By Nabin K. Dangal, Edgar Nieto, Stith Wiggs, Jose Gonzalez-Acuna, and Daren S. Mueller.
Foliar fungicides were evaluated across seven Iowa State University research and demonstration farms to determine their performance for disease and yield response on soybeans in 2023. These farms included the 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), McNay Memorial Research and Demonstration Farm (Chariton), and Southeast Research and Demonstration Farm (Crawfordsville).
A total of 18 fungicides containing single and multiple active ingredients were evaluated at each location, along with an untreated control (UTC). Fungicide treatments were laid out in a randomized complete block design with four replications. Plot size ranged from 25.5 to 40 ft long and 10 to 15 ft wide (4 rows 30-inch inter-row spacing). Soybean cultivar Pioneer P31T64E was planted at the McNay Research farm but NK28-T3XF was planted in all other locations. Corn was planted in the previous year in all the locations. Planting dates across the locations and other field activity details are provided in Table 1. All fungicides were sprayed 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 20 gallons per acre (Table 2). Foliar diseases were assessed when soybean plants reached the R6 growth stage (full seed). Septoria brown spot (caused by Septoria glycines) progression 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 severity index (BSX). A total of two assessments, one in each center row, were collected and averaged for a plot. Severity (%) of other foliar fungal diseases, including frogeye leaf spot (caused by Cercospora sojina) and Cercospora leaf blight (caused by Cercospora species), were assessed in ten randomly selected leaves from the upper canopy of each plot. The total seed weight per plot and moisture were measured with a 2009 Almaco SPC20 research plot combine. 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, previous crop, planting date, fungicide application date, disease assessment date, and harvest date for seven soybean fungicide trials throughout Iowa in 2023.
Table 2. Fungicide products and their application rate for the 2023 soybean statewide foliar fungicide trials, conducted in seven Iowa State research and demonstration farms throughout Iowa.
Result summary
In 2023, the precipitation pattern during the season was different across the locations. Ames received the most cumulative precipitation between May and September (>15”) among the seven locations, whereas Nashua received the least amount of precipitation (9”) (Figure 1). Precipitation in August, the critical month for foliar disease development, was low in most locations.
Frogeye leaf spot (FLS) and Septoria brown spot (SBS) were the two diseases observed most frequently, but both occurred at very low levels in all locations (overall mean of less than 1% severity for both diseases). FLS did show up late in the growing season (late August). The effect of fungicide applications was statistically significant for FLS severity and SBS severity index (Figure 2). Across the seven locations, Veltyma, Affiance, Miravis Top, Revytek, Miravis Neo, Lucento, and Propulse had significantly less FLS severity as compared to the UTC, Priaxor, and Aproach Prima. Similarly, Quadris Top, Adastrio, Delaro, Delaro Complete, and Regev HBX also had significantly less FLS than the UTC. For SBS, none of the fungicides showed a reduction of the severity index (BSX) compared to the UTC. Treatments did not show a significant effect on overall yield response across the seven locations (Figure 2). In 2023, overall soybean yield was less than in 2022 and 2021 (Nieto et al. 2023, Kandel et al. 2021).
Management recommendations
The application of foliar fungicides can provide yield benefits when there is significant foliar disease pressure. Some reports show yield benefit of fungicides at low levels of disease or even when disease was absent, but these results are not consistent. Cercospora sojina and Septoria glycines strains resistant to QoI fungicides were confirmed throughout Iowa a few years ago; thus, judicious use of fungicides is crucial. 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 all be adopted to prevent fungicide resistance development.
References
Kandel, Y. R., Mueller, D., and Wiggs, S. 2021. Statewide evaluation of foliar fungicides on soybeans in 2021. Online publication. ICM News, Iowa State University Extension and Outreach. February 14, 2022. https://crops.extension.iastate.edu/blog/daren-mueller-stith-wiggs-yuba-kandel/statewide-evaluation-foliar-fungicides-soybeans-2021
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
Daren Mueller Professor
Daren Mueller is an associate 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 a...