Antifungal activities of chitosans (CTS) with different molecular weights (M-w) and different concentrations against Fulvia fulva (cooke) ciffrri (F. fulva) causing leaf mold in tomato plants were studied in vitro and in vivo; the action mode and its inhibition at different stages during the life cycle of F. fulva were observed. The results showed that: (1) in vitro, CTS exhibited strong antifungal activity against F. fulva, especially for the medium M-w (213 and 499 KDa) CTS. Almost complete inhibition of F. fulva conidia germination and mycelia colony radial growth was found when CTS was at concentration of 0.5 and 2 mg mL(-1), respectively; however, inhibitory effect on sporulation was not very obvious for all CTSs tested in this experiment. In vivo, CTS of 213 KDa CTS at 6 mg mL(-1) concentration produced stronger antifungal effect than others. (2) The morphological study by scanning electron microscope (SEM) showed that CTS could induce the hyphal swelling, and the surface of hypha which was treated with low M-w (82 KDa) chitosan was smooth, but was rough treated with high M-w (1320 KDa) chitosan. The further study using a confocal laser scanning microscopy (CLSM) coupled with fluorescein isothiocyanate (FITC)fluorescence detection system showed fluorescence of the FITC-labeled chitosans of which M(w)s were below 500 KDa could enter into the inner of hypha; however, 1320 KDa chitosan was blocked off the outer of hypha. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 3127-3135, 2011