Microwave-assisted degradation of chitosan for a possible use in inhibiting crop pathogenic fungi

Degradation of chitosan by H2O2 under microwave irradiation was investigated. The oxidative degradation of chitosan was highly accelerated by microwave irradiation under the condition of low temperature and low concentration of H2O2. The degraded chitosans with low molecular weight (Mw) were characterized by gel permeation chromatography, Fourier-transform infrared spectroscopy, ultraviolet-visible spectroscopy, X-ray diffraction and elemental analysis. The decrease of Mw led to transformation of crystal structure and increase of water solubility, whereas no significant chemical structure change in the backbone of chitosan was observed. Antifungal activities of chitosans with different Mw against crop pathogenic fungi Phomopsis asparagi, Fusarium oxysoporum f. sp. Vasinfectum and Stemphylium solani were investigated at the concentrations of 100, 200 and 400 mg/L. All degraded chitosans with low Mw exhibited enhanced antifungal activity compared with original chitosan and the chitosan of 41.2 kDa showed the highest activity. At 400 mg/L, the chitosan of 41.2 kDa inhibited growth of P. asparagi at 89.3%, stronger than polyoxin and triadimefon, the inhibitory effects of which were found to be 55.5% and 68.5%. All the results indicated that oxidative degradation under microwave irradiation was a promising technique for large-scale production of low Mw chitosan for use in crop protection.