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As we look forward to next year, frogeye leaf spot’s expanding footprint is just one more reminder that soybean growers need to be more proactive than ever when it comes to planning fungicide applications.
Frogeye leaf spot (FLS), a common threat to soybean yields, varies in its impact seasonally due to changing environmental conditions. Disease development thrives in warm and humid soybean canopies at temperatures of 75–85°F, exacerbated by prolonged leaf wetness from rain, dew, or irrigation. These conditions promote fungal spore germination, which can reinfect neighboring soybeans and cause severe defoliation of a field within a single season. The risk of yield loss increases with the persistence of infected soybean residue and continuous cropping without rotation.
📸: BASF employee, Albre Brown, Ph.D. Technical Marketing Manager, Soybean Fungicides and Insecticides/Featuring frogeye leaf spot on a soybean leaf.
To mitigate the impact of FLS, farmers can adopt strategies such as planting resistant varieties, practicing crop rotation, and using fungicides. Employing strategic agronomic management and timely fungicide applications significantly reduces the potential impact of frogeye leaf spot in soybean fields.
Instead of waiting for the disease to show up and potentially compromising current and future growing seasons, we recommend using this winter to make a specific plan about when and how you’ll approach pressure from fungi like FLS this year. Maximizing the impact of a fungicide application depends on an understanding of field history, scouting, and the timing of application. We’ll expand on these key points in the sections below, but here are a few helpful numbers to keep in mind:
Effectively control fungi growth with quick uptake and outstanding residual
activity with these treatments.
📸: BASF internal photograph/Featuring a table with recommended treatment for frogeye leaf spot.
Optimum application stages are R2-R4. Apply proactively before damage from FLS infection becomes visible.
Frogeye leaf spot is a fungal disease named for its signature visual pattern: an eye-like circular or angular lesion on the leaves of infected plants. In rare cases, lesions can also be found on the stems of particularly susceptible plants. These spots, measuring 1–5 mm in diameter, appear in a burst scattered across the surface of leaves as if they resulted from rainfall.
As the infection takes hold, the spots grow. The centers of the spots turn pale gray, while their borders take on a red, brown, or purple color. In the middle of the pattern, you may be able to observe small black dots resembling a pupil in the “eye.” These are the spores of the fungus.
📸: BASF employee, Albre Brown, Ph.D. Technical Marketing Manager, Soybean Fungicides, and Insecticides/Featuring Frogeye leaf spot on soybean leaves. Frogeye leaf spot can be identified by eye-like circular or angular lesions on leaves.
Like many fungal infections, FLS tends to infect immature plants, including seedlings, and senescing flowers. An FLS infection can begin in several ways:
Frogeye leaf spot can be present in seeds at planting. The infection may become obvious as weaker, infected plants begin to grow—or fail to germinate altogether.
Leftover soybean residue carrying the fungus can decompose, leading to the release of new spores and the start of a new infection as warmer, humid air arrives.
Splashing rain, irrigation water, or wind can all help disperse spores over short distances.
FLS is caused by Cercospora sojina, a fungal species with a “polycyclic” infection cycle, meaning it has the potential to release spores several times throughout a single growing season. When temperatures rise above 77 °F and the air is humid from frequent rainfall or irrigation, the pathogen begins to produce spores. As more spores come in contact with a plant, the risk of infection goes up.
📸: BASF employee, Albre Brown, Ph.D. Technical Marketing Manager, Soybean Fungicides and Insecticides/Featuring late season frogeye leaf spot damage in a soybean field.
Once an infection takes hold in a soybean plant, it becomes a host for the fungus and a potential source of infection for nearby plants. Even after the plant is cleared, tillage has the potential to bury residual material, allowing the fungus to overwinter for two or three more years when it could resurface and lead to another infection.
In 2022, FLS was responsible for an estimated 4 million bushels lost.2 While these small spots may not seem aggressive or threatening at first, they have a compounding effect. Each spot eats away at the plant’s ability to grow, and each spot has the potential to spread the infection. If left unchecked, the infection can spread to the seeds, causing yield loss. Infected plant material will then overwinter in your field, leading to potential losses the following year. As temperatures around the world continue to rise, FLS is spreading into new territories in the U.S., with estimated yield losses of around 35%.3
Remember, if you saw FLS in your field last season, there is a high probability you’ll find it again this season. This is especially true if you found it after it was already too late to apply fungicide.
