Isolation and identification of antifungal metabolites produced by rice-associated antagonistic Pseudomonas spp

Antifungal secondary metabolites produced by six strains of rice-associated Pseudomonas spp. were determined. Five different media pigment production medium (PPM), 1/5M523, potato-dextrose agar (PDA), yeast malt (YM), and King's medium B (KMB), were used to determine the best medium for plate assays of these strains against Rhizoctonia solani AG1, the rice sheath blight pathogen. Large inhibition zones were obtained on PPM, KMB, and YM using two strains of P. cepacia, In-b-6854 and In-b-6858, originating from the rice seed. The other four strains showed varying degrees of inhibition zones in the media tested. Their antifungal metabolites were isolated and compared with metabolites from reference strains and known compounds by regular and reversed-phase thin-layer chromatography. Pyrrolnitrin was present in the cell extracts from the two P. cepacia strains. Phenazine-1-carboxylic acid and pyocyanine were produced by two strains of P. aeruginosa, In-b-109 and In-b-784. A strain of P. putida, In-b-1821, produced 2,4-diacetylphloroglucinol. Several unidentified compounds were produced by a strain of P. fluorescens, In-b-7-14. Ten microliters of eluted spots from In-b-6854 and In-b-6858 inhibited the mycelial growth of P. solani 20 to 24 mm away