Screening of suitable carriers for Bacillus amyloliquefaciens strain QL-18 to enhance the biocontrol of tomato bacterial wilt

- RCC is the best carrier for the Bacillus amyloliquefaciens strain QL-18 (Ba).
- RCC supports the survival of Ba in soil with plenty of available nutrients.
- The water extract of RCC discourages the growth of Ralstonia solanacearum (Rs).
- RCC does not significantly reduce disease incidence when used alone.
- RCC improves Ba performance in suppression of Rs in greenhouse and field.

Soilborne plant diseases seriously limit agricultural production. Due to environmental concerns, sustainable agricultural practices must involve the use of environmentally friendly alternatives, such as biocontrol microbes (BMs), to suppress various plant diseases. However, the success of BM application largely depends on the carrier formulation. An ideal carrier that can support the survival of a BM while discouraging the growth of the target pathogen is expected to enhance the performance of the BM in plant disease control. In this study, various commercial composts and peats were evaluated as carriers for Bacillus amyloliquefaciens QL-18, a BM used to control tomato bacterial wilt (TBW) caused by the soilborne bacterial pathogen Ralstonia solanacearum. Among the evaluated carriers, rapeseed cake compost (RCC) was found to be the best carrier, acting as both a substrate and food source for B. amyloliquefaciens QL-18. RCC facilitated the growth and survival of strain QL-18 by supplying adequate levels of dissolved carbon and nitrogen. Moreover, aqueous extracts of RCC alone inhibited R. solanacearum growth, suggesting that certain RCC components are toxic to the pathogen. Greenhouse and field experiments demonstrated that RCC improved the ability of B. amyloliquefaciens QL-18 to reduce the incidence of TBW and the population of the pathogen in the tomato rhizosphere. Although other carriers maintained large populations of B. amyloliquefaciens QL-18, similar to those obtained with RCC, they were not deemed suitable carriers because they constituted a poor nutrient supply for B. amyloliquefaciens QL-18 growth; thus, they had less of an impact on disease control. The principle of BM carrier selection should be adopted in the future when developing new bio-products for the effective control of soilborne plant diseases.

Suitable carrier enhances biocontrol microbe's performance. Ideal carrier should support the survival of BMs while discouraging the growth of the target pathogen. Therefore, choose B (carrier j) for better biocontrol efficacy.