Suppression of Bacterial Wilt Disease of Tomato by Root-dipping with Pseudomonas fluorescens PfG32. The Role of Antibiotic Substances and Siderophore Production.

Abstract

P. fluorescens strain PfG32 isolated from the rhizosphere of onion actively suppressed the occurrence of bacterial wilt disease of tomato (BWT, caused by P. solanacearum) in vermiculite amended natural soil and produced antibiotic substance(s) and siderophores. To investigate the role of these substances in biocontrol, several mutants defective in antibiotic substances or siderophore production were isolated from the spontaneous Rifr mutant, PfG32R, by Tn5 insertion. Based on pot experiments, the suppression of BWT by Ant-Sid+ (antibiotic non-producing but siderophore producing) mutants was lower than its parental strain and Ant+Sid- (siderophore non producing but antibiotic producing) mutants. Ant-Sid+ mutants still gave significant control. The suppression of BWT by PfG32R was correlated to the suppression of the pathogen population observed on root surfaces and to the delay in appearance of a detectable population of the pathogen in root tissues. It is suggested that the ability of PfG32 to produce antibiotics and siderophores is the important factor in determining its biological control and that other factors may be secondary.