SIDEROPHORE-MEDIATED COMPETITION FOR IRON AND INDUCED RESISTANCE IN THE SUPPRESSION OF FUSARIUM-WILT OF CARNATION BY FLUORESCENT PSEUDOMONAS SPP

The mechanisms of suppression of fusarium wilt of carnation by two fluorescent Pseudomonas strains were studied. Treatments of carnation roots with Pseudomonas sp. WCS417r significantly reduced fusarium will caused by Fusarium oxysporum f. sp. dianthi (Fod). Mutants of WCS417r defective in siderophore biosynthesis (sid-) were less effective in disease suppression compared with their wild-type. Treatments of carnation roots with Pseudomonas putida WCS358r tended to reduce fusarium wilt, whereas a sid- mutant of WCS358 did not. Inhibition of conidial germination of Fod in vitro by purified siderophores (pseudobactins) of both Pseudomonas strains was based on competition for iron. The ferrated pseudobactins inhibited germination significantly less than the unferrated pseudobactins. Inhibition of mycelial growth of Fod by both Pseudomonas strains on agar plates was also based on competition for iron: with increasing iron content of the medium, inhibition of Fod by the Pseudomonas strains decreased. The sid- mutant of WCS358 did not inhibit Fod on agar plates, whereas the sid- mutants of WCS417r still did. This suggests that inhibition of Fod by WCS358r in vitro was only based on siderophore-mediated competition for iron, whereas also a non-siderophore antifungal factor was involved in the inhibition of Fod by strain WCS417r. The ability of the Pseudomonas strains to induce resistance against Fod in carnation grown in soil was studied by spatially separating the bacteria (on the roots) and the pathogen (in the stem). Both WCS417r and its sid- mutant reduced disease incidence significantly in the moderately resistant carnation cultivar Pallas, WCS358r did not. It is concluded that the effective and consistent suppression of fusarium wilt of carnation by strain WCS417r involves multiple mechanisms: induced resistance, siderophore-mediated competition for iron and possibly antibiosis. The less effective suppression of fusarium wilt by WCS358r only depends on siderophore-mediated competition for iron.