of 56 strains of Pseudomonas syringae pv. phaseolicola, causal agent of halo blight disease on bean, were investigated by studying the phenotypic and genetic diversity. All P. s. pv phaseolicola strains tested were pathogenic on the bean cultivar 'Canadian Wonder'. Carbon substrate utilization using BIOLOG (Biolog GN2 Microplate test) clustered all the phaseolicola strains together. The use of the phaseolotoxin gene cluster as a polymerase chain reaction (PCR) target detected all the P. s. pv. phaseolicola strains identified as toxin producers (Tox+) except for one strain (NCPPB 2385) from Australia. Grouping of P. s. pv. phaseolicola by fatty acid composition suggested the existence of five clusters. A majority of the strains from the USA and Turkey were present in Cluster A while 12 of the 16 strains in group C were from Africa. PmeI and PacI enzymes were used for pulsed-field gel electrophoresis (PFGE) analysis of P. s. pv. phaseolicola. Digestion of chromosomal DNA from P. s. pv. phaseolicola with PmeI and PacI generated 29 and 28 groups, respectively. Determination of similarity coefficients and clustering by UPGMA revealed five clusters. More diversity was observed among strains with PFGE than with fatty acid profiling. The results obtained in this study suggest that although a number of strains formed small clusters based on their geographical origin, a clear segregation cannot be concluded. The uniformity of the strains in Turkey isolated in 1994 could possibly indicate a recent introduction of a population of closely related strains. Although only a limited number of strains from the USA were compared, it is interesting to note that the strains were phylogenetically closely related considering that they spanned approximately 20 years. The oldest strain, NCPPB 52 from Canada, which was collected in 1941, had identical PFGE patterns with several strains from South Africa and one strain in Turkey collected in the 1990s. The presence of identical PFGE groups in more than 1 year in South Africa and Germany and the phylogenetically closely related group in the USA coupled with the fatty acid data would indicate the likelihood for local seed sources and/or endemic populations.