The control of potato common scab with compound Bacillus formulations and synthesized fungicide

Potato common scab is a soil- and seed-borne disease caused by Streptomyces spp., resulting in significant economic losses to the potato industry. To identify an effective combination of Bacillus strains and fungicide for controlling potato common scab, five Bacillus strains were tested in various combinations to screen the optimal antagonistic consortium against three Streptomyces species. The combination of BEV2 + FM2-4 was identified as the most effective. Furthermore, the radish seedling assay confirmed that this Bacillus consortium exhibited significant antagonistic effect on S. scabies. The plate count method was used to determine the inhibitory activity of three synthesized fungicides (2.5% fludioxonil FS, 50% fluazinam SC, and 20% bronopol SL) against Streptomyces spp. The results showed that 50% fluazinam SC had the best inhibitory effect on S. scabies, with an EC₅₀ of 0.442 μg/mL, followed by 20% bronopol SL with an EC₅₀ of 0.536 μg/mL, and 2.5% fludioxonil FS with an EC₅₀ of 2.916 μg/mL. The control efficacy of 2.5% fludioxonil FS, 50% fluazinam SC, and 20% bronopol, and their combinations with Bacillus BEV2 + FM2-4 against potato common scab was determined in the field. The results showed that 2.5% fludioxonil FS + Bacillus BEV2 + FM2-4 has the best control efficacy against potato common scab (72.57%). Among the three chemicals, 50% fluazinam SC had the best control efficacy (66.50%). The control efficacy of Bacillus BEV2 + FM2-4 alone on potato common scab was 70.17%, which is much higher than the control agent B. amyloliquefaciens QST713 (56.77%). The integrated approach of combining microbial agents with chemical fungicides identified in this study offers an effective technical strategy for managing potato common scab in future agricultural practice.