By Dr. Daren Mueller, Iowa State University
Soybean foliar fungicides were evaluated for foliar disease management and yield response across seven Iowa State University research and demonstration farms in 2018. These 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).
Field experiments at each location 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 and additional details are listed in Table 1. A total of 18 fungicide products were applied at recommended rate (Table 2) with a self-propelled research sprayer at growth stage R3 (beginning pod) at all seven locations. Foliar diseases were assessed when soybeans reached at 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 and determining disease severity by estimating the percent of leaf area covered by the disease on 10 leaves in the upper canopy. Other foliar diseases were also assessed, including frogeye leaf spot (caused by Cercospora sojina). 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.
Table 1. Research location, cultivar, planting date, planted population, fungicide application (spray) date, disease assessment date, and harvest date for seven soybean fungicide trials throughout Iowa in 2018.
Table 2. Fungicide products and their application rate for seven soybean fungicide trials throughout Iowa in 2018 *All products were applied at R3 with nonionic surfactant (Induce at 0.3% v/v) unless otherwise noted. *All Group 11 fungicides are QoI's.
Overall, the northern and northeastern experiment locations received more precipitation than the other sites. However, a large amount of rainfall occurred during the months of August and September in many locations. This is important to note as August is the critical month for foliar disease development. Frogeye leaf spot was observed at greater levels in all the locations compared to previous years. This might be due to a combination of Quinone outside inhibitors (QoI) fungicide resistant strains of C. sojina, which were confirmed throughout the state in 2017, coupled with the high rainfall in August. Septoria brown spot was also observed at all locations.
Heavy rainfall in September and October challenged timely harvest in many locations. Average yield in untreated plots was 54.9 bu/A. Yield response to fungicide was statistically significant, and the yield response ranged from 0.8 to 7.4 bu/A (Figure 1).
Treatment effect was statistically significant for both diseases (Figures 2 and 3). The QoI fungicides were not effective, however, the fungicides with new chemistries and with multiple mode of actions showed better performance than QoI fungicides alone for disease management and yield response.
Figure 2. Frogeye leaf spot severity in upper soybean canopy in fungicide treatments combined across all seven experiment locations in 2018. Means followed by the same letter are not statistically significant from one another.
Fungicides with solo QoI active ingredients had limited efficacy against frogeye leaf spot and provided less yield benefit compared with fungicides with multiple chemistries, especially some of the newer chemistries. C. sojina strains resistant to QoI fungicides have been confirmed throughout Iowa. An integrated method of disease management that does not depend only on fungicides should be employed to maximize disease management to preserve efficacy of existing fungicides. Other disease management practices such as crop rotation, planting of disease-resistant cultivars, and application of fungicides with multiple modes of action should all be used as tools for effective soybean disease management.
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