The differences between chitosan (350 kDa) and oligochitosan (6 kDa) in inhibitory effect on phytopathogenic fungi and on decay control were investigated. Both chitosan and oligochitosan strongly inhibited spore germination and mycelial growth of Alternaria kikuchiana Tanaka and Physalospora piricola Nose. Relatively, chitosan and oligochitosan showed more obvious inhibitory effect on mycelial growth than spore germination. Although oligochitosan had better inhibitory effects on fungal pathogenicity in vitro, chitosan was more effective on disease control in pear fruit stored at 25 degrees C. When treated with oligochitosan, pear fruit increased the activities of chitinase (CHI) and beta-1,3-glucanase. Differently, chitosan treatment significantly increased peroxidase (POD) activity of pear fruit. These results suggested that chitosan and oligochitosan triggered different mechanism for pathogenicity inhibition and disease control. (C) 2010 Elsevier Ltd. All rights reserved.