With support from the soybean checkoff through the United Soybean Board, Iowa State University researchers evaluated foliar fungicides on soybean in 2020. Because dry weather affected all seven field locations, this year was really an evaluation of fungicides largely in the absence of disease. Seventeen fungicides were tested at the R3 (beginning pod) growth stage, at the recommended label rate. Unlike the previous few years, frogeye leaf spot (Cercospora sojina) was only identified at the McNay Memorial Research and Demonstration Farm (Chariton), and was very low severity (1%). No frogeye leaf spot was observed at the other six Iowa State research farms  — Northwest Research and Demonstration Farm (Sutherland), Northern Research and Demonstration Farm (Kanawha), Northeast Research and Demonstration Farm (Nashua), Central Iowa Research Farms (Ames), Armstrong Memorial Research and Demonstration Farm (Lewis), and Southeast Research and Demonstration Farm (Crawfordsville). The main foliar disease observed in 2020 was Septoria brown spot (Septoria glycines), although this was at very low levels (<3% in lower canopy) at all the locations.

Across the seven locations, Endura, Domark 230, Topguard EQ, Quadris, Quilt Xcel, Delaro, Preemptor, and Priaxor averaged less than 1.5 bu/ac yield response. The yield response for Lucento, Acropolis, Veltyma, Quadris Top, Revytek, and Stratego YLD averaged between 1.5-2.5 bu/ac and the yield response for Aproach Prima, Miravis Neo, and Affiance averaged between 2.5-3.2 bu/ac (Table 1).

This research is a good reminder that soybean foliar fungal diseases need moisture to develop. The primary purpose of foliar fungicides is to manage foliar diseases such as frogeye leaf spot, Cercospora leaf blight (Cercospora kikuchii), and Septoria brown spot. When these diseases are not present or are at extremely low levels, the yield response from a foliar fungicide application is minimal. We know that fungicide resistance issues for frogeye leaf spot and Septoria brown spot exist across Iowa. When disease risk exists, be sure to consult a current efficacy chart like the one produced by the Crop Protection Network to select a fungicide that will control the disease of concern.

Fungicides are very important tools for disease management, and it is critical to preserve their efficacy. In drier years like 2020, foliar diseases are not as much of a problem and so the need for foliar fungicides is also relatively limited. It is also important to consider economic gains from any fungicide application. Potential yield response must exceed the cost of fungicide product and application cost.

table of soybean fungicide yield results
Table 1. Yield response (bu/ac) from statewide foliar fungicide evaluation in 2020. All products were applied at R3 soybean growth stage. NS= Non-significant, CV = Coefficient of variance, and LSD = Least significant difference