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 flu orescent Pseudomonas strains were studied. Treatments of carnation roo ts with Pseudomonas sp. WCS417r significantly reduced fusarium will ca used by Fusarium oxysporum f. sp. dianthi (Fod). Mutants of WCS417r de fective in siderophore biosynthesis (sid-) were less effective in dise ase 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 germi nation of Fod in vitro by purified siderophores (pseudobactins) of bot h Pseudomonas strains was based on competition for iron. The ferrated pseudobactins inhibited germination significantly less than the unferr ated pseudobactins. Inhibition of mycelial growth of Fod by both Pseud omonas strains on agar plates was also based on competition for iron: with increasing iron content of the medium, inhibition of Fod by the P seudomonas strains decreased. The sid- mutant of WCS358 did not inhibi t Fod on agar plates, whereas the sid- mutants of WCS417r still did. T his suggests that inhibition of Fod by WCS358r in vitro was only based on siderophore-mediated competition for iron, whereas also a non-side rophore antifungal factor was involved in the inhibition of Fod by str ain WCS417r. The ability of the Pseudomonas strains to induce resistan ce against Fod in carnation grown in soil was studied by spatially sep arating the bacteria (on the roots) and the pathogen (in the stem). Bo th WCS417r and its sid- mutant reduced disease incidence significantly in the moderately resistant carnation cultivar Pallas, WCS358r did no t. It is concluded that the effective and consistent suppression of fu sarium wilt of carnation by strain WCS417r involves multiple mechanism s: induced resistance, siderophore-mediated competition for iron and p ossibly antibiosis. The less effective suppression of fusarium wilt by WCS358r only depends on siderophore-mediated competition for iron.