Indira Poudel, Madaleine Alessi, Sara Thomas-Sharma, Vinson Doyle and Chien-Yu Huang

Soybeans, one of the most economically important crops produced in Louisiana, are planted on more than 1 million acres annually in Louisiana and contributed more than $776 million to the state’s economy in 2024. Many fungal pathogens can infect soybeans and detrimentally impact yield. To enhance the resilience of soybean production against fungal diseases, a team of LSU graduate students and LSU AgCenter plant pathologists are exploring the use of naturally occurring, plant-derived molecules that have antimicrobial activities or defense-priming effects as novel tools to manage important fungal diseases of soybeans.

In the Midsouth region, Cercospora leaf blight, purple seed stain, frogeye leaf spot and aerial blight are diseases caused by fungal pathogens that have significant economic effects on soybean production. These pathogens all share a preference for the weather conditions characteristic of the southern states, such as high temperatures and high humidity during the growing season. Notably, Cercospora leaf blight and frogeye leaf spot are becoming more common in the midwestern and northern soybean production regions of the United States because of warming winter temperatures.

Cercospora leaf blight and purple seed stain in the Midsouth region are primarily caused by three known pathogens: Cercospora cf. flagellaris, Cercospora kikuchii and Cercospora cf. sigesbeckiae. In recent years, intensive disease surveys have shown that C. cf. flagellaris is the dominant Cercospora species in the southern states of the U.S. Frogeye leaf spot is caused by another species of Cercospora known as Cercospora sojina. All these Cercospora species can be transmitted by wind and through infected plant tissue or passed onto seeds. Aerial blight is caused by a soilborne pathogen, Rhizoctonia solani AG1-IA, which can remain in fields for many seasons due to its ability to form survival structures called sclerotia that can overwinter in the soil. Its ability to infect corn and rice, which are often grown in rotation with soybeans, also makes this pathogen difficult to control in the field. Each of these diseases is increasingly difficult to manage because the frequency of resistance to commonly used fungicides has increased in the pathogen populations.

In addition to the increasing levels of fungicide resistance among the pathogens causing Cercospora leaf blight and purple seed stain — which are different stages of the same disease — as well as frogeye leaf spot and aerial blight, many countries have reduced synthetic pesticide inputs for agricultural use because of their nontarget environmental effects and concerns about producing safe food. Thus, the development of safe, eco-friendly and effective fungicides to manage Cercospora leaf blight, frogeye leaf spot and aerial blight will help prevent the development of pathogen resistance and improve the marketability of soybeans.

The research team’s goal is to develop an effective biofungicide using naturally occurring, plant-derived lipid biosynthesis or metabolic inhibitors and peptides with antimicrobial activities or defense priming effects as new and original tools to manage Cercospora leaf blight, purple seed stain, frogeye leaf spot and aerial blight. Lipids, a group of molecules that includes fats, oils and steroids, are essential to all living organisms. They play a fundamental role in maintaining membrane integrity, regulating cell signaling and serving as energy reserves. In disease-causing Fungi, lipid metabolism — a process that includes the formation and breakdown of lipid molecules — is also linked to virulence, stress tolerance and fungicide resistance mechanisms.

Selective toxicity lipid biosynthesis inhibitors are compounds that disrupt the production of essential lipids in a target pathogen (the fungi that cause disease) while causing minor or no harm to the host and are used for the development of new antifungal compounds or as antimicrobial agents for fungicide applications. Importantly, the phytotoxin cercosporin, which is produced by species of Cercospora that cause Cercospora leaf blight and purple seed stain, was shown to be sequestered in the fungal lipid droplets. This showed that plant-derived lipid biosynthesis inhibitors can be effective solutions for managing fungal diseases in soybeans.

The team’s research focuses on exploring various plant-derived lipid biosynthesis inhibitors — which block the creation of lipids and include flavonoids, terpenes, diterpenoids and fatty acids — and their effects on fungal lipid biosynthesis and metabolism. By growth or cercosporin inhibition screenings, the team has identified a panel of compounds that can suppress fungal pathogens associated with Cercospora leaf blight, frogeye leaf spot and aerial blight.

This approach offers a promising and sustainable alternative to conventional fungicides, potentially reducing the risk of chemical resistance and environmental harm. Naturally occurring, plant-derived peptides with antimicrobial activities are emerging as potential biopesticides against a wide range of plant pathogens. Additionally, peptides (short-chain amino acids) with priming effects can elicit plant defense responses, which act as immunity inducers. When this occurs, a plant exposed to a priming stimulus initiates a temporary “primed state” within the plant to prepare it for future challenging events. If the plant is later attacked by a pathogen, its defense response would be launched significantly quicker and more robustly than a plant without the initial priming. Thus, peptides with priming effects can be used as protectants to prevent pathogen infection.

Functional peptides with antimicrobial activity or disease-response priming effects, such as SAMP (stable antimicrobial peptide) from finger limes and CAPE1 (CAP-derived peptide 1) from tomatoes, can be applied externally as seed treatments to prime the defense response and protect the plant from future infection. The team has tested seed treatments using SAMP and CAPE1 on soybean seeds inoculated with the causal agents of Cercospora leaf blight and purple seed stain and frogeye leaf spot, and both have shown promising results in the laboratory.

The team will continue to experiment with disease management from the perspective of inhibiting fungal pathogens as well as improving natural defense responses in plants. The efficacy of these compounds will be evaluated at different concentrations and through different application methods. The success of the compound is ideally determined by its combined ability to prime the defense response in the plant and suppress fungal growth. The compounds can be applied by spraying the leaves of the plants or by coating the seeds. These experiments will allow the team to determine the most effective concentration and application method of the compounds.

Currently, this research is conducted in the laboratory with the long-term goal of directly benefiting soybean farmers in the field. By identifying plant-derived natural molecules that effectively inhibit fungal growth, the researchers aim to develop safer and more sustainable options for managing major soybean diseases, including Cercospora leaf blight and purple seed stain, frogeye leaf spot and aerial blight. Once the optimal formulations are identified, they will be further tested in field applications as natural alternatives to fungicides.

These compounds have the potential to reduce farmers’ dependence on chemical fungicides, minimize resistance issues and lower production costs. In addition, the use of eco-friendly treatments will help maintain soil health, protect beneficial microorganisms, reduce environmental pollution and increase marketability. Ultimately, this research will enable Louisiana farmers to achieve more stable yields and sustainable soybean production, promoting long-term profitability and environmental stewardship.

Indira Poudel and Madaleine Alessi are graduate students in the LSU AgCenter Department of Plant Pathology and Crop Physiology. Sara Thomas-Sharma is an associate professor, Vinson Doyle is a professor and Chien-Yu Huang is an assistant professor, all in the Department of Plant Pathology and Crop Physiology. The project is supported by the Louisiana Soybean & Grain Research and Promotion Board and the LSU AgCenter.

This article appears in the winter 2025 edition of Louisiana Agriculture.

Indira Poudel, a graduate student in the LSU AgCenter Department of Plant Pathology and Crop Physiology, and undergraduate student Achun Jaden Thatthamla, who is majoring in microbiology at LSU, have tested the efficacy of plant-derived natural molecules with seed treatments. Photo by Chien-Yu Huang

Cercospora leaf blight is a disease caused by a fungal pathogen and has significant economic effects on soybean production. This pathogen has a preference for the weather conditions characteristic of the southern states, such as high temperatures and high humidity during the growing season. AgCenter file photo