Synergistic integration of melatonin, copper nanoparticles, and Bacillus velezensis mitigates anthracnose and microplastic stress in chili: A novel eco-friendly strategy for sustainable crop protection

Anthracnose disease, caused by Colletotrichum capsici, severely reduces chili pepper growth and yield worldwide. Although melatonin (MT), copper nanoparticles (CuNPs), and Bacillus velezensis exhibit individual antifungal activity, their combined efficacy under environmental stress has not been explored. Here, we report for the first time the synergistic application of MT (100 µM), CuNPs (75 mg/L), and B. velezensis (10 CFU/mL) (MCB) to suppress anthracnose under microplastic (MPs) stress, addressing both biotic and abiotic stress simultaneously. The MCB treatment significantly inhibited C. capsici by increasing fungal membrane permeability and reducing disease incidence. MPs and pathogen stress impaired plant morpho-physiological traits, while MT, CuNPs, and B. velezensis restored plant vigor. The MCB combination yielded the strongest improvements in photosynthetic capacity, chlorophyll content, hormonal balance, defense gene expression, antioxidant activity, and lignin accumulation, while reducing malondialdehyde and reactive oxygen species. Secondary metabolites and mineral contents, suppressed under stress, were also enhanced. This study presents a novel, integrative strategy combining biomolecule signaling, microbial biocontrol, and nanotechnology to mitigate fungal infection and MP stress simultaneously, offering a framework for resilient and sustainable crop protection in environmentally challenging agricultural systems.