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Experimental results proved that the development of Colletotrichum capsici B4 is extremely sensitive to water-soluble chitosan (WSC) and inhibitory effects increase with increasing concentrations of WSC. In the in vitro test, the results showed that the entire prevention of conidial germination and mycelial diameter development are recognized in a potato dextrose agar medium containing 0.4% and 0.8% WSC, respectively. WSC was more efficient in a potato dextrose broth where it absolutely prevented the mycelial development of C. capsici B4 at a concentration of 0.32%. WSC treatments considerably decreased disease prevalence and the lesion diameter of anthracnose disease on chilli pepper fruits in the in vivo experiments. Experiments in biochemistry indicated that the activities of the key defense-related enzymes in peel, consisting of chitinase, β-1,3-glucanase and total phenolic content, were increased by both C. capsici B4 infection and C. capsici B4 treatment, and was further treated with a 0.5% WSC concentration. Practical applications From the present studies we conclude that water-soluble chitosan (WSC) can directly inhibit the development of C. capsici B4 isolated from infected chili pepper fruits showing typical symptoms of anthracnose in in vitro and effectively control anthracnose in chili pepper fruit in in vivo with concentration-dependent antifungal effects. These findings suggest that WSC may be recognized as a feasible effective alternative to artificial fungicides for postharvest disease control of chili pepper fruits. [ABSTRACT FROM AUTHOR]