Oat Leaf Diseases: Secondary Metabolites of Pathogenic Fungi and Infected Plants and Their Toxicity

Oats (<i>Avena sativa</i>), a globally vital cereal and forage crop, incur heavy losses from foliar fungal pathogens, whose virulence is largely mediated by secondary metabolites. These compounds are toxic to the host plant, normal cells, and livestock. This review summarizes the current knowledge on 31 pathogenic fungi and their 310 secondary metabolites, which are classified into 9 chemical classes: lactones, benzene derivatives, terpenoids, nitrogen-containing compounds, peptides, flavonoids, pyranones, steroids, and organic acids. Of these 310 secondary metabolites, 68 compounds display phytotoxicity, and an additional 3, 3, and 2 exhibit cytotoxic, teratogenic, and carcinogenic activities, respectively. Pathogen-infected oat plants produce 16 inducible secondary metabolites as defense responses to pathogenic fungi, but their toxicity to plants, animals, and the agroecosystem remains unexplored. Therefore, integrating pathogen-derived toxins and host-derived phytochemicals is essential for developing eco-friendly disease management strategies and deepening our understanding of the chemical ecology underpinning oat-pathogen interactions.