Manejo da ferrugem da soja Phakopsora pachyrhizi com fungicidas e silício

Abstract

Currently, soybean rust, whose etiologic agent is Phakopsora pachyrhizi, is the most important disease of the soybean crop in Brazil. Control is accomplished basically with sistemic fungicides of the triazole and estrobilurins groups. The constant use of fungicides in an action specific manner can favor the selection of resistant populations of the pathogen. Thus, it is necessary to alternate products with different action mechanisms and furthermore, to develop other strategies in the control of the disease. The use of silicated fertilization provided reduction of diseases in several cultures, mono as well as dicotyledonous. There are also reports of the use of Si associated with fungicides indicating effectiveness in the reduction of diseases such as powdery mildew in pumpkin, leaf spot in St. Augutinegrass and soybean. Based on this, the objective was to evaluate the interaction of silicon x fungicides on the germination of P. pachyrhizi uredospores in vitro and this interaction on the intensity of the disease, in a greenhouse. In a second assay the objective was to evaluate the resistance mechanisms associated with Si. Experiments were done in a greenhouse at the Federal University of Lavras, where doses of Si were appraised using, as source, calcium silicate x fungicides and in another experiment doses of Si (potassium silicate). In the first assay, significant interactions among the doses of Si and fungicides were observed in relation to the intensity of the disease. The plants treated with 448 mg of Si kg-1 of soil, in the absence of a fungicide, reduced by 28.89% the number of lesioned leaves and presented 45% less pustules/cm2 compared to the control (without Si and without fungicide). In the second assay, with Si in a nutritious solution, a lineal decline of AUDPS with an increase of Si doses in the solution, and an increase of the chlorophyll b content, carotene and lignin was observed. Besides this a foliar analysis of macro and micro nutrients indicated that the levels of P, Ca, S and Zn, in the aerial part were influenced by the doses of Si. Through scanning electron microscopy it was verified that the epicuticular wax layer was more developed in plants that received Si in the solution compared with plants not supplied with that element.