Synthesis and effects of zinc-chitosan complex on morpho-growth and biochemical responses in Alternaria alternata, a cause of leaf blights and spots on plants.

The increasing resistance of fungal pathogens to synthetic fungicides necessitates eco-friendly alternatives. Zinc-chitosan (Zn-Ch) complexe offer a sustainable solution due to their biocompatibility and antifungal properties. This study synthesized and characterized Zn-Ch using UV-vis spectroscopy, Fourier transform infrared (FTIR) spectroscopy, dynamic light scattering (DLS), zeta potential, X-ray diffraction (XRD), and fluorescence spectral analysis, confirming strong Zn²⁺ interactions with chitosan. In vitro bioassays evaluated antifungal efficacy across different treatments: Mancozeb (chemical control), chitosan alone (Ch), non-complexed Zn (2.5 and 5.0 ppm) + Ch, and the Zn (5.0 ppm)-Ch complex. In all treatments, Mancozeb and Ch were tested at identical concentrations (0.5-2.5%) to ensure comparative analysis. Mancozeb and Ch exhibited dose-dependent inhibition, reaching 66.67% and 78.57% at 2.5%, respectively. Non-complexed Zn (2.5 and 5.0 ppm) + Ch demonstrated enhanced antifungal activity, with 5.0 ppm achieving 89.03% inhibition, while the Zn (5.0 ppm)-Ch complex exhibited the highest antifungal efficacy (99% inhibition) with severe hyphal distortion and delayed sporulation. Biochemical assays revealed an initial increase in catalase (CAT), peroxidase (POX), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL) activities, peaking at 1.0-1.5%, followed by significant declines at higher concentrations. The Zn (5.0 ppm)-Ch complex caused the steepest enzymatic decline at 2.0-2.5%, suggesting oxidative stress-induced fungal suppression. Collectively, these findings confirmed the potential of the Zn-Ch complex in enhancing antifungal efficacy by altering fungal morphology, suppressing growth, and activating enzymatic defense responses in plants. Zn-Ch emerged as a promising, biocompatible alternative to synthetic fungicides, warranting further in vivo validation for its application in sustainable crop protection strategies.