Characterization of antifungal metabolites of Pseudomonas fluorescens and their effect on mycelial growth of Magnaporthe grisea and Rhizoctonia solani.

Fluorescent Pseudomonads have been successfully employed in controlling major fungal plant diseases due to their antifungal metabolites. The present study focuses on characterization of secondary metabolites produced by Pseudomonas fluorescens and their in-vitro antagonistic ability against rice blast caused by Magnaporthe grisea and sheath blight caused by Rhizoctonia solani. Among 20 strains of P. fluorescens isolated from rhizosphere of rice seedlings, one particular strain P.f 05 was highly effective in inhibiting mycelial growth of rice pathogens. To further characterize the production of antifungal metabolites by strain P.f.05, several growth media were used. Among the media tested, King’s B medium at 120 rpm with pH of 7.0 and 40% of dissolved oxygen incubated at 28 C yielded maximum amount of secondary metabolites. Among them, four secondary metabolites were identified through thin layer chromatography with Rf values of 0.22, 0.35, 0.42 and 0.51. Of these four, one particular metabolite was found to inhibit the mycelial growth of two rice pathogens significantly higher compared to other three metabolites. Of interest, this metabolite was further characterized by HPLC, NMR and Mass Spectroscopy and identified as 2, 4-diacetyl-phloroglucinol (DAPG). The melting point of 2, 4 DAPG was between 143-175C. The infrared spectroscopy spectrum of 2, 4 DAPG showed carbonyl group at 1636 cm, other functional groups at 1639 cm and OH group at 3434 cm. The molecular weight was estimated at M/z 210 by mass spectroscopy which agreed with the composition of C10H10O5 for 2, 4 DAPG. These results offer a scope of identifying superior strains of P. fluorescens with high potential of antifungal metabolite production in rhizosphere of rice ecosystem for successful management of rice blast and sheath blight diseases.