Foliar application of a soluble source of silicon, alternated or combined with fungicide, for soybean rust management
Authors:   , Quadros Leonardo Packer 
Published: November 2023

Soybean rust (SR) is one of the most aggressive diseases in soybeans. Thus, alternative strategies for SR management aiming to reduce yield losses and the selection pressure exerted on the Phakopsora pachyrhizi population by the abusive use of fungicides deserves to be searched. This study evaluated the effectiveness of the foliar application of a soluble source of silicon [Silício Forte ® (10% potassium oxide, 25% silicon, 3% magnesium oxide, and 0.01% calcium oxide), Verde Fertilizantes Ltda, Brazil; 40 g/L and syrup volume of 100 L/ha; referred to silicon source (SiS) afterward], combined or alternated with fungicide [Trifloxistrobina (375 g/L) + Ciproconazol (160 g/L); Sphere Max® - Bayer S.A., Brazil], for controlling SR under field conditions. The experiments were carried out in the 2021/22 [Experiment 1 (E1)] and 2022/23 [Experiment 2 (E2)] growing seasons and installed in a randomized block design with four replications and six treatments. The treatments conducted in E1 and E2 were as follows: (1) non-spray of SiS or fungicide (control), (2) three sprays of fungicide (F), (3) three sprays of SiS combined with fungicide (SiS+F), (4) fungicide, SiS, and fungicide, respectively, in the first, second, and third spray (F/SiS/F), (5) SiS, fungicide, and SiS, respectively, in the first, second, and third spray (SiS/F/SiS), and (6) three sprays with SiS (SiS). Plants in each plot were evaluated for severity, plant defoliation, yield assessments (yield and weight of 1000 grains), chlorophyll a fluorescence parameters, and concentration of photosynthetic pigments (chlorophyll a+b and carotenoids). Severity data was used to calculate the area under the rust progress curve (AURPC). The SiS inhibited the germination of P. pachyrhizi urediniospores from 12 to 55% as the rates increased from 20 to 80 g/L compared to the control treatment. There was no significant difference between SiS and control treatments and among F, SiS+F, and F/SiS/F treatments for AURPC, plant defoliation, weight of 1000 grains, and yield. There was a significant difference among treatments for AURPC in E1 and E2, plant defoliation in E1 (higher values for SiS and control treatment followed by treatment SiS/F/SiS, and F, SiS+F, and F/SiS/F treatments), plant defoliation in E2 (higher values for SiS/F/SiS, SiS, and control treatments followed by F, SiS+F, and F/SiS/F treatments), weight of 1000 grains and yield in E1 (higher values for F, SiS+F and F/SiS/F treatments, followed by SiS/F/SiS treatment as well as SiS and control treatments), in addition to the weight of 100 grains and yield in E2 (higher values for F, SiS+F, F/SiS/F, and SiS/F/SiS treatments followed by SiS and control treatments). The effectiveness of SR control with SiS, alternated or combined with fungicide, ranged from 44 to 97%. The AURPC was significantly reduced by 6% for SiS treatment compared to the control treatment in E1. Plants submitted to F+SiS, F/SiS/F, and SiS/F/SiS treatments displayed less impairment on their photosynthetic apparatus (great maximum photosystem II quantum yield, effective photosystem II quantum yield, and electron transport rate values linked to lower quantum yield of non-regulated energy dissipation values) and higher concentrations of Chl a+b and carotenoids compared to plants from the control treatment. In conclusion, the present study brings a body of evidence highlighting the potential of spraying the SiS, alternated or combined with fungicide, to reduce SR severity and plant defoliation due to better physiological performance of infected plants considering that more energy could be allocated for biochemical pathways involved in host defense reactions with a positive impact on yield. Keywords: Glycine max. Rust. Mineral nutrition. Photosynthesis. Integrated disease management.