LSU AgCenter researchers exploring new approaches for managing Cercospora leaf blight are learning more about what triggers toxin production, when mitigation efforts are most successful, how the fungus is spread in the field and how to speed screening for resistance in soybeans.

Sucrose triggers cercosporin toxin production

AgCenter plant pathologist Zhi-Yuan Chen is studying the correlation between soybean leaf tissue sugar levels and the production of the Cercospora toxin.

“Sucrose production might be the signal that triggers an increase in cercosporin toxin production by the pathogen,” Chen said.

Doctoral student Maria Zivanovic, who is working with Chen, has found that soybean leaf sucrose levels increased significantly from the R3 to the R5 developmental stage.

Using leaf extracts in the lab, Zivanovic discovered that the Cercospora pathogen will produce more of the toxin in the R5 extracts than in R3 extracts, and sucrose added to extracts from the R3 stage will produce as much or more of the toxin than in the R5 samples.

“Sucrose had been reported to induce toxin production on other pathogens, but not yet in Cercospora,” Chen said.

Zivanovic also found that adding ammonium phosphate to leaf extracts from R5 will suppress toxin production by the pathogen in culture.

Early foliar application of ammonium phosphate when sugar production in the leaves begins to increase may be a solution for managing Cercospora disease, and it has the added benefit of increasing yield, Chen said. However, this is still in the early stage of field trials, and yield estimates will not begin until the project moves to larger field trials.

Chen is also in the third year of a study using a host-induced gene-silencing mechanism-based molecular fungicide to suppress Cercospora leaf blight.

He has isolated more than 10 gene sequences that have shown encouraging results in suppressing the Cercospora and soybean rust pathogens in the studies conducted in the lab.

Rapid lab screening is moving forward

Plant pathologist Sara Thomas-Sharma is continuing to develop a rapid lab protocol for screening for Cercospora leaf blight resistance in different soybean varieties. The results could give breeders a new tool to focus on the number of soybean varieties to include in field trials.

Researchers are measuring the toxicity of cercosporin on smaller leaf discs in the lab to predict disease resistance in the field, where conditions are far more complex. More than 50% of the 25 varieties tested last year in the lab showed consistent disease ratings, Thomas-Sharma said.

Current lab testing will include 25 of the varieties in official variety trials conducted across the state to provide a comparison of each variety tested with disease ratings from field locations.

“Anything we can develop to give some consistency would be a step forward,” Thomas-Sharma said. “Something is better than nothing in this scenario because there is no alternative beyond field variety trials.”

Iron formulations for managing Cercospora leaf blight

Thomas-Sharma also is looking at iron applications as an alternative to fungicides to help improve soybean resistance to the disease.

“We know that cercosporin toxin can bind to iron, and iron is known to make a plant more resistant to disease by triggering other defense systems within the plant,” she said.

Previous work on this project showed that an iron concentration of 300 parts per million in soybean leaves is needed to see any effect against Cercospora leaf blight.

Researchers have recently established baseline data for iron concentrations in soybeans to show how iron content in the plant changes during growth stages. While the lower canopy had higher concentrations of iron, the upper canopy only reached about 100 to 200 parts per million during R3 to R5 growth stages.

“In Louisiana, iron deficiency is not a problem, so what we are trying to do is push iron concentrations over what is normal for the plant to try to improve resistance,” Thomas-Sharma said.

Research trials have produced variable results, in part because of changes in commercially available iron formulations. The rate and timing of foliar applications may have also played a role, she said.

Cercospora likely not solely seedborne

AgCenter mycologist Vinson Doyle has identified seven different hosts, in addition to soybeans, for the most prevalent species associated with foliar symptoms of Cercospora leaf blight in Louisiana. It is one piece of evidence that this pathogen may not be solely seedborne.

Regardless of the dominant species found on the seed, the pathogen species prevalent on blighted leaves is always Cercospora flagellaris, and it shows up on harvested seed, too, he said.

Doyle hopes to confirm that the inoculum for Cercospora leaf blight can be airborne. If it is, he wants to determine when it is moving in the field and how that information can be used to develop better management practices.

Nelomie Galagedara, a doctoral student advised by Doyle and Thomas-Sharma, is monitoring spore traps in the field to collect samples and develop a diagnostic test to rapidly assess the different species on each sample.

“We think if we can identify when the peaks of spore production occur and when the spores are out there at the highest density, then it will be the optimum time to apply fungicides,” Doyle said.

Most of the alternative hosts are found in field margins on trees and weed species, such as pokeweed, giant ragweed and even Venus’ looking-glass. The study is also trying to determine how long the Cercospora pathogen will survive on plant debris.

This story is featured in the Louisiana Soybean and Grain Research and Promotion Board 2020 Report.

LSU AgCenter plant pathologist Vinson Doyle looks at a sample of the Cercospora disease under a microscope. An enlarged image of Cercospora is displayed on the computer monitor. Photo by Olivia McClure

The soil turns red as research associate Clark Robertson sprays a pre-emergent soil application of iron at the AgCenter Macon Ridge Research Station in Winnsboro. Photo by Sara Thomas-Sharma