Bacillus subtilis isolated from medicinal plants rhizosphere effectively controls Cercospora leaf spot and improves plant growth in mung bean (Vigna radiata)

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Abstract
Background: Mung bean is an important leguminous crop, which is reported to face devastating yield losses of up to 70% due to Cercospora leaf spot (CLS) disease. Traditional methods, such as the application of agrochemicals and fungicides, have been used to control CLS, but their intensive use has toxic effects on edible crops. Methods: To find out a sustainable alternative, this study characterizes a strain, Bacillus subtilis Medicinal_04, isolated from Cannabis sativa rhizosphere and explores its role as an eco-friendly biofungicide and biostimulant. The species level identification of the isolate was confirmed by Average Nucleotide Identity (ANIb) and a digital DNA–DNA hybridization (dDDH). The antagonistic efficacy of B. subtilis Medicinal_04 against Cercospora canescens was evaluated in vitro as well as in planta assays. Results: ANIb of 97.80% and a dDDH score of 85.90% against the reference B. subtilis str. 168. confirmed this isolate as B. subtilis. The in-vitro results showed that B. subtilis robustly inhibited C. canescens growth by 81.5%, strongly correlated with positive chitinolytic activity and a diverse genomic array of secondary metabolite biosynthetic gene clusters. The in planta results demonstrated that B. subtilis seed priming reduced disease incidence by 80 and 71.4%, while foliar application resulted in reductions of 90 and 85.7% for NM-51 and NM-20-21 varieties, respectively. Furthermore, fungicide application successfully reduced disease, however it caused noticeable phytotoxic reductions in root-shoot architecture and chlorophyll content. In contrast, biological interventions completely bypassed these trade-offs as B. subtilis application displayed improved root-shoot length, pod number, and chlorophyll content, while simultaneously enhancing antioxidative enzyme activities (SOD, POD, and CAT) and PR-1 gene expression. Conclusion: These findings demonstrate that B. subtilis Medicinal_04 has the potential to serve as a multifunctional biocontrol agent and is capable of securing high-level disease suppression and optimizing plant productivity, offering a valuable toolkit for climate-smart, sustainable agriculture.

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