Be on the lookout for frogeye leaf spot following hot, humid summers. The spots it causes become visible anytime from R1 to R6, but it thrives under a thick canopy. If you intend to manage the infection, prioritize scouting for it around R1 when the plants begin to bloom. We recommend scouting at least a few plants in every 10 acres of field. Bear in mind that the amount and severity of the disease can differ greatly between different plots of your field due to environmental conditions. As an example, rows that experience early morning shade will stay wet longer from dew or irrigation, making a more humid and hospitable habitat for the fungus to grow.
📸: BASF employee, Albre Brown, Ph.D. Technical Marketing Manager, Soybean Fungicides and Insecticides/Featuring Frogeye leaf spot lesions on soybean plants that have started to darken to brown.
FLS on leaves can sometimes be mistaken for “application droplets,” also known as herbicide drift. These terms refer to places where herbicide may have landed on a plant previously. If you are unsure, pay particular attention to the spot’s purplish border, as this is a strong indicator of FLS.
These spots can also develop on stems and pods, though they are harder to diagnose. Lesions in these places start out reddish in color and darken to brown and develop a sunken center with age. The fungus that causes this disease can even invade the pod walls, ultimately reaching the seed.
Your strategy for treating and preventing frogeye leaf spot in your fields should depend on your goals, risk levels, and the severity of the infection you’ve seen so far. Preventive strategies should revolve around careful seed selection with a trusted expert from your seed retailer, a nearby extension office agent, or your regional BASF agent. Meanwhile, treatment will likely require a holistic strategy that incorporates seed selection, fungicide application, and crop rotation.
Several genes are commonly used to combat FLS. The Rcs3 gene is reported to be resistant to all known races of FLS pathogens in the U.S.4 To set your soybean fields up for success, always speak with your seed selection expert for more personalized guidance.
For unparalleled power and proven plant health protection in a soybean fungicide, choose Revytek® Fungicide. When it comes to treating frogeye leaf spot, you’ll need a fungicide with strong curative action, fast uptake, and long-lasting residual performance. This treatment boasts a +6-bu/A yield increase over untreated soybean fields thanks to its unique active ingredient, Revysol®, which is available only through BASF.
A new soybean fungicide innovation from BASF, Revylok™ fungicide is designed for growers who are losing valuable bushels to disease. The complementary systemic effects of Revysol® and Xemium® fungicidal active ingredients are powerful against pathogens that have developed resistance.
The ideal window for disease management is between R2-R4 soybean growth stages. The period of foliar fungicide application with the highest rate of success is during the early R3 stage. Ideally, you’ll want to apply at least two to three modes of action to effectively manage pathogen populations that have developed resistance to older chemistries.
📸: BASF employee, Albre Brown, Ph.D. Technical Marketing Manager, Soybean Fungicides and Insecticides/Featuring defoliated soybean field with frogeye leaf spot.
Crop rotation with nonhost crops like corn, sorghum, and small grains can disrupt FLS’ life cycle, though it will require several growing seasons to greatly reduce the chance of infection. Reduced tillage systems also tend to have more instances of infection since FLS overwinters in plant residue.
If FLS continues to spread after an appropriately timed fungicide application, you may be dealing with a fungicide-resistant infection. In the past, Group 11 fungicides have been widely used for the management of FLS. However, since 2010, reports of FLS with resistance to Group 11 fungicides continue to expand across states like Nebraska and Iowa.
Professor Daren Mueller, a plant pathologist from Iowa State University, lists Group 11 fungicides as “High Risk” because of their tendency to produce resistance. He goes on to classify DMI fungicides (Group 3) as “medium risk” and SDHI fungicides (Group 7) as “medium to high risk.”5
As mentioned above, a strong prevention and management plan will include several modes of action and a residual fungicide to provide a broad spectrum of control.
In a warming world, frogeye leaf spot is one of many threats to soybean yield worth monitoring. The key takeaways on the subject are:
With diseases like FLS under control, you’ll be free to reap the benefits of soybeans thriving in warm, humid weather. Just be sure to check in often for updates on evolving diseases and resistances.
See other examples of techniques to produce resilient soybean fields:
Experts are available to help you make your decisions. Reach out to your fungicide retailer, a nearby extension office agent, or a fungicide company professional like your regional BASF representative.
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Endnotes
Always read and follow label directions. Grow Smart, Revytek, Revysol, and Xemium are registered trademarks of BASF. Revylok is a trademark of BASF. Copyright 2024 BASF Corporation. All rights reserved.
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January 8, 2025
