Research Highlights

Investigating the Variability in the Pathogen Behind Sudden Death Syndrome

By Laura Temple

Pathogenicity, or the ability of a pathogen to cause disease, can vary dramatically within a species, much like human genetics do. This applies to Fusarium virguliforme and other Fusarium species, the soil-borne fungal pathogens that can cause sudden death syndrome, or SDS, in soybeans. This disease can be a significant yield robber.

“Understanding the species composition and genetic diversity of Fusarium presently found in association with SDS in Ohio will help us determine how best to protect soybean yields,” explains Horacio Lopez-Nicora, assistant professor, soybean pathology and nematology, at Ohio State University. “Those differences include how much damage they can cause to soybeans, how much they can tolerate fungicide and how they interact with other pathogens.”

He is leading research funded by the Ohio Soybean Council aimed at learning about genetic diversity in strains of the fungus causing SDS across the state. The study started during the 2022 growing season when SDS appeared in many soybean fields, including some that used a seed treatment fungicide at planting to protect against the disease. 

In 2023, soybeans with SDS symptoms appeared earlier than usual in the growing season. Lopez-Nicora points to planting soybeans in cool, wet soils in the spring, followed by a period of drought and then a period of heavy rains as factors that may have induced those early symptoms.

His team used samples from diseased soybean plants gathered during these seasons to begin exploring Fusarium genetic diversity and address three study objectives:

1. Identify the species and genetic diversity of Fusarium associated with SDS in Ohio.
2. Test those strains for pathogenicity and fungicide resistance.
3. Investigate how Fusarium strains interact with soybean cyst nematode, or SCN.

Disease Identification

The first step in understanding a pathogen is correctly identifying the disease it causes. Symptoms for SDS appear similar to other soybean diseases, like brown stem rot or red crown rot, which are caused by very different pathogens.

“We gathered or requested that disease samples include the entire soybean plant, including the root system, so we could correctly identify the disease,” Lopez-Nicora says. “From the Fusarium isolates retrieved from the samples, we started a collection. Do all Fusarium isolates behave the same?”

He adds that the collection may show differences between strains of the pathogen throughout Ohio. As his team learns more about each isolate, they will be able to better advise soybean farmers how to protect against the most common strains in their areas.

Isolate Screening

From collected samples of Fusarium species, Lopez-Nicora’s research team grew each fungus in the lab, creating pure isolates for further study. They exposed soybeans to those isolates to see their pathogenicity in causing SDS and the potential severity. They also subjected the isolates to fungicides, to test if any strains have developed resistance.

“We received samples of soybeans with SDS symptoms from farmers who reported protecting the seed with a fungicide seed treatment,” he says. “Our screening will help determine if the soybeans outgrew the fungicide protection or if the pathogen is truly developing fungicide resistance.”

He notes that these pathogens are difficult to eradicate because they live in the soil. The detailed information about strains found in the plant samples will tell farmers specifically what challenges they are dealing with in given fields. 

Interaction with Soybean Cyst Nematode

Research documents interactions between SDS and SCN. In the greenhouse, the research team used isolates grown from these samples to infest soils also containing SCN. They planted soybeans in these soils and observed any changes in soybean root development and SCN populations.

“This part of the research will help us learn more about how SDS and SCN work together to damage soybeans,” Lopez-Nicora explains. “For example, do some types of Fusarium virguliforme cause more root rot? Do some interact with SCN more easily?”

He believes a better understanding of the multiple threats soybeans face will help farmers make better management decisions. As this work continues, he expects to have better answers to farmers’ questions about their SDS symptoms and potential interactions with SCN. 

Published: Dec 4, 2023