The search for a soybean variety resistant to Cercospora leaf blight (CLB) has proven to be incredibly difficult for LSU AgCenter researchers.

A major pest of soybeans, particularly in the southern U.S., it can cause significant yield loss and reduce seed quality. The seed disease purple seed stain is caused by the same pathogen responsible for CLB.

According to Blair Buckley, a plant breeder at the AgCenter Red River Research Station in Bossier City, finding a Cercospora-resistant soybean variety has been a struggle.

“It’s definitely been a tough nut to crack,” Buckley said.

He and others have dedicated years of research working to find resistance to CLB.

“Some lines have looked promising at some locations, but not at others,” he said. “The resistance has not held up over time, which just makes it that more difficult.”

That’s not to say some progress hasn’t been made. Researchers started looking at 500 different breeding lines and have since pared that down to 57 lines that have shown promising degrees of resistance.

Brian Ward, a postdoctoral researcher in the LSU Department of Plant Pathology and Crop Physiology, was involved in examining the lines. He helped catalogue the results from three years of studies to identify the most promising varieties.

“Continuing to look at 500 different lines was really too unwieldy. Consolidating the best lines should give us a better picture and allow us to replicate trials better for more clarity,” Ward said.

According to Ward, these lines originated in the United States, Japan, China, Turkey and other countries.

Ward said that there are eight varieties of the 57 consolidated groups that seem to stand out and may help unlock the secret.

“I’m confident that we are going to find resistance in this group,” he said.

One obstacle is that CLB remains somewhat of a mystery. There are multiple Cercospora species involved with the disease. Is it nutrient related? What role does the soil play? How does weather affect it? The answers to these questions could play a key role in expediting the process.

One reason for the urgency is that growers have limited effective management tools for CLB. Fungicide efficacy has significantly declined, and current fungicide strategies have offered limited success in combating the disease.

AgCenter plant pathologists Boyd Padgett and Trey Price are conducting fungicide trials to develop treatment plans to minimize the effects of CLB. A major hurdle to overcome is that strobilurin fungicides are no longer effective on the disease.

Price and Padgett are testing fungicides at five research stations and several on-farm locations. A Mud Master plot sprayer rig is allowing them to replicate tests on both large and small plots.

“Some of the newer SDHI (succinate dehydrogenase inhibitors) products have showed some success, which is encouraging,” Price said.

Price indicated that what constitutes success is still somewhat limited.

“We have seen fungicide applications provide 10% to 15% yield preservation in small-plot trials, but that is not common,” he said.

Price believes finding sources of resistance to CLB in old soybean varieties and incorporating those genetics into high-yielding varieties adapted to the MidSouth is going to be the key to overcoming it. There will be a place for fungicides as a management tool, but they will not be a long-term solution.

“Fungicides are a temporary solution,” Price said. “As with herbicides and insecticides, Mother Nature will win over time.”

One novel approach scientists are examining is looking at the role iron plays in reducing CLB. High levels of iron in leaves has been associated with less CLB. Foliar sprays of iron at the R5 growth stage have been successful in increasing the iron content in leaves, so in theory, this should help reduce CLB.

A caveat to this methodology is in parts of the northern central United States, where some varieties have been bred to be resistant to iron deficiency chlorosis (IDC), a condition common to the area. Researchers plan to test these IDC-resistant varieties to see if the varieties can accumulate more iron and become less susceptible to CLB. If they can, these varieties can be crossed with other lines and help provide resistance to CLB.

Craig Gautreaux

This soybean variety trial at the Dean Lee Research and Extension Center near Alexandria is part of an effort by the LSU AgCenter to develop resistant soybean lines to Cercospora leaf blight (CLB). This disease is a primary pest among soybeans grown in the Midsouth, and finding resistance to CLB has been difficult. Photo provided by Trey Price