Increasing Soybean Resistance Against Rust Using a Product Containing Calcium and Nitrogen Complexed With Polyphenols

Among soybean diseases, rust, caused by Phakopsora pachyrhizi, stands out as one of the most destructive. Resistance inducers may be a great alternative to reduce the yield losses caused by this disease from the perspective of more sustainable agriculture. In this study, soybean plants were sprayed with water (control) or with Cautha [referred to as induced resistance (IR) stimulus after that] and inoculated or non-inoculated with P. pachyrhizi. The germination of urediniospores was significantly reduced by 22%, 26%, 19%, and 25% for the IR stimulus rates of 2.5, 5, 10, and 20 mL/L, respectively. Rust severity was significantly reduced by 27%, 19%, 23%, 25%, and 41% at 7, 9, 11, 13, and 15 days after inoculation, respectively, and the area under the disease progress curve significantly decreased by 27% for IR stimulus-sprayed plants compared to water-sprayed plants. For infected plants, foliar concentrations of Ca, N, chlorophyll a + b, and carotenoids were higher for IR-stimulus sprayed plants than for water-sprayed plants. Lower concentrations of malondialdehyde (less cellular damage) and reactive oxygen species (hydrogen peroxide and superoxide anion radical) along with great activities of antioxidative enzymes (ascorbate peroxidase, catalase, glutathione reductase, and superoxide dismutase) helped to reduce rust symptoms for IR-stimulus sprayed plants. On top of that, these plants also showed greater foliar concentrations of total soluble phenols and lignin as well as increased activities of defence-related enzymes (chitinase, β-1,3-glucanase, phenylalanine ammonia-lyase, peroxidase, polyphenoloxidase, and lipoxygenase). These results strongly support the potential of using this IR stimulus to increase soybean resistance against infection by P. pachyrhizi and, at the same time, to act directly against the germination of the urediniospores.