Genetic diversity and biological control activity of novel species of closely related pseudomonads isolated from wheat field soils in South Australia

Rhizobacteria closely related to two recently described species of pseudomo nads, Pseudomonas brassicacearum and Pseudomonas thivervalensis, were isola ted from two geographically distinct wheat field soils in South Australia. Isolation was undertaken by either selective plating or immunotrapping util izing a polyclonal antibody raised against P. brassicacearum. A subset of 4 2 isolates were characterized by amplified 16S ribosomal DNA restriction an alysis (ARDRA), BIOLOG analysis, and gas chromatography-fatty acid methyl e ster (GC-FAME) analysis and separated into closely related phenetic groups, More than 75% of isolates tested by ARDRA were found to have >95% similari ty to either Pseudomonas corrugata or P. brassicacearum-P. thivervalensis t ype strains, and all isolates had >90% similarity to either type strain. BI OLOG and GC-FAME clustering showed a >70% match to ARDRA profiles. Strains representing different ARDRA groups were tested in two soil types for biolo gical control activity against the soilborne plant pathogen Gaeumannomyces graminis var. tritici, the causative agent of take-all of wheat and barley. Three isolates out of 11 significantly reduced take all-induced root lesio ns on wheat plants grown in a red-brown earth soil. Only one strain, K208, was consistent in reducing disease symptoms in both the acidic red-brown ea rth and a calcareous sandy loam. Results from this study indicate that P. b rassicacearum and P. thivervalensis are present in Australian soils and tha t a level of genetic diversity exists within these two novel species but th at this diversity does not appear to be related to geographic distribution. The result of the glasshouse pot trial suggests that some isolates of thes e species may have potential as biological control agents for plant disease .