INSIGHTS
- Fungicide applied at R3 consistently increased soybean yield in the trial.
- Yield response to a fungicide applied at the V4 growth stage was minimal.
- Soybean varieties responded similarly to foliar fungicide applications.
Introduction
Foliar fungicide application timing in soybeans is a critical aspect of crop management that can significantly influence disease control efficacy and overall yield potential. Historically, the R3 growth stage (beginning pod) has been the optimal timing to apply fungicide (Figure 1). At this critical stage, soybeans are particularly vulnerable to various fungal pathogens. Applying fungicides at R3 maximizes spray coverage of the plant canopy, effectively protecting both existing and newly forming pods. This timing has consistently demonstrated yield improvements.
Figure 1. Soybeans at V4 growth stage on top and R3 growth stage on bottom from 2024 trials. Recent changes to soybean production including higher yield potential, earlier planting dates, rising disease pressure, and advancements in fungicide technology has prompted growers and agronomists to ask if there is a potential yield benefit to applying a fungicide earlier during vegetative growth stages.
Agronomy In Action Research Trial
In 2024, the Golden Harvest Agronomy in Action Research team implemented a trial at six locations across Illinois, Iowa, Kansas, Nebraska and Wisconsin to evaluate the effect of a fungicide application during the V4 growth stage (four unfolded trifoliate leaves, Figure 1). The fungicide application at V4 was compared to a traditional application timing at R3 and a no fungicide or untreated check. A combination treatment of a fungicide applied at V4 and R3 was also included. All fungicide applications used Miravis® Neo fungicide applied at 13.7 oz/acre.
- Check – no fungicide applied
- V4 Fungicide
- R3 Fungicide
- V4 + R3 Fungicide
Four soybean varieties were planted at each location depending on geography (Table 1).
Table 1. Variety groups planted at each location based on relative maturity (RM). Yield Results
Despite environmental conditions conducive for disease for much of the growing season, overall disease pressure was low at all sites. Sclerotinia stem rot (white mold) was present at Malta, IL and was likely the reason soybean yields were lower there. Janesville, WI had septoria brown spot but symptoms did not progress up the canopy.
Yield environments ranged from 66 Bu/A at Malta, IL to 87 Bu/A at Clinton, IL. There was no interaction between variety and fungicide treatment, so all results are averaged across varieties. On average across all locations, fungicide applied at the R3 growth stage significantly increased yield by 2.5 Bu/A compared to the untreated check (Graph 1). Fungicide at R3 statistically increased yield at 2 out of the 6 locations with a 3.8 Bu/A response at Clay Center, KS and 4.5 Bu/A response at Malta, IL. Yield response to R3 fungicide applications ranged from 1.5 – 4.5 Bu/A depending on the location.
Fungicide application at V4 did not significantly increase yield at any location. Clinton, IL (1.7 Bu/A) and Janesville, WI (1.0 Bu/A) were the only locations with a numerical response of at least 1 Bu/A (Graph 1). On average across all locations, combining V4 and R3 fungicide timings did not out yield R3 application alone (Graph 1). Although not statistically different, only Janesville, WI, observed greater yields with two application timings compared to the R3 timing alone.
Graph 1. Soybean yield response to foliar fungicide timing at six locations averaged across four varieties in 2024.Summary
Results from this study confirm the consistency in yield response from R3 fungicide applications and why it should be considered as part of a comprehensive soybean management program. Even in environments with low disease pressure, positive responses to fungicide at the R3 timing was observed. In contrast, fungicide applied during the vegetative stage of V4 had minimal effect on yield. There may be environments and situations where an early season fungicide application is necessary to prevent heavy disease pressure, however this timing should not be a widely used management strategy.
Fungicide application timing in soybeans is critical and the R3 growth stage continues to be the optimal timing to maximize spray coverage and protect soybeans from disease at the most vulnerable period in their life cycle.
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