Pseudomonas fluorescens 2-79 suppresses take-all, a major root disease of wheat caused by Gaeumannomyces graminis var. tritici. The bacteria produce an antibiotic, phenazine-1-carboxylic acid (PCA), and a fluorescent pyoverdin siderophore. Previous studies have established that PCA has an important role in the biological control of take-all but that antibiotic production does not account fully for the suppressiveness of the strain. To define the role of the pyoverdin siderophore more precisely, mutants deficient in production of the antibiotic, the siderophore, or both factors were constructed and compared with the parental strain for control of take-all on wheat roots. In all cases, strains that produced PCA were more suppressive than those that did not, and pyoverdin-deficient mutant derivatives controlled take-all as effectively as their respective fluorescent parental strains. Thus, the phenazine antibiotic was the dominant factor in disease suppression and the fluorescent siderophore had little or no role. The siderophore also was of minor importance in a second strain, P. fluorescens M4-80R, that does not produce PCA. Strains 2-79 and M4-80R both produced substances distinct from the pyoverdin siderophore that were responsible for fungal inhibition in vitro under iron limitation, but these substances also had, at most, a minor role in disease suppression in situ.