Statewide evaluation of foliar fungicides on soybeans in 2025 in Iowa

In 2025, commercial foliar fungicides were evaluated across six 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), Wyffles Farm (Central Iowa Research and Demonstration Farms, Ames), Armstrong Memorial Research and Demonstration Farm (Lewis), and Southeast Research and Demonstration Farm (Crawfordsville).

A total of 16 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 to 15 feet wide (4 or 6 rows at 30-inch inter-row spacing). At all locations, the cultivar was XO 2865E (Xitavo Soybean Seed), which was planted at 140,000 seeds/acre in a field that was corn the previous year. 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. Sudden death syndrome (SDS) was observed at Ames, Crawfordsville, and Armstrong. SDS was assessed using the Southern Illinois University (SIU) rating system, where disease incidence (Di) represents the percentage of plants exhibiting SDS symptoms, and disease severity (Ds) is scored on a 1–9 scale. The foliar disease index (FDX) was calculated using the formula FDX = (Di × Ds) / 9. 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.

Results 

In 2025, the precipitation pattern during the season was different across the locations. Nashua received the most cumulative precipitation from planting to harvesting (31.1 inches) among the six locations. Ames, Kanawha, Armstrong and Sutherland also received at least 20 inches of precipitation whereas Crawfordsville received the least amount of precipitation (17.48 inches) during the season (Figure 1). Precipitation was very low during August and September, which might have affected the development of diseases.

Septoria brown spot was observed at all locations but at low levels. SDS was observed at Ames, Crawfordsville and Armstrong at low levels. Frogeye leaf spot (FLS) was not detected during the growing season. Fungicide applications had no statistically significant effect on BSX (P = 0.051); however, they significantly affected yield (P = 0.020) across six locations (Figure 2). Delaro Complete, Delaro, and Revytek produced significantly higher yields—averaging 3.7 bu/ac greater than the untreated control. Lucento, Adastrio, Veltyma, Revytek, and Cortina Xtra also yielded 2.6–3.3 bu/ac higher than the control. Overall soybean yield in 2025 exceeded that of the previous three years (Dangal et al. 2024; Mueller et al. 2025; Nieto et al. 2023).

Foliar fungicide applications can improve yield, especially under high foliar disease pressure. In this study, some fungicides increased yield even under low disease conditions; however, responses remain inconsistent when disease levels are minimal. Cercospora leaf blight was reported late in the season, suggesting that some infections may have developed after data collection. The detection of QoI-resistant Cercospora sojina and Septoria glycines further underscores the need for prudent fungicide use. Disease management should follow an integrated approach—combining crop rotation, resistant varieties, and fungicides with multiple modes of action—to reduce resistance risks and sustain yield potential.

References

Mueller, D., Dangal, N., Nieto, E., Wiggs, S., and Gonzalez-Acuna, J. 2025. Statewide evaluation of foliar fungicides on soybeans in 2024 in Iowa. Online publication. ICM News, Iowa State University Extension and Outreach. January 9, 2025. 

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.

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.