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The rhamnolipid biosurfactant (RL-DS9) extracted from the bacterial strain Pseudomonas aeruginosa DS9 was evaluated for its antifungal activity against Colletotrichum falcatum that causes red rot in sugarcane. The surface tension (ST) reduction, biosurfactant production, and antifungal activity of biosurfactant against C. falcatum were investigated by using the medium with different carbon sources and it was found to be maximum in glucose. Moreover, highest reduction of ST and production of biosurfactant was achieved at 4.5% (w/v) concentration of glucose. The efficacy of RL-DS9 was compared with a commercially available rhamnolipid (RL-R95) using microtitre plate assay. Results showed that at 100 μg ml(-1) concentration RL-DS9 exhibited 86.6% inhibition against C. falcatum spore germination, and in the same concentration RL-R95 showed 83.3% inhibition. From liquid chromatography-mass spectrometry (LC-MS) analysis, it was revealed that only two similar congeners Rha-(C10 ) and Rha-Rha-(C10:1 ) were found to be in common among both the rhamnolipids. In the plant bioassay test, it was noted that red rot incidence was reduced on the sugarcane plants treated with RL-DS9. This is the first report that rhamnolipid biosurfactant produced by Pseudomonas aeruginosa DS9 could be able to control red rot disease of sugarcane caused due to the infection with the fungus Colletotrichum falcatum.

Pseudomonas aeruginosa RS29, the native biosurfactant‐producing strain isolated from the oil fields of Assam, India was used to investigate the influence of the carbon nitrogen ratio on production of the biosurfactant. The biosurfactant producing ability of the strain was measured based on surface tension (ST) reduction of the culture medium and the emulsification (E24) index. Production was greatly influenced by the sources of nitrogen and carbon as well as the carbon to nitrogen (C/N) ratio. Sodium nitrate was the best nitrogen source and the water miscible carbon source, glycerol was observed as the best carbon source for maximum biosurfactant production. The C/N ratio 12.5 allowed the maximum production of biosurfactant by the RS29 strain. At this C/N ratio, 55 % ST of the culture medium was reduced by the produced biosurfactant. Concentrations of crude and rhamnolipid biosurfactant obtained at this particular C/N ratio were 5.6 and 0.8 g/l respectively. The RS29 strain was novel as it was able to produce a sufficient amount of biosurfactant utilizing a much lower amount of the water miscible carbon source, glycerol. Extraction of the biosurfactant by a chloroform–methanol (2:1) mixture was the best method to obtain the highest biosurfactant from the culture medium of the strain. The biosurfactant was confirmed as a mixture of mono and di‐rhamnolipid congeners, Rha–C10–C10–CH3 being the most abundant one. The biosurfactant was a good foaming and emulsifying agent.

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