Synergistic Effects of <i>Bacillus amyloliquefaciens</i> <scp>SDTB009</scp> and Pyraclostrobin on Walnut Brown Spot Disease

ABSTRACT Walnut brown spot, caused by the soil‐borne pathogen Alternaria alternata , significantly reduces the quality of walnuts, posing a serious threat to the walnut industry. To combat fungal diseases in fruit trees, the combined use of biological agents and fungicides has emerged as a practical strategy. Bacillus amyloliquefaciens SDTB009 effectively inhibits A. alternata , Cytospora juglandicola and Colletotrichum acutatum , with 10 8 CFU/mL of SDTB009 fermentation broth showing the strongest inhibitory effects against these pathogens. Among the nine fungicides tested, pyraclostrobin and boscalid demonstrated superior efficacy in inhibiting the mycelial growth of A. alternata ASF1, with EC 50 values of 0.395 and 0.439 mg/L, respectively. These fungicides also showed good inhibitory effects on strains ASC2 and AMGT3. Additionally, SDTB009 grew normally on plates containing 250 mg/L of pyraclostrobin or boscalid, demonstrating good compatibility with both fungicides. The crude lipopeptide extracted from SDTB009, when combined with pyraclostrobin or boscalid, showed the best synergistic effects at ratios of 8:1 and 6:1, respectively. Field trials indicated that pyraclostrobin provided control efficacy of 71.9% to 82.1% against walnut brown spot disease. Treatment with 25% pyraclostrobin suspension (250 mg/kg active ingredient) plus 5 × 10 9 spores of SDTB009 per gram after the first and second applications resulted in control efficacies of 87.5% and 86.3%, respectively, indicating a strong synergistic effect between the two. This combined solution has proven to be an effective method for controlling brown spot disease in fruit trees.