Evaluation of Bacillus subtilis YLX6 for biocontrol of Alternaria alternata , the causal agent of leaf spot in Lonicera caerulea

BACKGROUND: Blue honeysuckle (Lonicera caerulea) is an economically important berry crop in Heilongjiang Province, China (> 2000 ha under cultivation). Severe leaf spot threatens yield and quality, while chemical control raises environmental and pathogen resistance concerns, driving demand for sustainable biocontrol solutions. RESULTS: Alternaria alternata was confirmed as the causal agent via morphological and multilocus phylogenetic analyses. Endophytic Bacillus subtilis YLX6, isolated from healthy blue honeysuckle leaves, exhibited strong antagonism against A. alternata and broad-spectrum antifungal potency. Its lipopeptides suppressed A. alternata conidial germination (55.9-61.5%), sporulation (77.0-79.7%), and mycelial growth (50.8-60.0%), elicited distinct hyphal abnormalities, and maintained robust stability across temperature, pH, and ultraviolet (UV) gradients. Field trials achieved 65.0% (fermentation broth) and 58.1% (purified lipopeptides) control efficacy. Preparative chromatography combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) identified antifungal component Y-2 as a mixture of C13-C15 surfactin and C14-C15 iturin A structural homologs. CONCLUSIONS: Bacillus subtilis YLX6 is a highly promising biocontrol agent for blue honeysuckle leaf spot. Its lipopeptide metabolites - characterized by potent antifungal activity and exceptional environmental resilience - are the core bioactive determinants. Incorporating YLX6 into integrated disease management offers a viable approach to reduce reliance on synthetic fungicides. © 2026 Society of Chemical Industry.