Best Time of Day for Corn and Soybean Fungicide Applications

INSIGHTS

• Changing weather conditions throughout the day can impact fungicide efficacy.
• Minimal yield increases were noticed with applications made at cooler times of day.
• Targeting fungicide applications to the crop stage and disease pressure will influence yield more than time of day.

Introduction

Routine fungicide applications have gained popularity, and ideally applications should be made just prior to favorable disease development conditions to minimize infections. In addition, applying just prior to growth stages in which the crop starts to become more vulnerable to infection can also improve results. Because of these factors, most corn and soybean fungicides are applied at the R1 and R3 growth stages respectively. The narrow window for applying fungicides at the right crop stage prevents most growers from even considering potential benefits from spraying at specific times of the day to improve efficacy. If you own a sprayer and have the flexibility to spray at specific times, there may be some benefit to adjusting your application time.

Application Factors Influencing Fungicide Effectiveness

Maximizing fungicide uptake into the leaf will undoubtedly increase the potential for yield response. Weather conditions associated with the specific time of day applications are made can play an important role in dictating how well a fungicide is taken up by the leaf. Consider the following factors related to the time of day applications are made:

1. Wind Speed: No wind and low wind speeds help maximize leaf coverage and minimize drift. Light winds can also help prevent fine spray particles from being trapped in temperature inversion layers and drifting away.

2. Leaf dryness: Heavy morning dew on leaves could facilitate fungicide runoff, reducing its uptake by the plant. Dry leaves minimize this, however light morning dew could still be advantageous when spray gallons per acre are very low, acting to spread fungicide across the leaf surface more evenly.

3. Temperature/drying conditions: High temperatures that promote fast drying of spray solutions may increase evaporative losses and reduce leaf uptake. Calm, cloudy days and cooler late afternoon to dusk conditions can help leaf stomates stay open longer after application, potentially enhancing uptake of fungicides.

2024 Timing of Fungicide Application Trials

The Agronomy in Action research team implemented trials designed to investigate corn and soybean responses to the impact of the time of day the fungicide application was made. Corn trials were carried out in Malta, IL, and Slater IA, and soybeans trials were at Slater, IA, and Waterloo, NE. Trials were designed to compare the effectiveness of early morning, noon, mid-afternoon, and evening/night application times to an untreated check. Fungicide applications were made over the duration of the same day at each site, using Miravis^® Neo fungicide at a rate of 13.7 oz/A. Treatments were applied by ground at the R3 stage in soybean trials (20 GPA) and with a DJI Agras T20P spray drone at the R1 stage in corn trials (5 GPA).

Results

When averaged across locations, corn yields increased to some degree with all fungicide application timings as compared to the untreated checks (Graph 1). The largest fungicide application timing responses in corn yield were observed with early morning (12.7 Bu/A) and late evening (9.1 Bu/A) application timings. Noon and mid-afternoon applications tended to be somewhat less responsive to fungicide applications, although the results were not statistically significant.

When averaged across soybean trial locations, there were similar fungicide timing responses for morning, mid-afternoon and evening applications (1.1 to 1.4 Bu/A). The noon timing was the least responsive (0.4 Bu/A) of the four timings (Graph 2). Interestingly, when looking at Slater, IA, and Waterloo, NE, responses independently, Slater was the most responsive when applied in the evening (2.1 Bu/A), whereas Waterloo was most responsive with morning applications (3.2 Bu/A, data not shown). For both locations, noon application responses continued to be the least responsive of all timings.

Graph 1. Corn yield response to different fungicide time-of-day application timings averaged across trial locations in 2024.

Graph 2. Soybean yield response to different fungicide time-of-day application timings averaged across trial locations in 2024.

Summary

This study showed a potential for better yield response to fungicide by making applications in the early or later parts of the day when temperatures and wind speed are typically lower. Low disease pressure and dry conditions during late summer likely reduced overall fungicide response in both 2024 trials. While there was a slight yield advantage to early morning and evening applications, data across all timings in both studies indicate very small yield differences across the duration of a single day. As mentioned earlier, factors such as temperature, leaf dryness and wind speed are dynamically related and difficult to predict. Timely fungicide applications targeting proper crop growth stage and applied just prior to disease onset will likely have a greater influence on yield than the time of day the application is made.