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.
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.
|YEAR||COUNTY||APPLICATION DATE (GS)x||DISEASE DRIVING YIELD RESPONSEy||AVERAGE TRIAL YIELD (BU/A)y||ESTIMATED GAIN FROM FUNGICIDE (BU/A)z|
|2020||Lincoln||June 16 (R2.5)||—||NA||—|
|June 16 (R3)||—||60.1 (NS)||1.1|
|2020||Lonoke||June 17 (R2.5)||Septoria brown spot*||33.8***||8.9|
|2020||Chicot||June 18 (R3)||—||71.1 (NS)||0.8|
|2020||Phillips||June 30 (R3)||—||NA||—|
|2020||Desha||July 14 (R5)||—||NA||—|
|2020||White||July 15 (R4)||—||49.4 (NS)||-1.5|
|2020||Arkansas||July 23 (R2)||Aerial blight**||55.7***||12|
|2020||Prairie||August 4 (R2.5)||Soybean rust**||70.1*||1.8|
|2020||Ashley||August 10 (R2.5)||Frogeye leaf spot||51.2*||4.4|
|2021||Lincoln||June 29 (R3)||51 (NS)||2|
|2021||Arkansas A||June 30 (R3)||Septoria brown spot***||75.7***||4.9|
|2021||Jefferson||July 8 (R3)||—||98.9 (NS)||-0.5|
|2021||Chicot||July 9 (R3)||—||64.1 (NS)||1.4|
|2021||Lawrence||July 26 (R3)||—||79.3 (NS)||1.2|
|2021||White||July 27 (R3)||Frogeye leaf spot***||56.6***||4.6|
|2021||Arkansas B||July 29 (R3)||Target spot***||72.2***||8|
|2021||Prairie||August 5 (R3)||Septoria brown spot***||44.5 ***||2.2|
|2021||Ashley||August 12 (R3)||—||NA||—|
|2021||Arkansas C||August 16 (R3)||Frogeye leaf spot / aerial blight***||44.7***||9.3|
x Date of fungicide application and growth stage of the field at application (GS)y Significantly different by treatment at *P = 0.05, **P = 0.01, or ***P < 0.001, NS = not significantly different, NA = yield data was not available
z Average of yields from all fungicide treatments applied – yield of nontreated
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.