2019 Soybean Foliar Fungicide Trials Show Decreasing Efficiency in QoI Class

January 31, 2020
ICM News

Iowa State University researchers recently published their annual fungicide trial data for soybeans in 2019, revealing a continued decline in efficiency among the quinone outside inhibitors (QoI) class of fungicides in preventing foliar diseases in soybeans.

Among the six Iowa State research farms  — Northwest Research and Demonstration Farm (Sutherland), Northern Research and Demonstration Farm (Kanawha), Central Iowa Research Farms (Ames), Armstrong Memorial Research and Demonstration Farm (Lewis), McNay Memorial Research and Demonstration Farm (Chariton), and Southeast Research and Demonstration Farm (Crawfordsville) — the top two foliar diseases observed in the 2019 growing season were frogeye leaf spot (Cercospora sojina) and Septoria brown spot (Septoria glycines). Researchers tested 16 fungicide compounds and their efficacy in controlling these two diseases. All research locations had natural inoculum for both diseases. All fungicide applications were made at the R3 (beginning pod) growth stage, at the recommended label rate. Researchers note that the Northeast Research Farm data was not included in its results because of missing data.

Per the fungicide trial results, overall fungicidal control of frogeye leaf spot was statistically significant at the Ames and Armstrong locations (Table 1). The trial results among the other research farms did not illustrate a significant change in disease control with fungicide use. In analyzing individual fungicidal performance, the one product that was solely a QoI (Quadris) had decreased efficacy in controlling frogeye leaf spot, compared to all the other fungicides, which were a premix containing multiple fungicidal modes of action.

Frogeye leaf spot severity (% in upper canopy) in fungicide treatments across the six locations in 2019. Note, all fungicides are premixes except for Quadris.
Table 1. Frogeye leaf spot severity (% in upper canopy) in fungicide treatments across the six locations in 2019. Note, all fungicides are premixes except for Quadris. All products were applied at R3 with nonionic surfactant (Induce at 0.3% v/v) unless otherwise noted.
NS= Non-significant, CV = Coefficient of variance, LSD = Least significant difference

Overall fungicidal control of Septoria brown spot was statistically significant at Ames, Armstrong, Kanawha, and McNay (Table 2). Results for managing brown spot with fungicides was less consistent across locations. 

Septoria brown leaf spot index (% in lower canopy) recorded on fungicide treatments across the six locations. Note, all fungicides are premixes except for Quadris.
Table 2. Septoria brown leaf spot index (% in lower canopy) recorded on fungicide treatments across the six locations. Note, all fungicides are premixes except for Quadris. 
All products were applied at R3 with nonionic surfactant (Induce at 0.3% v/v) unless otherwise noted. 
NS= Non-significant, CV = Coefficient of variance, LSD = Least significant difference

These research results confirm Iowa State findings that foliar fungal disease continues to develop resistance to QoI. Fungicides reliant solely on QoIs had decreased efficacy against both frogeye leafspot and Septoria brown leaf spot, which are the main foliar diseases of concern in Iowa, and did not provide profitable yield increases. Therefore, researchers conclude that unnecessary use of foliar fungicides should be minimized and an integrated method of disease management that does not depend solely on foliar fungicides should be employed. If a fungicide application is deemed necessary, 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. Other disease management practices such as crop rotation and planting of disease-resistant cultivars should be used to minimize dependency on fungicides.

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Daren Mueller Associate Professor

Dr. Daren Mueller is an associate professor and extension plant pathologist at Iowa State University. He is also the coordinator of the Iowa State University Integrated Pest Management (IPM) program. Dr. Mueller earned his bachelor's degree from the Univ...