Production of Pseudomonas fluorescens P-5 and P-6 for bean damping-off disease.

Growth and efficacy of two biological control agents, Pseudomonas fluorescens Flugge, P-5 and P-6, were evaluated in combinations of two carbon (sucrose & molasses) and two nitrogen (urea & yeast extract) sources to optimize control of Rhizoctonia solani Kuhn, which is a causal agent of bean damping-off. Both strains were grown in five liquid media including: sucrose + yeast extract, molasses + yeast extract (2:1 w/w), molasses + yeast extract (1:1 w/w), molasses + urea and nutrient broth at an initial inoculation of 1×10 CFU mL. Cells from overnight cultures were used to inoculate soil at 1×10 CFU cm soil. At the same time, fungal inoculum was added to soil at the rate of 2 mg cm soil. The medium containing molasses and yeast extract (1:1 w/w) supported rapid growth and high cell yields in both strains. In greenhouse conditions, the influence of the media on the biocontrol efficacy of P-5 and P-6 was the same and P. fluorescens P-6 in molasses + yeast extract (in two different ratios) reduced the severity of disease from 90.9% to less than 28%. On the other hand, there were significant differences on bean growth promotion in greenhouse conditions, after one month. Strain P-5 in molasses + yeast extract (1:1 w/w) was more effective on bean growth promotion as compared to the other media.