Plant Secondary Metabolites for the Control of Plant Diseases: Antiviral, Antifungal, and Antibacterial Activities and Their Biotechnological Applications

Viral, fungal, and bacterial pathogens substantially reduce crop productivity, postharvest quality, and agricultural sustainability. Although synthetic pesticides remain widely used, concerns over environmental persistence, residues, and pathogen resistance have increased interest in safer alternatives. Plant secondary metabolites (PSMs) are promising candidates, but their application is often limited by poor water solubility, volatility, rapid degradation, and inconsistent field performance. This review evaluates the antiviral, antifungal, and antibacterial activities of major PSM classes, including phenolics, terpenoids, nitrogen-containing compounds, and sulfur-containing metabolites, and discusses their biotechnological applications in plant disease management. Recent studies were assessed with emphasis on metabolite sources, target pathogens, mode of action, formulation strategy, and biological efficacy. Evidence indicates that PSMs act through multiple mechanisms, including membrane disruption, inhibition of viral replication, suppression of fungal growth, interference with bacterial metabolism, and activation of plant defense responses. Nanoformulations, encapsulation systems, controlled-release carriers, and green-synthesized nanoparticles can improve stability, delivery, and bioefficacy. Despite this potential, broader agricultural implementation requires further field validation, formulation standardization, long-term stability assessment, and regulatory harmonization.