Biocontrol efficiency of native Bacillus sp. HC-9 on honeysuckle leaf spot caused by Alternaria alternata

Honeysuckle leaf spot disease, caused by <i>Alternaria alternata</i>, severely affects the yield and quality of honeysuckle crops. Biological control using native antagonist microorganisms offers a promising alternative to chemical fungicides. In this study, strain HC-9, previously isolated from honeysuckle leaves, was molecularly identified as <i>Bacillus</i> sp. This strain exhibited broad-spectrum antifungal activity, effectively inhibiting eight phytopathogenic fungi, including <i>A. alternata</i>, with an <i>in vitro</i> inhibition rate of 70.91 %<i>.</i> The cell-free supernatant of HC-9 fermentation broth demonstrated strong antifungal activity (72.79 % inhibition after 48 h of fermentation), which was concentration-dependent and stable under high temperature (100°C), UV exposure (1 h), and acidic conditions, though it was sensitive to alkaline environments. The supernatant also markedly suppressed <i>A. alternata</i> conidial germination by 82.02 % and induced significant hyphal morphological abnormalities<i>.</i> Enzymatic profiling showed that HC-9 produced extracellular enzymes such as protease, cellulase, chitinase, and β-1, 3-glucanase, which are likely contributors to fungal cell wall degradation. Moreover, treatments with HC-9 induced the accumulation of reactive oxygen species and malondialdehyde in honeysuckle leaves and enhanced the activity of defense-related and antioxidant enzymes, suggesting the activation of systemic resistance. Both pot and field trials showed that HC-9 significantly reduced disease severity, achieving biocontrol efficiencies of 66.52 %-71.28 % under greenhouse conditions and up to 80.52 % in the field. These findings highlight the potential of <i>Bacillus</i> sp. HC-9 as a promising biocontrol agent for the sustainable management of honeysuckle leaf spot disease.