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To determine whether glucose in growth medium affects secondary metabolite production and biocontrol efficacy of Pseudomonas chlororaphis O6.The secondary metabolites pyrrolnitrin and phenazines antagonize phytopathogenic fungi. The expression of the prnA gene encoding tryptophan halogenase, the first step in pyrrolnitrin biosynthesis, required the stationary-phase sigma factor, RpoS. Mutations in rpoS and prnA in Ps. chlororaphis O6 eliminated antifungal activity against Rhizoctonia solani and Fusarium graminearum. Pyrrolnitrin production was reduced by glucose in growth media, whereas phenazine levels were increased. The efficacy of Ps. chlororaphis O6 in the biocontrol of tomato late blight was reduced by addition of glucose to the growth medium.Regulation by glucose of pyrrolnitrin production influenced the efficacy of the biocontrol of tomato leaf blight.The nutritional regulation of secondary metabolite production from a soil pseudomonad may account, at least in part, for the variability of biocontrol under field conditions.