Potential biological control activity and genetic diversity of Pseudomonas syringae pv. syringae strains

A total of 71 Pseudomonas syringae pv. syringae strains obtained from herbaceous and woody plant species were evaluated for the in vitro inhibition activity against a broad spectrum of bacteria and fungi, and were classified into two distinct groups. The 53 strains of group A inhibited, to a different extent, all the tested micro-organisms: Rhodotorula pilimanae, Bacillus megaterium, Salmonella sp., Listeria monocytogenes, P. syringae, Botritys cinerea, Penicillium digitatum, Alternaria alternata, Aspergillus ochraceus, Fusarium solani. The 18 strains of group B inhibited the growth only of B. megaterium, L. monocytogenes and P. syringae. The culture filtrates of strains of group A, unlike those of group B, induced the necrosis of bean pods and showed in vitro antimicrobial activity. Strains of group A, on the contrary to those of group B, were moderately to highly virulent when artificially inoculated on citrus fruits. All strains of groups A and B were virulent on bean pods. No strain of the two groups was virulent on apple fruits and grapes. Applications of P. s. pv. syringae strains reduced postharvest green and blue mould in citrus and apple fruits, respectively. The efficacy of disease control by strains of group A was higher than that of strains of group B. One strain of group A and one from group B showed interesting prerequisites for use as a biocontrol agent towards P. digitatum and P. expansum on orange and apple fruits, respectively. The syrB gene was present in almost all strains. Repetitive-sequence polymerase chain reaction (PCR) using Enterobacterial Repetitive Intergenic Consensus (ERIC) primers and unweighted pair-group method with average linkage (UPGMA) analysis showed that the strains varied genetically and could be grouped into seven clusters.