Effect of Chitosan on Cucumber Plants: Suppression ofPythium aphanidermatumand Induction of Defense Reactions

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Biological activity of chitosan on Pythium rot of cucumber was investigated. Growing cucumber plants in the presence of chitosan 100 or 400 microgram/ml) controlled root rot caused by Pythium aphanidermatum and triggered several host defense responses, including the induction of structural barriers in root tissues and the stimulation of antifungal hydrolases (chitinase, chitosanase, and beta-1,3-glucanase) in both the roots and leaves. Whereas chitosan did not cause any apparent phytotoxicity to cucumber plants, it adversely affected the growth of P. aphanidermatum. A close examination of hyphal cells revealed that chitosan caused wall loosening, vacuolation, and, in some cases, protoplasm disintegration. This may, in part, explain the limited ability of the pathogen to colonize root tissues in the presence of chitosan. Ultrastructural study of root tissue from chitosan-treated plants showed that fungal cells were mainly restricted to root surfaces. The interplay of the antifungal and eliciting properties of chitosan makes chitosan a potential antifungal agent for the control of root rot of cucumber caused by P. aphanidermatum.