Efeito de fungicidas e silício no controle da mancha foliar e na produção de metabólitos secundários em Megathyrsus maximus

Abstract

Brazil has an extensive pasture area dedicated to cattle feeding but remains vulnerable to pathogens such as leaf spot caused by Bipolaris maydis in Megathyrsus maximus (syn. Panicum maximum). Research on chemical control in forage grasses is still limited, which encourages the search for sustainable alternatives such as silicon, a beneficial element that strengthens the physical barrier of the cell wall and can modulate the synthesis of secondary metabolites. This study aimed to evaluate the effect of silicon and fungicides on leaf spot severity and on the induction of isoflavones, compounds involved in plant defense responses against pathogens, in M. maximus cv. Tanzania, a cultivar known for its susceptibility to B. maydis. The field experiment was carried out at Embrapa Gado de Corte, using a randomized block design with four replications and six treatments, and applications started when the disease reached an average of 5% of the leaf area. The treatments were: (T1) Water (control); (T2) Pyraclostrobin + Epoxiconazole; (T3) Azoxystrobin + Cyproconazole; (T4) Trifloxystrobin + Cyproconazole; (T5) Mancozeb; and (T6) Commercial Si. Disease severity was monitored, and the area under the disease progress curve (AUDPC), forage mass production, and secondary metabolites were evaluated. Fungicides significantly reduced AUDPC compared with the control (T1), while silicon (T6) showed a positive effect, comparable to fungicide T2. Treatments T4 and T3 increased total isoflavones, T5 and T4 enhanced genistein, T3, T4, and T5 induced daidzein, and T4 increased daidzin. T6 did not induce isoflavones, indicating that its effect on Megathyrsus maximus is mainly as a physical barrier and an indirect stress modulator. The results indicate that fungicides not only contribute to disease control but also stimulate the production of secondary metabolites. Silicon (T6) showed performance similar to treatment T2, representing a promising alternative for more sustainable phytosanitary management.