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.