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

Honeysuckle leaf spot disease, caused by Alternaria alternata, 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 Bacillus sp. This strain exhibited broad-spectrum antifungal activity, effectively inhibiting eight phytopathogenic fungi, including A. alternata, with an in vitro inhibition rate of 70.91 %. 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 A. alternata conidial germination by 82.02 % and induced significant hyphal morphological abnormalities. 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 Bacillus sp. HC-9 as a promising biocontrol agent for the sustainable management of honeysuckle leaf spot disease.