SUPPRESSION OF FUSARIUM-WILT OF CARNATION BY PSEUDOMONAS-PUTIDA WCS358 AT DIFFERENT LEVELS OF DISEASE INCIDENCE AND IRON AVAILABILITY

Treatment with Pseudomonas putida WCS358r, a rifampicin-resistant derivative of strain WCS358, significantly reduced fusarium wilt of carnation grown in rockwool if disease incidence was moderate, but not if disease incidence was high. Differences in disease incidence could intentionally be established by varying the inoculum density of the pathogen Fusarium oxysporum f. sp. dianthi (Fod). The effectiveness of disease suppression by WCS358r increased with decrease of inoculum density and consequently decrease of disease incidence. WCS358r and a Tn5 marked derivative of WCS358 (B243) reduced fusarium wilt of carnation most effectively if a low iron availability for the pathogen was established by adding unferrated or only partially ferrated ethylenediamine [di(o-hydroxphenylacetic) acid]. A Tn5 mutant of WCS358 defective in siderophore biosynthesis (JM218) did not reduce disease incidence. Siderophore production and inhibition of Fod by WCS358r in vitro decreased with increasing iron availability supporting the more effective disease suppression by strains WCS358r and B243 at low iron availability, Siderophote-mediated competition for iron was shown to be the mechanism of suppression of fusarium wilt of carnation by P. putida WCS358. Its effectivity was highest at a low iron availability and at a moderate disease incidence.