Worth of Foliar Fungicides in Soybean Production
An article titled “A machine learning interpretation of the contribution of foliar fungicides to soybean yield in the north-central United States” by Shah et al. appears in Scientific Reports 2021, 11:18769, (https://doi.org/10.1038/s41598-021-98230-2), and presents results from an analysis of the contribution of foliar fungicide application to soybean yields in the north-central U.S. Major points from the article follow.
• Reported results are based on grower surveys conducted in 2014-2016 that provided a database of 2738 non-irrigated fields, of which 30% had been sprayed with foliar fungicides.
• Surveyed locations were spread across the U.S. north-central region at latitudes from 37.11N to 48.97N. (For reference, Jackson, Tenn. is at latitude 35.65 and Stoneville, Miss. is at latitude 33.42).
• Latitude and planting date were the two most important factors associated with yield, while application of foliar fungicides ranked 7th out of 20 factors in relative importance to final yield.
• More yield benefit was gained from using foliar fungicides in late-planted fields at more southern latitudes in the survey.
• There was a greater yield response to foliar fungicide use in higher-yield environments. Also, financial return attributed to use of foliar fungicide was positive and significant from the 100 highest yielding fields, while the opposite was true from the 100 lowest yielding fields in the survey.
• Results from the survey indicate the following. 1) Foliar fungicides should not be applied indiscriminately, but rather as a response to disease scouting and forecasting, and integrated pest management. 2) Environmental considerations and potential loss of product efficacy associated with indiscriminate use of foliar fungicides should be weighed against the potential yield penalty associated with not using them.
The results reported in the above article are based on surveys from locations much farther north than the Midsouth soybean production region. Thus, the below information from recent research conducted in Arkansas should be considered for decisions about using foliar fungicides in the Midsouth.
Results from on-farm research that was conducted in Arkansas in 2020 and 2021 and reported by Dr. Terry Spurlock, Univ. of Arkansas plant pathologist, appear in the article titled “Soybean On-Farm Fungicide Trials Summary, 2020-2021". These results provide up-to-date information about using currently-available foliar fungicides in the Midsouth. Key points from that research follow.
• On-farm trials were conducted in 10 Arkansas counties in both 2020 and 2021. All trials were irrigated to ensure high yield environments.
• Fungicides with FRAC codes 3+7 or 3+7+11 were applied at the R2.5-R3 stage of development. Thus, applied materials had multiple modes of action.
• Disease presence was assessed prior to fungicide application and again at R6.
• Foliar fungicide applications resulted in significant yield increases at all locations where disease pressure was moderate to severe.
• Where disease pressure was not moderate to severe, applying a foliar fungicide did not result in a $ return above the application cost.
• Frogeye leaf spot, Septoria brown spot, aerial blight, and target spot were the yield-limiting diseases that were associated with significant yield increases resulting from fungicide applications. These diseases are all common in Midsouth soybean fields, and are likely to occur in fields where soybeans have been grown.
• Across both years, significant yield responses to fungicide application were most often obtained from fields that were later-maturing (R2.5-R3 from July 23-Aug. 10 in 2020 and July 27-Aug. 16 in 2021).
Here is the take-home message supported by results reported in the above articles, and results from earlier research.
• Efficacious foliar fungicides should only be applied when pressure from targeted disease pathogens is moderate to high, or is expected or forecast to be at that level. Click here for an article on this website that provides information about how the disease triangle works for forecasting how/when a disease pathogen might inflict economic losses to soybeans.
• An economic yield response from application of foliar fungicides to soybeans in the Midsouth is more likely to occur in late-planted fields, where later-maturing varieties are grown, and in high-yield environments such as early-planted fields that are irrigated.
• Indiscriminate use of foliar fungicides without a verified reason based on scouting results and/or forecasting likely will lead to quicker loss of product efficacy. This means that the automatic prophylactic application of foliar fungicides to soybeans at stage R3 in the Midsouth should be evaluated against this potential occurrence.
• The documented and continuing reduced efficacy of foliar fungicides against common soybean disease pathogens indicates that plant breeders and geneticists must increasingly work toward discovering genetic resistance to these pathogens, and incorporating identified resistance sources into newly developed and released soybean varieties.
Composed by Larry G. Heatherly, Feb. 2022, larryh91746@gmail.com