Gallic acid-oriented modification of chitosan hydroxyl groups for the control of Phytophthora capsici

To improve the solubility and enhance the antifungal and antioxidant properties of chitosan, three gallic acid (GA)-modified chitosan derivatives were synthesized by grafting GA onto the C₆-OH (retaining -NH₂; PCSGA), C₂-NH₂ (NCSGA) sites, or both (CSGA). Structural confirmation was achieved using FTIR, ¹H NMR and XRD. Antifungal assays showed that PCSGA exhibited optimal activity against Phytophthora capsici (EC₅₀ = 68.69 μg/mL), which was attributed to the retained amino groups and GA integration. Antioxidant evaluations revealed that GA grafting significantly enhanced free radical scavenging and reducing power. Antioxidant activity evaluation revealed that gallic acid grafting significantly enhanced the free radical scavenging capacity and reducing power of the chitosan derivatives. The ABTS⁺ scavenging rate reached 99.7 ± 0.30% at a concentration of 8 μg/mL, while the DPPH free radical scavenging rate attained 93.64 ± 0.06% at 80 μg/mL. SEM/TEM imaging revealed that PCSGA induced severe membrane damage in pathogens. This was corroborated by increased cellular electrolyte permeability and exudation of cytoplasmic contents. In vivo studies demonstrate that PCSGA exhibits significant curative and protective efficacy against the disease of chili pepper and for preserving mango after harvest. At a concentration of 200 μg/mL, PCSGA (39.21 ± 0.34%) exhibits protective efficacy similar to metalaxyl. After dipping mangoes in the solution, the fruit stays fresh for 12 days longer. The study establishes the structure-activity relationships and proposes PCSGA as a multifunctional biopolymer for agricultural applications.