This study aimed to prepare oligochitosan by gamma radiation. Irradiated oligochitosan was prepared in solid state in air at room temperature. The molecular weight of irradiated oligochitosan in relation to different irradiation doses were analyzed and assess the efficiency of irradiated oligochitosan on Colletotrichum gloeosporioides (C. gloeosporioides) causing chili anthracnose in Thailand. The viscosity average molecular weight of irradiated oligochitosan was carried out by using Ubbelohde viscosmeter. Gamma irradiation led to significant reduction of the viscosity-average molecular weight (Mv) of chitosan. The apparent Mv of oligochitosan decreased with increasing radiation dose. Lactic acid was used as a solvent instead of acetic acid which is a common solvent due to the inhibition ability of acetic acid against C. gloeosporioides. Potato Dextrose Agar (PDA) incorporated with oligochitosan at different concentrations (0.001, 0.005, 0.01, 0.05, 0.1, 0.5 or 1.0%) was used as treatments for in vitro tests. The result shows that PDA mixed with 0.05 – 1% irradiated oligochitosan were inhibited or retarded the growth of C. gloeosporioides.
