Chitosan, a hydrophilic biopolymer industrially obtained by N-deacetylation of chitin, can be applied as an antimicrobial agent. The current review of 129 references describes the biological activity of several chitosan derivatives and the modes of action that have been postulated in the literature. It highlights the applications of chitosan as an antimicrobial agent against fungi, bacteria, and viruses and as an elicitor of plant defense mechanisms.

Recently, much attention has been paid to chitosan as a potential polysaccharide resource. Although several efforts have been reported to prepare functional derivatives of chitosan by chemical modifications, few attained their antimicrobial activity against plant pathogens. The present paper aims to present an overview of the antimicrobial effects, mechanisms, and applications of a biopolymer chitosan and its derivatives in crop protection. In addition, this paper takes a closer look at the physiochemical properties and chemical modifications of chitosan molecule. The recent growth in this field and the latest research papers published will be introduced and discussed.

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Novel N,O-acyl chitosan (NOAC) derivatives were synthesized to examine their fungicidal activity against the gray mould fungus Botrytis cinerea (Leotiales: Sclerotiniaceae) and the rice leaf blast fungus Pyricularia oryzae (Teleomorph: Magnaporth grisea). The fungicidal activity was evaluated by the radial growth bioassay. NOAC derivatives were more active against the two plant pathogens than chitosan itself, and the effect was concentration dependent. Against B. cinerea, 4-chlorobutyryl chitosan (EC50=0.043%), decanoyl chitosan (EC50=0.044%), cinnamoyl chitosan (EC50=0.045%), and p-methoxybenzoyl chitosan (EC50=0.050%) were the most active (12-13-fold more active than chitosan). (Un)-substituted benzoyl chitosan derivatives were more active against B. cinerea than most of these with N,O-alkyl derivatives. Against P. oryzae chitosan derivatives with lauroyl, methoxy acetyl, methacryloyl and decanoyl were the most active.