Fungicide applications can protect soybean yield when foliar diseases are present. However, Missouri soybean fields do not typically have high foliar fungal disease pressure. Yet we still see approximately 60% to 65% of Missouri soybean acreage treated with fungicide, based on a recent informal survey of retailers, crop advisers and extension specialists in the state.
What is the return on investment (ROI) for these foliar fungicide applications to Missouri soybeans?
The Missouri Soybean Merchandising Council (MSMC) partnered with MU Extension in 2018 to address this question through on-farm research. The study has continued annually and involves replicated strip trials on Missouri soybean fields, alternating fungicide treated and non-treated soybean strips. Tests have been conducted on areas throughout the state, and the fungicide application is at growth stage.
The median (or mid-point) yield increase is 1.5 bushels per acre (bu/a) for fungicide-treated soybeans compared to non-treated soybeans in the 54 completed strip trials. University scientists across nine states and Ontario, Canada, found a similar number when analyzing more than 240 small plot research trials. Fungicide applications were projected to increase yield by 2.7%, a 1.6 bu/a increase in a field that averages 60 bu/a.
Both studies support a yield increase that corresponds with fungicide applications to soybeans. However, the odds of a fungicide application resulting in a positive ROI are low. For example, if we estimate fungicide application costs at $40 per acre and soybean
prices at $13 per bushel, we need approximately a 3.1 bu/a yield increase to break even. Of the 54 Missouri strip trials, 22% reached or exceeded that break-even point. Figure 1 shows the average yield difference between fungicide-treated and non-treated soybeans for each strip trial. The best-case scenario was a 12 bu/a yield increase in fungicide-treated soybeans; the worst was a 3 bu/a yield loss in fungicide-treated soybeans.
We are still learning what factors contribute to these different outcomes. Both scenarios occurred in east-central Missouri in fields with similar agronomic practices, low disease pressure and application of a product with two modes of action: Group 7 DMI (demethylation inhibitor) plus Group 11 QoI (quinone outside inhibitor). One difference between the trials was rainfall following fungicide application. The black bars in Figure 2 indicate locations where detectable precipitation occurred within 48 hours of application; gold bars indicate a lack of rain. In general, precipitation within 48 hours of application corresponded with less risk of yield losses due to fungicide application.
This also seems consistent with what we know about fungicide injury to soybeans, which is more likely to occur under hot and dry conditions. Results from 2023 strip trials should provide additional insights into the importance of precipitation given the arid growing season.
What are common soybean diseases that fungicides can control?
The confirmation of soybean rust disease in the U.S. in 2004 resulted in a substantial increase in fungicide applications to soybeans, which were minimal before that time. Soybean rust occurs in south-east Missouri occasionally and is a severe threat when present
due to its ability to spread rapidly. However, most diseases in Missouri soybeans are not as aggressive as soybean rust, and there are opportunities to scout before making fungicide applications.
Frogeye leaf spot (FLS) is generally the most concerning foliar soybean disease across Missouri. We observed the condition in 11 of the 54 strip trials. Symptoms appeared late in the season at deficient levels in 10 plots with no effect on yield. However, symptoms appeared early in one plot, resulting in a 4 bu/a yield loss in non-treated soybeans. The best way to determine whether a fungicide application is warranted for FLS management is to know how the soybean variety is rated for FLS resistance and scout. A fungicide application should be considered if disease symptoms appear during the vegetative or early reproductive stages. Fungicide resistance is already an issue with this disease, and so it is important to avoid unnecessarily exposing the pathogen to fungicides.
Septoria brown spot is the most common disease of Missouri soybeans, but it does not typically cause yield losses that warrant fungicide applications. As expected, we observed the condition in 100% of strip trials. Symptoms usually develop early in the season, with red to brown spots forming on leaflets of the lower canopy. Warm and wet weather can spread the disease to the upper canopy when concerns over yield losses can arise.
Cercospora leaf blight, target spot and white mold are diseases becoming more common in Missouri, and soybean yield may be better protected with fungicides under certain conditions.
What diseases are not controlled with fungicide applications?
Viruses and bacteria can cause foliar soybean diseases but are not managed by fungicides. We observed bacterial blight and more soybean vein necrosis virus (SVNV) than usual during 2023.
Diseases such as sudden death syndrome, Phytophthora root rot and brown stem rot cannot be controlled by foliar fungicides. These can be tricky because disease symptoms can appear on leaves, and fungi or fungi-like pathogens cause the disease. However, those pathogens live in the soil and infect roots. Foliar fungicide applications do not reach the target pest.
What fungicides are most effective?
The Crop Protection Network, which consists of state extension pathologists across the U.S., publishes a fungicide efficacy guide annually and includes the University of Missouri input. Products with multiple modes of action are more likely to delay the onset of fungicide resistance.
Final thoughts: Fungicides are a crucial component of disease management in soybeans. They can provide a positive ROI when used to protect soybean yield from disease. It’s crucial to preserve the effectiveness of fungicides. This requires minimizing unnecessary applications that expose the pathogen to fungicides and increase the risk of fungicide resistance. Farmers already must manage herbicide-resistant waterhemp. No one wants to try to control a resistant pathogen growing inside the soybean plant.
Helpful websites:
- MU Strip Trials Annual Report: https://ipm.missouri.edu/cropPest/stripTrial.pdf
- Soybean Fungicide Efficacy Guide: https://cropprotectionnetwork.org/publications/fungicide-efficacy-for-control-of-soy-bean-foliar-diseases
- MU Field Crop Pathology Website: https://bishm.mufaculty.umsystem.edu/home
- MU Plant Diagnostic Clinic: extension.missouri.edu/programs-plant-diagnostic-clinic
