EVect of Chitosan on Growth and Toxin Production by Alternaria alternata f. sp. lycopersici

The antifungal activity of chitosan, a biopolymer of beta-1-4 glucosamine, against Alternaria alternata f. sp. lycopersici, causal agent of black mold of tomato, was investigated. Chitosan was incorporated into potato-dextrose broth at concentrations of 100-6400 mu g ml(-1), and the growth and toxin production by the fungus were assessed after 15 days of incubation. At the higher concentrations, chitosan significantly affected both fungal growth and toxin production. However, at lower concentrations toxin production was affected more than growth. The fungus sporulated excessively in the presence of chitosan, but the spores were less viable. Chitosan also induced aggregation, abnormal shape, excessive branching and hyphal contortion of fungal cells, and leakage of proteins. The virulence of the toxin in culture filtrates of the fungus grown on different concentrations of chitosan was assessed by administering toxin on tomato disks. The phospholipid content, electrolyte leakage and activities of xylanase and pectin methylesterase were measured in the tomato tissue administered with culture filtrates containing fungal toxin. Decreased trends in the tendency to cause electrolyte leakage, phospholipid degradation and activation of xylanase and pectin methylesterase in the tomato tissue were observed with increasing concentrations of chitosan. The results showed that toxin produced in the presence of chitosan was less effective in causing degradation of tomato tissue compared with the control. Thus, chitosan is a potential antifungal agent which can interfere with the pathogenic factors of the fungus.