RHIZOSPHERE BACTERIA AS POTENTIAL BIOCONTROL AGENTS AGAINST SOFT ROT CAUSED BY VARIOUS PECTOBACTERIUM AND DICKEYA spp STRAINS
D.M. Krzyzanowska, M. Potrykus, M. Golanowska, K. Polonis, A. Gwizdek-Wisniewska, E. Lojkowska, S. Jafra
In vitro screening was used to select potential biological control agents (BCAs) against soft rot caused by Pectobacterium and Dickeya spp. (including the newly characterised D. ‘solani’). Out of 1165 rhizobacteria screened, 18 potential BCAs were selected on the basis of their in vitro ability to inhibit the growth (antibiosis) of at least one pathogenic strain (group I) or to inactivate acyl-homoserine lactones (AHLs) involved in the quorum sensing of Pectobacterium spp. and Dickeya spp. (group II). The AHL-degrading Bacillus spp. were also screened by PCR for the presence of the aiiA gene, and this report is the first to document the presence of this gene in Bacillus simplex. The 18 antagonists, all identified as Pseudomonas or Bacillus, were further characterised for motility, production of siderophores, and biosurfactants. Although most group I isolates suppressed maceration of potato tuber slices by Pectobacterium spp. strains, the ability to cause antibiosis in vitro did not guarantee the ability to suppress disease in planta. Despite the poor efficacy against Dickeya spp. on potato slices, group I isolates were highly effective on chicory leaves, which indicates that the host plant is an important criterion in the selection of BCAs against Dickeya spp. The AHL-inactivating isolates of group II were active against a broad spectrum of both Dickeya spp. and Pectobacterium spp. strains. Significantly, the efficacy of some BCAs of both groups differed among pathogen species and pathogen strain within species. The selected rhizobacteria warrant additional study as potential BCAs for the control of soft rot.