Abstract
In this study the taxonomy of the opportunistic plant pathogenic soft rot Pseudomonas bacteria has been investigated, also in order to clarify the status of the species name Pseudomonas marginalis which is used either for soft rot strains resembling P. fluorescens biovar 2 (= P. marginalis sensu stricto) or for any fluorescent, oxidase positive soft rot pseudomonad (= P. marginalis sensu lato). It was also tried to verify the possible soft rot activity of P. aeruginosa, isolated from plant, animal and man. In total 164 saprophytic and pathogenic strains, including 14 type or pathotype strains were studied. Conventional physiological and biochemical tests, pathogenicity tests on potato disks and chicory heads and whole cell fatty acid analysis (FAA) were applied. Of the non-reference strains 25 were identified phenotypically as P. aeruginosa, 81 with some difficulty, due to many variations, as biovars, intermediates of these biovars or new forms of P. fluorescens, two as P. aureofaciens, two as P. cichorii, 11 as P. putida, 21 as unknown fluorescent pseudomonads, two as P. stutzeri and seven as P. viridiflava. Oxidase positive soft rot strains could not be distinguished by biochemical tests from P. fluorescens, P. putida or P. aureofaciens. There was only a low correlation between pectolytic activity and ability to cause soft rot. In multivariate analysis of fatty acid patterns four clusters could be distinguished, viz.: one heterogeneous (super) cluster containing strains of P. aureofaciens, P. chlororaphis, P. fluorescens, P. putida and P. tolaasii and strains received as P. marginalis; one cluster comprising the oxidase negative P. syringae and P. viridiflava and the related oxidase positive P. cichorii; one containing P. stutzeri and P. mendocina; one tight cluster with all but one P. aeruginosa strains. All oxidase positive soft rot strains were found in the P. fluorescens supercluster and all the oxidase negative ones (phenotypically P. viridiflava) in the cluster containing P. syringae, P. viridiflava and P. cichorii. Within the P. fluorescens supercluster there were no clusters discriminating soft rot strains (P. marginalis sensu stricto or sensu lato), they were found scattered among saprophytic strains. The absence of clustering of soft rot strains as a separate taxon in phenotypic or fatty acid analysis renders the species name P. marginalis artificial and its use should be abandoned. The taxonomic implications of finding a continuum of strains within the P. fluorescens complex (including P. aureofaciens, P. chlororaphis, P. putida and P. tolaasii) using FAA or phenotypic tests by us and other authors is discussed, also in relation to future naming of soft rot pseudomonads. The possible soft rot activity of P. aeruginosa could not be confirmed, only a few strains occasionally caused some necrotizing action on plant tissue. Finally pathogenic strains of P. fluorescens not able to produce soft rot, causing bacterial leaf stripe of Iris spec. are described.
Published Version
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