Mechanism of Chitosan Oligosaccharides in Inhibiting Phytophthora infestans and Synergistic Enhancement with Conventional Fungicides: A Strategy for the Management of Potato Late Blight

Chitosan oligosaccharides (COSs) show broad-spectrum antimicrobial activity, yet their mechanism against Phytophthora infestans, the causal agent of potato late blight, remains unclear. This study evaluated the inhibitory effects, mechanisms, and field performance of COS against P. infestans. Results showed that COS significantly inhibited mycelial growth in three strains (T30–4, 3928A, 88069), with IC50 values of 0.10, 0.11, and 0.11 g/L, and at 0.5 g/L, it inhibited sporangial germination in T30–4. Transcriptomics revealed 727 differentially expressed genes (DEGs) enriched in fatty acid metabolism, membrane biogenesis, and transmembrane transport. Combined application with sublethal stress (high salt, high temperature, and high oxygen) or chemical fungicides (infinito and mancozeb) enhanced antioomycete efficacy. COS also induced systemic resistance in potatoes without phytotoxicity. Field trials confirmed that COS combined with infinito increased the yield by 31% and reduced disease severity by 35%. These findings support COS as a promising biocontrol agent for sustainable management of potato late blight.