In in vitro assays, a chitosan polymer caused differential growth inhibition of the following pathogens isolated from tobacco: Phytophthora parasitica Dastur var. nicotianae (Ppn), Pythium aphanidermatum (Edson) Fitzp, Rhizoctonia solani Kühn, and Sclerotium rolfsii Sacc. The most sensitive were P. aphanidermatum and S. rolfsii, the growth of which was fully inhibited at a chitosan dose of 1.5 g L -1 ; the growth of Ppn was fully inhibited at 2 g L -1 . In vivo assays involving plants grown from seeds immersed in chitosan, as well as plants sprayed with this product, were performed to detect the induction of defence response markers in the leaf and consequent resistance to disease. Although defence/resistance marker enzyme activities varied, activation was greater in the chitosan-treated plants than in controls. Marker enzyme activities in the sprayed plants were generally equal to or stronger than those recorded in the seed immersion-treated plants, except for phenylalanine ammonia-lyase activity at the lowest immersion concentration tested. Although there were no statistical differences among treatments with respect to resistance against Ppn, the greatest protection was afforded by the spray treatments, in which the infection index was reduced between 17 and 19% compared to the controls. In conclusion, this chitosan polymer directly inhibited the growth of several tobacco pathogens and caused the induction of defence enzymes in leaves, but was not able to protect tobacco plantlets against Ppn infection via the activation of induced resistance. This work demonstrated the potential of chitosan in protecting tobacco plants against soil-borne pathogens.Additional key words: antifungal activity, induced resistance, Nicotiana tabacum, Phytophthora parasitica, Pythium aphanidermatum, Rhizoctonia solani, Sclerotium rolfsii.
Resumen Evaluación de quitosano como inhibidor de patógenos del suelo y elicitor de marcadores defensivos y resistencia en plantas de tabacoEn ensayos in vitro, se demostró la inhibición diferenciada, causada por un polímero de quitosano, sobre el crecimiento de los siguientes patógenos del suelo aislados de tabaco: Phytophthora parasitica Dastur var. nicotianae (Ppn), Pythium aphanidermatum (Edson) Fitzp, Rhizoctonia solani Kühn y Sclerotium rolfsii Sacc. Los más sensibles fueron P. aphanidermatum y S. rolfsii, ya que la inhibición total del crecimiento ocurrió con 1,5 g L -1 de quitosano, mientras el crecimiento de Ppn fue totalmente inhibido a 2 g L -1 . En ensayos in vivo, se determinó la inducción foliar de marcadores defensivos y resistencia en plántulas de tabaco previamente tratadas por inmersión de semillas y aspersión foliar del quitosano. La activación enzimática fue mayor en los tratamientos con quitosano. Al analizar las dos
