Fosfito de potássio na indução de resistência a Verticillium dahliae Kleb., em mudas de cacaueiro (Theobroma cacao L.)

Abstract

There are several reports concerning with the use of phosphites for controlling of plant diseases, through direct action, antifungal, and indirect, by resistance induction. The performance of phosphites as inducer of resistance is questioned, not finding in many of those papers, evidences of defense responses activated by these salts. The present work aimed at studying the effect of doses (0.62; 1.25; 2.5 and 5.0 mL.L–1 of water) of potassium phosphite on the protection of cocoa seedlings against V. dahliae, besides investigating the possible mechanisms involved in the defense response. Experiments were accomplished at the laboratory of Physiopathology and in a greenhouse at the Plant Pathology Department – UFLA. Foliar applications of phosphite were carried out 7 days before inoculation and severity assessments at 20, 30, 40, 50 and 60 days after inoculation. Experiments were also set to verify the toxic effect of phosphites and to evaluate the action of the enzymes peroxidases and polyphenoloxidases and the lignin content. The treatment with the potassium phosphite (1.25 mL.L–1 of water) provided only 10% reduction in the area under the disease progress curve (AUDPC), 60 days after spraying, not differing from none of the doses, nor from the dose zero. All the used doses presented antifungal effect, inhibiting the germination of V. dahliae conidia. The application of the potassium phosphite (1.25 mL.L–1 of water) did not induce increase in the activities of the enzymes peroxidases and polyphenoloxidases in relation to the control. Treated plants with that product presented a small increment in the lignin content, not presenting a significant difference to the absolute control.