Abstract
Fluorescens pseudomonads can suppress soil-borne plant pathogens by employing several mechanisms such as competition for iron by means of siderophores. Three indigenous strains Pseudomonas fluorescens (UTPF5, UTPF61 and UTPF76), Pseudomonas aeruginosa 7NSK2 and its pyoverdin mutant, MPFM1, were used for investigation the effect of iron competition on bean damping-off suppression. The rates of siderophore produced by strains were determined quantitatively. Results demonstrated that 7NSK2 with 625.29 µM had the most ability of pyoverdine production. Study of the role of different iron and zinc concentration on siderophore and HCN production exhibited that iron increased HCN and decreased siderophore production significantly. Appling of iron chelates and purified pyoverdine indicated that susceptibility to iron competition is correlated to type of chelates and fungi species. Pythium aphanidermatum and R. solani exhibited the most and the lowest susceptibility, respectively. Effects of iron chelates and zinc sulphate on biocontrol activity of strains were assessed in greenhouse condition. By providing of iron and subsequently decrease of iron competition, chelates reduced biocontrol activity especially in the case of UTPF5. This strain suppressed disease and increased plant growth in absence of iron chelates. Pyoverdine mutant, MPFM1, showed moderate biocontrol activity in compartion with wild type.

Keywords: Bean, Fluorescent Pseudomonads, Rhizoctonia solani, Siderophore