Soybean On-Farm Fungicide Trials Summary, 2020-2021

by Terry Spurlock, Extension and Research Plant Pathologist - January 26, 2022

Key Points

• Frogeye leaf spot, Septoria brown spot, and aerial blight were yield limiting diseases at multiple trial locations on soybean.
• Septoria brown spot was yield limiting in two fields where applications were made earlier in the year. The disease moved to the upper third of the canopy and defoliated during pod fill.
• Foliar fungicides provided good control and protected yield at all locations where disease was moderate to severe.
• The likelihood of yield response by fungicide treatment increased in later maturing fields.
• Fields where a yield response was observed had lower temperatures on average (about a degree F) and higher average humidity (between 1-2 %) than those where a yield response was not observed.

Methods

Ten large block fungicide trials were arranged in randomized complete block designs and either planted on 30-inch or 38-inch rows with each fungicide treatment replicated three times, and a non-treated control included in each replication. Trial sizes varied from 15-55 acres each. Fungicide treatments in 2020 were Miravis Top and Revytek applied at 13.7 fl oz/A and 8 fl oz/A, respectively.  In 2021, Miravis Top was applied at 13.7 fl oz/A at all locations and Revytek was applied at 7 fl oz/A at all locations except Ashley County and ‘Arkansas County C’. Trivapro was applied at 13.7 fl oz/A as a substitute at these two locations. Additionally, Approach Prima at 6.8 fl oz/A and Priaxor at 4 fl oz/A +Tilt at 4 fl oz/A were applied at Jefferson County and Chicot County. Each trial was furrow irrigated except for ‘Arkansas A’ which was irrigated with a center pivot.  Fungicides were applied at R3 with a 30ft boom mounted on a ground-driven sprayer in a total water volume of 10 gal/A at 40 psi using TeeJet XR11002VS tips at 5.0 mph. Five points were georeferenced approximately equidistant throughout each block for disease assessments. Disease severity data were collected at a 5-foot radius around each point on percentage scale in 2020, 0-100% where a higher percentage indicated more severe disease, or a 0-9 scale in 2021, where a rating of 9 represented the most severe. Diseases were assessed prior to treatment and again at R6. The grain was harvested with a commercial combine and yield data collected using either a yield monitor or weigh wagon. Yields were adjusted to 13% moisture content for comparison. Prior to analysis, field disease ratings were treated as ordinal data and rank transformed using the rank function in R. Georeferenced yield data was buffered and cleaned. All resulting data were averaged within each fungicide block and analyzed by nested ANOVA followed by means separation of fixed effects using Tukey’s honest significant difference test (HSD) at P=0.05.  Weather data at each location was collected by subscription service to DTN.

Results

Across both years, significant yield responses to fungicide application were most often observed in later maturing fields.  These fields were at R2.5-R3 from July 23 – August 10 in 2020 and July 27 to August 16 in 2021. Weather data indicated differences in average temperature and humidity when locations with a significant yield response were compared to those without a yield response.  The diseases found in fields where a yield response occurred are all common to Arkansas soybean production and the fungi causing these diseases are very likely to be found in many if not most fields where soybeans have been grown for any length of time.  The weather (environment) and the susceptibility of the soybean to a certain disease dictated disease severity. Where disease was not moderate to severe, a fungicide application did not add value to the crop above the application cost (based on a cost of $21).

Table 1.  Yield responses of fungicide trials on soybean completed in 2020 and 2021.

 

Figure 1.  Yield limiting Septoria brown spot from an area in the nontreated control plots of a fungicide trial, 2021.

 

Figure 2.  Yield limiting frogeye leaf spot from an area in the nontreated control plots in a fungicide trial, 2021.

Acknowledgements

We thank the Arkansas Soybean Promotion Board for generously providing funding for this project. We also thank the cooperating farmers for providing field locations for the trials and the participating county extension agents for assistance with the trials.