In 2024, we’ve witnessed a shift from near-perfect field conditions in March to acceptable planting conditions in April, which then turned cool and wet and have remained so into May. When soil conditions are cool and wet, several concerns arise such as the impact on herbicide applications, the prevalence of seedling diseases that thrive in cool/wet soil, and a range of emergence issues that can be linked to equipment. Understanding these challenges is crucial for effective crop management.

Emerged soybean with damage to hypocotyl and cotyledons. Photo Credit – Aaron Hager, University of Illinois

I would like to specifically focus on the use of PPO Inhibitors or Group 14 Mode of Action herbicides. Many of these PPO-inhibiting herbicides are the foundation of preemergence herbicide applications. Beyond having residual properties that may prevent troublesome weeds from emerging like waterhemp, they often have great burndown properties to control weeds that may be present at the time of application. Herbicides below are examples of active ingredients (A.I.) that may contribute to crop responses in cool/wet conditions simply because the soybean seedling is taking longer to metabolize the herbicide.

The list below ranks from one, potentially the most troublesome, to four, which may cause the least amount of concern.

1. Flumioxozin – Valor
2. Saflufenacil – Sharpen
3. Sulfentrazone – Spartan
4. Fomesafen – Flexstar

It’s worth noting that during periods when soybeans are actively growing, you’re unlikely to observe any negative responses to these herbicide A.I.s. In fact, some of the most effective weed control with lasting residual can be achieved with products like Flumioxozin. Generally, if you’re planning early-season target applications, it is advised to avoid products higher up on the list for use as tank mix components. However, as the season progresses, the higher-ranked products can offer excellent weed control without potentially sacrificing crop safety.

Symptoms of PPO injury from germination through the VC growth stage, when the cotyledons are fully expanded and the unifoliate leaves are unfolded, may include any of the following:

• Reddish-colored lesions or spots on the hypocotyl and/or cotyledons. In case of severe injury, lesions on the hypocotyl can weaken the soybean seedlings, which may cause soybean death as they are trying to emerge, especially if crusting conditions are present.
• Reddish-colored tissue near the soil surface.
• Injury to only cotyledons that may discolor or even misform the shape of cotyledons. Typically, when injury only occurs to cotyledons, young soybean plants recover without any issues with stand establishment.

Often, the observed conditions of soybeans may mimic one thing but be caused by a completely different causal agent. Items may include any of the following, but they are not limited to just one:

• Soybean response to SDS seed treatments, particularly iLevo from Bayer CropScience. This is often purely cosmetic, and visible symptoms quickly disappear once the first trifoliate emerges. This is commonly referred to as the “Halo” effect because of the halo-like appearance, which is often reddish brown in color on the soybean cotyledon.

Credit – J. Rees, University of Nebraska-Lincoln

• Seedling pathogens that typically affect soybeans during cool, wet conditions could also mimic PPO injury and should be properly identified. One common soybean pathogen that may thrive in cool, wet conditions is the Pythium species, which has a wide range of hosts, including corn. Key symptoms may include water-soaked lesions, spongy or soft tissue, or even a foul order of rotting soybeans that may lead to entire seedling death or damping off.

Credit – University of Minnesota Extension

• Frost injury can also be a culprit that prolongs PPO injury or maybe the single cause of injury. It may have a nipping-type appearance on the cotyledons, a water-soaked appearance, or even complete seedling death that may result in a black appearance of soybeans.

In conclusion, PPO herbicides may raise some alarms of unsightly-looking soybeans as they emerge and begin to grow. It is important to consider all possibilities of possible causal agents. My advice to growers, agronomists, and field scouts whenever going to the field is to take a moment and get a 360° view. This may prevent fixating on one potential issue. Look at different areas of the field, dig up some plants (some that look good and bad), ask questions, and consider field history (even last year). Please take a moment and consider the big picture.

Sources:

The following products references in this blog are registered trademarks of their respective trade names.

1. Valor – Valent U.S.A. LLC
2. Sharpen – BASF Corporation
3. Spartan – FMC Corporation
4. Flexstar- Syngenta Corporation
5. iLevo – Bayer CropScience LLC

Information and pictures displayed in this article were referenced from the following Land Grant Universities:

Hager, A. “Soybean Injury from Soil-Applied Herbicides.” Department of Crop Sciences, University of Illinois at Urbana-Champaign, May 20, 2014.

Malvick, D. “Soybean seed and seedling diseases” University of Minnesota Extension, 2018

Rees, J., Jhala, A., Jackson-Ziems, T., “Q & A: What is Causing Problems with Soybean Emergence?” University of Nebraska, Lincoln, June 4, 2020