Biocontrol potential of Ralstonia sp. TCR112 and Mitsuaria sp. TWR114 against tomato bacterial wilt

In this study, we aimed to identify potential biocontrol agents capable of suppressing tomato bacterial wilt caused by Ralstonia pseudosolanacearum. In total, 441 bacteria were isolated from the rhizosphere soil of tomato, Chinese chive, and Welsh onion. Based on the results of the in vitro antibacterial activity assay, 275 isolates were selected and further evaluated using a tomato seedling bioassay. Eighteen isolates that belonged to that the genera Ralstonia and Mitsuaria exhibited a relatively higher disease suppression (>50% reduction in disease severity) than the other isolates. The isolate TCR112 of Ralstonia and 10 isolates of Mitsuaria were assessed for their biocontrol effect in a series of pot experiments. Among the isolates, TCR112 (identified as Ralstonia sp.) and TWR114 (identified as Mitsuaria sp.), which showed a consistent disease suppression in pot experiments, were selected as final candidates for further evaluation under field conditions. The results showed that soil drenching at weekly intervals with isolates TCR112 and TWR114 reduced the wilt incidence in the first year by 57.2% and 85.8%, and in the second year by 57.2% and 35.3%, respectively, indicating that these isolates were promising biocontrol agents of tomato bacterial wilt. The isolates effectively reduced the pathogen population in the rhizosphere and crown of pot grown tomatoes. Monitoring the population dynamics of biocontrol isolates revealed that both isolates have stable rhizosphere and endophytic colonization capacities. Furthermore, the in vitro assay for siderophore, indole-3-acetic acid, protease, and polygalacturonase production revealed that TCR112 produces the former three substances and TWR114 produces the latter three substances. Altogether, the results suggest that both isolates suppress tomato bacterial wilt by preventing pathogen multiplication and infection via direct antagonism and/or indirect effects such as competing for nutrients and inducing resistance in tomato plants. Furthermore, this is the first study reporting the potential of Mitsuaria as a biocontrol agent against tomato bacterial wilt.