Indução biológica de resistência e controle de Sclerotinia sclerotiorum (Lib.) de Barry na cultura da soja

Abstract

White mold (Sclerotinia sclerotiorum) is one of the main diseases of soybean, favored by conditions of high humidity and moderate temperatures, which can result in severe reductions in productivity. The pathogen survives in the soil for long periods in the form of sclerotia, making its control challenging. In this sense, Bacillus subtilis and Trichoderma harzianum have shown themselves to be promising alternatives to chemical control, being a more sustainable and less aggressive approach to the environment. However, much of the information about the antagonistic activity of these agents available in the literature concerns the use of pure cultures in biocontrol, which may justify different antagonistic responses due to the use of commercial formulations because they present readily available metabolites. Given this context, the objective of this study was to evaluate the antagonistic characteristics of T. harzianum and B. subtilis isolates against S. sclerotiorum, comparing their efficacy with commercial formulations in vitro. Furthermore, we sought to investigate the differences in efficacy between microbiological products and pure cultures of antagonists on photosynthetic metabolism and induction of resistance to white mold in soybean in vivo. In the first chapter, three in vitro assays were conducted to evaluate the antagonistic effect of five treatments (control, B. subtilis, T. harzianum, Raiz Performance® and BioGreen®). The first assay analyzed the efficacy of volatile compounds on mycelial growth and sclerogenesis over time in two isolates of S. sclerotiorum (DC18 and PS16). In the second, the action of direct confrontation was evaluated on the same parameters for both phytopathogenic isolates. The third assay addressed the antagonism in carpogenic germination and sclerotia survival of the DC18 isolate of S. sclerotiorum. The phytopathogen isolates demonstrated different sensitivities to treatments at the evaluated intervals. Thus, DC18 showed lower sensitivity to T. harzianum in direct confrontation, with significance starting 60 hours after incubation, while commercial products exhibited greater antagonistic effect in the volatile compounds assay, with inhibition above 58% for both pathogen isolates. In both assays, BioGreen® and T. harzianum reduced the number and mass of sclerotia for both isolates DC 18 and PS16. On the other hand, commercial products stimulated carpogenic germination with germination averages above 33% and differentiation of stipes into apothecia above 24.38%, indicating changes in the physiology of the pathogen under the in vitro biocontrol condition. In the second chapter, an in vivo assay was conducted using two soybean cultivars (MSOY 6410 and DM64i63). The seven treatments evaluated in the experiment consisted of four biological agents (Bacillus subtilis, Trichoderma harzianum, Raiz Performance® and BioGreen®), one chemical inducer (Acibenzolar-S Methyl – Bion®) and two controls, one inoculated with the pathogen Sclerotinia sclerotiorum and the other non-inoculated. Seven days after application, inoculation with the pathogen was performed using the petiole method, with evaluations of gas exchange and photochemical potential quantum efficiency at 3 and 5 days after inoculation (DAI), while at 7 DAI the chlorophyll index, pathometry indices and antioxidant enzyme activity were evaluated. Plants under biological treatment reduced lesion size although in the area below the curve they presented inferior performance to the chemical inducer. The application of the chemical inducer promoted an increase in the photosynthetic rate at 3 DAI; however, the effect was not maintained at 7 DAI, and the treatment with B. subtilis and BioGreen® promoted metabolic adjustments in the components associated with the photosynthetic apparatus. In terms of total protein, plants treated with Raiz Performance® presented results similar to the non-inoculated control. The antioxidant metabolism was systemically influenced by the application of biological products, with a reduction in the activity of superoxide dismutase, with different responses of the cultivars in the activity of peroxidase. The results of these studies partially validated the hypothesis since microbiological products (Raiz Performance® and BioGreen®) stood out in the reduction of mycelial growth in vitro, although the performance varied among the isolates of Sclerotinia sclerotiorum. Regarding resistance induction, there were no clear differences between isolates and formulas in lesion reduction and area under the curve, but photosynthetic metabolism was favored by B. subtilis and BioGreen®. In addition, the microbiological isolates and products modulated antioxidant metabolism in a systemic manner, restricting disease progression and strengthening soybean resistance to white mold.