The rhizosphere soil of diseased tomato plants as a source for novel microorganisms to control bacterial wilt

Plant growth promoting rhizobacteria (PGPRs) are used for biocontrol of bacterial wilt caused by Ralstonia solanacearum. They are commonly isolated from the rhizosphere of healthy plants and are scarce in the rhizosphere of diseased plants. We hypothesized that a pathogen-prevalent environment, such as the rhizosphere of infected plants, would be a good or better source for isolating PGPRs than the rhizosphere of healthy plants. In order for these PGPRs to survive successfully in a pathogen-prevalent environment, they must have particularly well-developed survival strategies under the stresses exerted by pathogen activities, which would be of value for their use as biocontrol agents. To test this hypothesis, R. solanacearum-antagonistic bacteria were screened from the rhizospheres of diseased and healthy tomato plants. In total, 110 rhizobacteria were isolated, 18 of which showed antagonism to R. solanacearum in vitro. Among the 18 antagonistic strains, 11 (out of 60) were from the rhizosphere of diseased plants, with inhibition diameter zones ranging from 11.2 to 15.2 mm, whereas 7 (out of 50) were from the rhizosphere of healthy plants, with inhibition diameter zones ranging from 11.5 to 30.5 mm. Strains WR4, WR21, and WR42 from diseased plants rhizosphere, and HR61, HR62, and HR92 from healthy plants rhizosphere, were chosen to investigate their biocontrol efficacies (BCEs) in greenhouse condition. Results showed that WR-isolates performed better in reducing disease incidence (DI) than those HR-isolates. Population densities of R. solanacearum in the rhizosphere soil and crown section of tomato plants were lower in WR-isolate treatments than those in HR-isolate treatments. The best biocontrol effect was achieved by inoculating the strain WR21, followed by WR4, WR42, HR92, HR62, and HR61. Root colonization test showed WR21 had the highest root-colonizing capacity compared with 5 other antagonists. BCEs were positively (r = 0.747) correlated with root-colonizing capacities, but were negatively (r = 0.797) correlated with inhibition zones. In conclusion, the rhizosphere of diseased tomato plants is a good reservoir of biocontrol bacteria. (C) 2013 Elsevier B.V. All rights reserved.