For decades, scientists studying Cercospora leaf blight have sought to better understand the fungal disease that causes tens of millions of dollars in damage to Louisiana soybean crops each year.

A series of studies by LSU AgCenter researchers in recent years has answered many important questions.

Now, scientists can work toward two long-term goals: developing more precise recommendations for fungicide use and studying which varieties may carry resistance to the disease.

“It just took a lot of this foundational work to even know if it is worth it to do this kind of study,” said Vinson Doyle, head of the AgCenter Department of Plant Pathology and Crop Physiology. “We’ve finally gotten to that point.”

Cercospora leaf blight is the main foliar disease for Louisiana soybeans and is common throughout the Midsouth. The disease causes leaf yellowing, lesions on stems and leaves, and miscolored seeds and can lead to a yield reduction in soybeans.

Until recently, many thought that one Cercospora fungus species responsible for the disease was transmitted by seeds. A series of studies by Sara Thomas-Sharma, an AgCenter plant pathologist, proved that the fungus travels by spores through the air. Thomas-Sharma has led efforts to trap Cercospora spores in Louisiana and 20 other states and then examine the spores.

“Cercospora leaf blight is a very local problem in a sense that it is a Midsouthern disease problem,” Thomas-Sharma said. “It’s not considered a big problem in the Midwest in bigger soybean growing regions, but based on our spore trapping work across the U.S., it is detected almost everywhere.”

Doyle and other scientists have now identified more than 10 species of Cercospora throughout North and South America that can cause CLB. Another obstacle in studying Cercospora leaf blight was recently solved by a doctoral student in the AgCenter Department of Plant Pathology and Crop Physiology who found a way to infect plants with the fungus and reproduce symptoms in a controlled setting, Doyle said.

With these major obstacles cleared, Thomas-Sharma and Doyle can proceed with the research they have pursued for years. Over the past several years they have sought the resources and now have established a growth chamber facility at the greenhouses near the LSU Baton Rouge campus, where they can soon begin testing commercial soybean varieties and research breeding lines for resistance to Cercospora leaf blight.

“It seems like a simple question, an obvious question that you would answer, but there have been so many other foundational questions that we’ve had to deal with or problems that we’ve had to deal with to get to this point,” Doyle said. “Screening varieties for resistance for a lot of diseases is step one, but for this one, you couldn’t do it.”

Thomas-Sharma has also begun studying environmental conditions that contribute to the disease, both in soybean fields and in climate-controlled greenhouses. Not all plants that carry the fungus develop symptoms or lose yield, she said. Heat and humidity appear to create conditions needed for the disease to flourish.

“We don’t know if the conditions for the different species are different,” Thomas-Sharma said. “The ultimate goal is to try to figure out what environmental conditions might be associated with spore release in the field through greenhouse trials.”

In the future, Thomas-Sharma, in collaboration with researchers across the U.S., hopes to create a predictive modeling system to allow soybean producers to better understand when to apply fungicides based on the risk created by environmental conditions and the likely presence of Cercospora spores. Similar tools for other crops exist on the Crop Protection Network website, a project created by a consortium of land-grant universities.

“There’s still a lot of testing before we actually recommend growers to change application timing,” Thomas-Sharma said. “We need to test to see if our predictions actually control the disease better.”

After much work, AgCenter researchers say they are eager to develop more science-backed recommendations that can help produce better crops.

“We’re much closer to understanding or predicting when disease outbreaks are going to occur and also identifying the genetic basis of resistance,” Doyle said. “The best tools are going to be finding resistant varieties. We’re finally at a point where we can do some of that work. I think we have much better prospects now than we did 10 years ago of really trying to figure out when farmers need to put their energy toward addressing this disease and when they can ignore it.”

New technologies and years of scientific breakthroughs have contributed to a promising future for Cercospora leaf blight studies, the researchers said.

“People have been working on this disease for a long time,” Doyle said. “Now we’re finally able to make progress, and much of that has come from support from the soybean board.”

Cercospora leaf blight is a disease caused by a fungal pathogen and has significant economic effects on soybean production. LSU AgCenter file photo