Foliar fungicides were evaluated across seven Iowa State University research and demonstration farms to determine their performance for disease and yield response on soybean in 2021. 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 15 fungicides containing single and multiple active ingredients were evaluated at each location. Fungicide treatments were laid out in 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 NK S25-E3 was planted in all locations except in Crawfordsville, where Merschmans Arthur 2230E was planted.
Corn was planted in 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).
Foliar diseases were assessed when soybeans reached the R6 (full seed) growth stage. 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. A total of two assessments, one in each row (a couple plants per row), were collected and averaged for a plot.
Other foliar fungal diseases were also assessed, including frogeye leaf spot (caused by Cercospora sojina) and Cercospora leaf blight (caused by Cercospora species) if present in 10 leaves in the upper canopy of each plot. Total seed weight/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.
In 2021, the precipitation pattern during the season was different across the locations, Ames and Southerland received less cumulative precipitation than the other locations (Figure 1). However, the total precipitation and precipitation occurred in August, the critical month for foliar disease development, was fairly low in most of the locations compared to the year like 2018 when severe foliar disease symptoms was observed throughout the state.
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Frogeye leaf spot and Septoria brown spot were the two diseases observed most frequently but both occurred at very low levels in all locations. Frogeye leaf spot severity was less than 1% and Septoria brown spot severity was about 6% in check plots (data not shown). Fungicide effect was not statistically significant for both diseases, perhaps because of the very low level of disease. No other foliar disease were observed at significant level in any locations.
Yield response to fungicide also was not statistically significant (Figure 1). In 2021, overall soybean yield was greater than 2020 (Mueller et al. 2020). Check plots, when all the locations were combined, yielded 64.3 bu/A. No fungicides produced statistically greater yield than check although yield in some fungicide treatment was numerically greater.
Soybean fungicides can provide yield benefits when there are foliar diseases. Some reports show yield benefit of fungicides at low levels of disease or when disease was absent but this is not consistent. Cercospora sojina and Septoria glycines strains resistant to QoI fungicides were confirmed throughout Iowa a few years ago.
Judicious use of fungicides and an integrated method of disease management that does not depend only on fungicides, should be used to manage the diseases and preserve the efficacy of existing fungicides. Disease management practices such as crop rotation, planting disease-resistant cultivars, and application of fungicides with multiple modes of action can all be used slow selecting for fungicide resistance.