Statewide evaluation of foliar fungicides on soybeans in 2024 in Iowa

January 9, 2025 2:35 PM
Blog Post

In 2024, commercial foliar fungicides were evaluated across seven Iowa State University research and demonstration farms to determine their effect on disease control and yield response on soybeans. The locations included the Northwest Research and Demonstration Farm (Sutherland), Northern Research and Demonstration Farm (Kanawha), Northeast Research and Demonstration Farm (Nashua), South Woodruff Farm (Central Iowa Research and Demonstration Farms, Ames), Armstrong Memorial Research and Demonstration Farm (Lewis), McNay Memorial Research and Demonstration Farm (Chariton), and Southeast Research and Demonstration Farm (Crawfordsville).

A total of 18 fungicides containing single or multiple active ingredients were evaluated at each location, along with a non-treated control (NTC). Fungicide treatments were laid out in a randomized complete block design with four replications. Plot size ranged from 24 to 35 feet long and 10 feet wide (4 rows at 30-inch inter-row spacing). The cultivar was NK28P6XF (Syngenta) and corn was planted in the previous year in all the locations except Nashua where the previous crop was oats. Planting dates across the locations and other field activity details are provided in Table 1. All fungicides were applied using a self-propelled research sprayer at recommended rates at the beginning of pod (R3 growth stage) with nonionic surfactant (Induce at 0.3% v/v) and at a volume of 20 gallons per acre (Table 2). Foliar diseases were assessed between the R5 (beginning seed) and R6 growth stage (full seed). Septoria brown spot (caused by Septoria glycines) was assessed by measuring the height of the highest infected leaf in the canopy, and the disease severity was estimated as the percent of leaf area covered by the disease on the highest infected leaves which was converted to Septoria brown spot severity index (BSX). A total of two assessments, one in each center row, were collected and averaged per plot. Severity (%) of frogeye leaf spot (caused by Cercospora sojina) was assessed in ten randomly selected leaflets from the upper canopy of each plot. No other disease symptoms were observed. The two central rows of each plot were harvested at crop maturity with a 2009 Almaco SPC20 research plot combine and the total seed weight per plot and moisture content were measured. Seed weight was adjusted to 13 percent moisture, and yield was calculated in bushels per acre.

Table 1. Research farm location, seed rate, soybean cultivar, planting date, fungicide application date, disease assessment date, and harvest date for seven soybean fungicide trials throughout Iowa in 2024.
Table 1. Research farm location, seed rate, soybean cultivar, planting date, fungicide application date, disease assessment date, and harvest date for seven soybean fungicide trials throughout Iowa in 2024.

Table 2. Fungicide products, FRAC group, manufacturer and their application rate for the 2024 soybean statewide foliar fungicide trials, conducted in seven Iowa State research and demonstration farms throughout Iowa.
Table 2. Fungicide products, FRAC group, manufacturer and their application rate for the 2024 soybean statewide foliar fungicide trials, conducted in seven Iowa State research and demonstration farms throughout Iowa.

Results summary

In 2024, the precipitation pattern during the season was different across the locations. Nashua received the most cumulative precipitation between May and September (24.6 inches) among the seven locations. Ames, Sutherland and Armstrong also received at least 20 inches of precipitation whereas McNay received the least amount of precipitation (14.7 inches) (Figure 1). Precipitation was very low during August and September, which might have affected the development of diseases.

Figure 1. Monthly and cumulative precipitation from May to September and average Septoria brown spot severity index (BSX) at the nontreated control plots across the seven research locations in Iowa from May to September in 2024.
Figure 1. Monthly and cumulative precipitation from May to September and average Septoria brown spot severity index (BSX) at the nontreated control plots across the seven research locations in Iowa from May to September in 2024.

Figure 2. Average disease severity of Septoria brown spot index (BSX) (yellow dots) and average soybean yield in bushels per acre (blue bars) in plots applied with fungicide products and non-treated control (NTC) combined across all seven experiment locations in 2024. All fungicide products were applied at their label recommended rate at the beginning of pod (R3) with a nonionic surfactant (Induce at 0.3% v/v). The treatments were not statistically different for both BSX (P = 0.96) and yield (P = 0.31). The
Figure 2. Average disease severity of Septoria brown spot index (BSX) (yellow dots) and average soybean yield in bushels per acre (blue bars) in plots applied with fungicide products and non-treated control (NTC) combined across all seven experiment locations in 2024. All fungicide products were applied at their label recommended rate at the beginning of pod (R3) with a nonionic surfactant (Induce at 0.3% v/v). The treatments were not statistically different for both BSX (P = 0.96) and yield (P = 0.31). The blue horizontal dotted line represents the mean yield value of NTC.

Septoria brown spot (SBS) was the only disease observed in all locations but at very low levels (< 3 BSX). FLS did not show up throughout the growing season. The effect of fungicide applications was not statistically significant for BSX (P = 0.96). Similarly, treatments did not show a significant effect on overall yield (P = 0.31) across the seven locations (Figure 2). In 2024, overall soybean yield was greater than in 2023 and 2022 (Dangal et al. 2024, Nieto et al. 2023).

Management recommendations

Applying foliar fungicides can enhance crop yields, particularly under significant foliar disease pressure. While some studies indicate yield benefits even at low disease levels or in the absence of disease, these findings are inconsistent. Previous reports of QoI fungicide-resistant strains of Cercospora sojina and Septoria glycines should push us towards more judicious use of fungicides. Moreover, an integrated method of disease management that does not rely only on fungicides should be incorporated to manage crop diseases and preserve the efficacy of existing fungicides. Disease management practices such as crop rotation, planting disease-resistant cultivars, and applying fungicides with multiple modes of action can help prevent fungicide resistance development and minimize crop loss.

References

Dangal, N., Nieto, E., Wiggs, S., Gonzalez-Acuna, J., and Mueller, D. 2024. Statewide evaluation of foliar fungicides on soybeans in 2023 in Iowa. Online publication. ICM News, Iowa State University Extension and Outreach. March 12, 2024. https://crops.extension.iastate.edu/blog/daren-s-mueller-edgar-nieto-jose-gonzalez-acuna-nabin-dangal-stith-wiggs/statewide-evaluation

Nieto, E., Dangal, N. Mueller, D., and Wiggs, S. 2023. Statewide evaluation of foliar fungicides on soybeans in 2022. Online publication. ICM News, Iowa State University Extension and Outreach. January 13, 2023. https://crops.extension.iastate.edu/blog/daren-s-mueller-edgar-nieto-nabin-dangal-stith-wiggs/statewide-evaluation-foliar-fungicides

Category: 
Diseases
Tags: 
foliar fungicide
soybeans
Authors: 

Nabin Dangal

Edgar Nieto

Stith Wiggs

Jose Gonzalez-Acuna

Daren Mueller Professor

Daren Mueller is a professor and extension plant pathologist at Iowa State University. He is also the coordinator of the Iowa State Integrated Pest Management (IPM) program. Daren received his bachelor's degree from the University of Wisconsin-Madison in 1996, and his master's degree and doctorat...