Mathematical Assessment of Three-Year Field Performance and Growth Promotion Efficiency of Bacillus pumilus and Bacillus thuringiensis in Brassica juncea

Yellow leaf disease caused by Fusarium equiseti has emerged as a serious constraint to mustard ( Brassica juncea L.) production, resulting in substantial reductions in plant growth and yield. The present study aimed to identify the causal pathogen of yellow leaf disease and evaluate the plant growth-promoting, biocontrol, and induced systemic resistance (ISR) potential of two rhizospheric bacterial isolates. Diseased mustard leaves collected from Uttar Dinajpur district, West Bengal, India, yielded a fungal isolate (MUSLD-01), which was identified as Fusarium equiseti through morphological, scanning electron microscopic, and molecular analyses. Two rhizospheric bacterial isolates were identified as Bacillus pumilus (GenBank accession ON783696) and Bacillus thuringiensis (GenBank accession OQ415951) based on 16S rDNA sequencing. Both isolates exhibited multiple plant growth-promoting traits, including indole-3-acetic acid production, phosphate solubilization, siderophore production, and catalase activity. In vitro dual culture assays demonstrated pronounced antagonistic activity against F. equiseti , with B. thuringiensis exhibiting comparatively greater inhibition of mycelial growth. GC–MS analysis of ethyl acetate extracts of bacterial culture filtrates revealed diverse antifungal bioactive metabolites associated with pathogen suppression. A three-year field evaluation (2023–24 to 2025–26) involving three mustard varieties (B-9, B-54, and NYC) demonstrated consistent improvements in plant height, leaf length, root length, and seed yield following PGPR inoculation. Among the tested treatments, B. pumilus produced the greatest growth promotion and yield enhancement, while both bacterial isolates substantially reduced the adverse effects of F. equiseti under field conditions. Mathematical assessment of growth response indicated approximately 19–20% improvement in vegetative growth, 34–36% enhancement in seed yield, and about 31–33% greater root development in PGPR-treated plants compared with untreated controls. Enhanced activities of phenylalanine ammonia-lyase, peroxidase, chitinase, and β-1,3-glucanase further confirmed activation of induced systemic resistance. Collectively, the integration of microbiological, biochemical, mathematical, and multi-year field evidence demonstrates that B. pumilus and B. thuringiensis are promising eco-friendly biocontrol agents and plant growth-promoting rhizobacteria for the sustainable management of mustard yellow leaf disease and improvement of crop productivity.