Soil sterilization, pathogen and antagonist concentration affect biological control of Fusarium wilt of cape gooseberry by Bacillus velezensis Bs006

BACKGROUND AND AIM: Fusarium wilt (FW) is the major constraint on cape gooseberry (Physalis peruviana L.) production. Fungicides have been ineffective in disease control and alternative tools are not available. Bacillus velezensis (formerly Bacillus amyloliquefaciens) strain Bs006 has an antagonistic potential against Fusarium oxysporum f. sp. physali (Foph). However, results of in vivo tests have been variable. We examined the effect of biotic sources of variability on the biocontrol activity of Bs006. METHODS: Pot experiments in greenhouse were carried out to determine the influence of soil sterilization and concentration of both pathogen and antagonist in soil on biocontrol activity and the effect of pathogen on plant growth promotion by Bs006. RESULTS: Efficacy of Bs006 against FW was significantly lower under sterile than non-sterile soil condition. Diluted liquid culture of Bs006 at 1 × 10⁶ and 1 × 10⁷ cfu.mL⁻¹ reduced FW by up to 97% under low Foph inoculum pressure (10² to 10⁴ cfu.g⁻¹ of soil) but at 1 × 10⁸ cfu.mL⁻¹ biological treatment significantly reduced FW only when the concentration of Foph was 1 × 10⁴ cfu.g⁻¹ by 71%. The evaluation of biomass of Bs006 (1 × 10⁸ cfu.mL⁻¹) and supernatant free of bacteria added at 10% allowed to observe that the supernatant was an additional source of biocontrol variability, since high volumes of supernatant favored the development of the disease. Plant growth promoting activity by Bs006 was reduced by the presence of Foph in the soil. Bs006 grew endophytically in cape gooseberry and had high population levels in the rhizosphere inoculated with Foph. CONCLUSIONS: The efficacy of Bs006 to reduce FW was affected by soil sterilization, the concentration of both antagonist and pathogen, and high volumes of supernatant. This work has practical implications for the design of control strategies based on B. velezensis Bs006.