Combination of Pseudomonas aeruginosa and Pochonia chlamydosporia for Control of Root-Infecting Fungi in Tomato

A plant growth‐promoting rhizobacterium, Pseudomonas aeruginosa strain IE‐6, and a fungal antagonist, Pochonia chlamydosporia, were tested for their ability to inhibit mycelial growth of root‐infecting fungi under laboratory conditions including Macrophomina phaseolina, Fusarium oxysporum, F. solani and Rhizoctonia solani. Biocontrol effectiveness of the bacterium and the fungus alone or in combination was also determined for the control of root‐infecting fungi under field conditions. In a dual‐culture plate assay, the colonies of P. chlamydosporia and P. aeruginosa met each other and no further growth of either organism occurred. Against M. phaseolina, F. solani and R. solani, an ethyl acetate extract of the culture filtrates of P. aeruginosa inhibited fungal growth greater than the hexane extract, but against F. oxysporum the hexane extract caused greater inhibition of fungal growth. By contrast, against M. phaseolina, F. oxysporum and F. solani, the hexane extract of P. chlamydosporia was more effective in the inhibition of fungal growth than the ethyl acetate fraction. Ethyl acetate extracts of P. aeruginosa at 1.0 mg/ml not only inhibited the radial colony growth of R. solani but also lysed the fungal mycelium. P. aeruginosa produced siderophores and hydrogen cyanide under laboratory conditions. Field experiments conducted in 1997 and repeated in 1998 revealed that Pochonia chlamydosporia and P. aeruginosa significantly suppressed the root‐infecting fungi M. phaseolina, F. oxysporum, F. solani and R. solani and that the combination of the two caused greater inhibition of the fungal pathogens than either alone. Application of P. chlamydosporia and P. aeruginosa as a soil drench also resulted in enhanced growth of tomato plants.