Rhizosphere Competency of Bacillus subtilis B4 and B. amyloliquefaciens B7 and their Biocontrol Efficacy against Rhizoctonia solani and Streptomyces scabies under Field Conditions

Background: Potato (Solanum tuberosum L.) is susceptible to soil-borne pathogens such as Rhizoctonia solani AG-3 and Streptomyces scabies, which cause black scurf and common scab, respectively. These pathogens persist in the soil and are difficult to control using conventional chemical methods. Therefore, biocontrol agents such as Bacillus subtilis B4 and B. amyloliquefaciens B7 have emerged as promising alternatives due to their strong antagonistic activity and plant growth-promoting properties. Methods: Field trials were carried out using talc-based formulations of Bacillus spp., applied through tuber dipping (10 g and 15 g/L) and soil application (2.5 kg and 3.5 kg/25 kg of FYM), either individually or in combination. Rhizosphere soil samples were collected at 0, 15, 30, 60, and 90 days after sowing to determine colony-forming units (cfu/g) and evaluate rhizosphere persistence. Results: The combined treatment of tuber dipping (15 g/L) and soil application (3.5 kg) resulted the highest CFU values. B. subtilis B4 reached 6.4 × 10⁹ and 7.3 × 10⁹ cfu/g in 2019 and 2020, respectively. Though populations declined over time, viable counts of approximately ~ 5.9 log₁₀ CFU/g were still detected at 90 days. ANOVA significant effects (p<0.001) of treatment, bacterial strain, and sampling time, while yearly differences were non-significant. The same treatment enhanced emergence, plant vigor, and yield (213.75 qt/acre), while reducing disease severity compared with the control (143.00 qt/acre). Conclusion: Bacillus subtilis B4 and B. amyloliquefaciens B7 confirmed strong rhizosphere competency and disease suppression under field conditions. The integrated application method continued bacterial populations and improved plant performance, indicating that Bacillus-based bioformulations are a viable eco-friendly approach for controlling major potato diseases.