Sistemicidade e mecanismos de defesa ativados por fungicidas no controle da ferrugem da soja

Abstract

Brazil is the world's largest producer of soybeans. Despite its high production, this legume faces losses of up to 90% due to Asian Soybean Rust (ASR). Among the management strategies for ASR, chemical control with a mixture of triazole and strobilurin molecules,known as "Strobimix," stands out. However, it is necessary to understand the mechanisms of their systemicity. Thus, the objective of this study was to evaluate the translocation of triazoles and strobilurins via the root, their systemicity in the upper and lower thirds of leaflets, sprayed only in the central region, and quantify the levels of the enzyme phenylalanine ammonia-lyase (PAL), total soluble phenolic compounds, and soluble lignin in soybean plants inoculated with rust before and after the application of these fungicides. In all experiments, seeds from the soybean cultivar Olimpo IPRO® were used, and an active ingredient from the triazole and strobilurin groups was evaluated. The experimental design was a randomized block, consisting of six treatments and four replications. In experiment (I), Hoagland and Arnon's (1950) nutrient solution was used with two plants per pot. Four doses of fungicides were applied: 15, 25, 50, and 100% of the label dose, with 100% corresponding to the recommended dose of the active ingredient (a.i.). Every three days, disease severity was evaluated in all trifoliates using the diagrammatic scale of Franceschi et al. (2020), totaling ten evaluations. Severity values were integrated into the Area Under the Disease Progress Curve (AUDPC), according to the equation proposed by Shaner & Finney (1977). Control efficiency was calculated using Abbott's equation (1925). At the end of the experiment, the shoot dry weight (SDW) and root dry weight (RDW) of all plants were measured. In the second experiment, the fungicides were applied to the adaxial surface of the central leaflet of the third trifoliate, at the V4 stage, ensuring no runoff, to investigate systemicity towards the upper (acropetal) and lower (basipetal) thirds. After the first signs of the pathogen, disease severity was evaluated in the central trifoliate every three days, using the same diagrammatic scale as in the previous experiment, totaling six evaluations. The central leaflets were collected and scanned using ImageJ® software v. 1.54g. In the third experiment, five plants were used for collection at five time points for biochemical analysis. Fungicides were applied at the V4 phenological stage on the entire plant, on both leaf surfaces, with the same a.i.s and doses as in experiment II. In all experiments, plants were inoculated with a suspension of 1.0 x 105 urediniospores of Phakopsora pachyrhizi/mL in water + 0.05% Tween 20 seven days after the product application. Statistical analyses were performed using R Studio with the Scott-Knott test (p<0.05). In experiment I, the lowest phytotoxicity was observed for strobilurins at the lowest dose, i.e., 15% of the field dose. Thus, the pathogen was inoculated five days after the a.i.s were added to the nutrient solution. At this dose, all fungicides showed systemicity from the roots and reduced disease severity (p<0.05). In experiment II, when evaluating translocation by disease intensity reduction, control efficiency was observed in all treatments. Strobilurins showed systemicity in both directions, acropetal and basipetal, but triazole also achieved this efficiency. Before translocating via xylem, this product also translocates via the mesostemic pathway, allowing basipetal control. In experiment III, peaks of PAL enzyme activity occurred at 72 and 96 hours after inoculation. For soluble lignin, changes were observed only at 72 and 96 hours after inoculation.