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 deri vative of strain WCS358, significantly reduced fusarium wilt of carnat ion grown in rockwool if disease incidence was moderate, but not if di sease incidence was high. Differences in disease incidence could inten tionally be established by varying the inoculum density of the pathoge n Fusarium oxysporum f. sp. dianthi (Fod). The effectiveness of diseas e suppression by WCS358r increased with decrease of inoculum density a nd consequently decrease of disease incidence. WCS358r and a Tn5 marke d derivative of WCS358 (B243) reduced fusarium wilt of carnation most effectively if a low iron availability for the pathogen was establishe d by adding unferrated or only partially ferrated ethylenediamine [di( o-hydroxphenylacetic) acid]. A Tn5 mutant of WCS358 defective in sider ophore biosynthesis (JM218) did not reduce disease incidence. Sideroph ore production and inhibition of Fod by WCS358r in vitro decreased wit h increasing iron availability supporting the more effective disease s uppression by strains WCS358r and B243 at low iron availability, Sider ophote-mediated competition for iron was shown to be the mechanism of suppression of fusarium wilt of carnation by P. putida WCS358. Its eff ectivity was highest at a low iron availability and at a moderate dise ase incidence.