The effect of ten chitosan products, obtained from shrimp shells, with a viscosity-average
molecular weight ranged from 22 to 387 kDa on antimicrobial activity against different plant
pathogenic bacteria and fungi, was investigated in vitro and compared with standard high molecular
weight chitosan (846 kDa). The antibacterial activity was estimated by broth microdilution and agar
dilution techniques as a minimum inhibitory concentration (MIC) against Agrobacterium tumefaciens,
Erwinia carotovora, Corynebacterium fascians, and Pseudomonas solanacearum. The MIC was dependent on the
bioassay test and the bacterial reduction rate increased with the decreasing of the molecular weight (MW). The antifungal
activity against Alternaria alternata, Fusarium graminearum, F. oxysporum, F. solani, Phytophthora infestans, and
Rhizoctonia solani was evaluated by mycelia radial growth technique. The effective concentration producing 50% mycelia
growth inhibition (EC50) of the chitosans ranged from 480 to 3037 mg/L depending on the fungus tested and the MW of
the chitosan products. F. graminearum, F. oxysporum, and R. solani were relatively less susceptible, while A. alternata, F.
solani, and P. infestans were relatively more sensitive to the chitosan compounds.