Cercospora leaf blight (CLB) continues to challenge soybean growers across the southern U.S., but researchers at the LSU AgCenter are making strides in understanding and managing this complex disease.

Plant pathologists Vinson Doyle and Sara Thomas-Sharma are leading efforts to identify the pathogens responsible, track their spread and develop more effective control strategies.

Doyle, who has studied CLB since 2014, explained that early assumptions about the disease’s cause were incomplete.

“We assumed that it was a single species, Cercospora kikuchii,” Doyle said. “What my research has turned up is that there are actually many different species of Cercospora that cause the disease Cercospora leaf blight.”

This discovery has reshaped how researchers approach the disease. Doyle said there are different fungi that cause the same set of symptoms, and it isn’t easy to distinguish just by looking at the plant or the symptoms.

His team has since focused on identifying these species and understanding their biology.

“We started doing some studies to try to understand whether the pathogens that cause the disease could be found on other host plants and how they might be transmitted,” he said.

One major breakthrough came with the development of reliable inoculation protocols.

“We were able to generate spores in vitro and do the inoculations,” Doyle said. “Now we can finally start to understand or identify resistant germplasm.”

Thomas-Sharma’s work complements Doyle’s by focusing on the timing and environmental conditions that influence disease outbreaks.

“We have spore trappings that we do on a regular basis which we then analyze to see when we get peaks of the spores of the pathogen,” she said. “If we can try to predict when the spores might be in the air, we could potentially target our fungicide applications.”

Traditionally, fungicides are applied based on plant growth stages, but Thomas-Sharma’s research suggests a more precise approach. If fungicides can be applied before a predicted spore peak, the crop may be better protected.

Spore traps have been deployed at three AgCenter research stations — Macon Ridge, Dean Lee and Doyle Chambers Central Station.

She noted that the traps pick up Cercospora flagellaris in the highest numbers and most frequently during the season. Other species like Cercospora sigesbeckiae and Cercospora kikuchii appear less frequently and in lower numbers.

Despite these advances, challenges remain with fungicide efficacy and whether high levels of spores will mean high levels of disease in the field.

Soybean leaves symptomatic of Cercospora leaf blight in a soybean field at the LSU AgCenter Macon Ridge Research Station in Winnsboro. Photo by Sara Thomas-Sharma

To address this, the team is moving into growth chamber studies.

“We want to confirm that if there is a known source of spores, the spore traps are picking it up, and if you put healthy plants there, will it get diseased if you gave the right conditions?” she said.

Doyle emphasized the importance of these controlled studies.

“We can infect leaves that are present and as the plant continues to develop, and we can still find the fungus there,” he said. “We can find the fungus in the seed later in development, which suggests that this fungus is just growing through the plant.”

The new growth chamber facility on campus will allow researchers to simulate specific environmental conditions.

“If the humidity is 65%, do we get infection? Yes or no?” Doyle said. “Do we get more infection when humidity is 85%? If we maintain wetness on the leaves for 48 hours versus 24 hours, what is the impact?”

Both researchers agree that understanding the environmental triggers and identifying resistant varieties are key to long-term management.