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

LSU plant biology professor Zhi-Yuan Chen is studying the correlation between soybean leaf tissue sucrose 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.

LSU doctoral graduate student Maria Zivanovic is working with Chen and found that soybean leaf sucrose levels increased significantly from the R3 developmental stage to R5.

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, and sucrose added to extracts from the R3 stage will produce as much or more of the toxin than found in R5 samples.

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

Zivanovic also found that a foliar application of ammonium phosphate on leaf extracts at R5 will suppress the toxin production.

Early 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, he said. However, yield estimates will not begin until the project moves to larger field production trials.

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

Chen has isolated over 10 gene sequences that have shown efficacy in suppressing the Cercospora and soybean rust pathogens in the lab.

Iron formulation rates increased to manage Cercospora leaf blight

AgCenter field crop pathology assistant professor Sara Thomas-Sharma 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 systems within the plant,” she said.

Previous work on this project by retired AgCenter plant pathologist Ray Schneider showed that an iron concentration of 300 parts per million in soybean plant leaves is needed to see any effect against the disease.

Recent work has been completed to establish baseline data for iron concentration in soybeans to show how iron content in the plant changes during growth stages.

Results showed clearly that while the lower canopy easily met the higher level of concentration, the upper canopy only reached about 100 to 200 parts per million during R3 to R5 growth stages, which correlates with the disease progression.

“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 made in commercially available iron formulations. The rate and timing of foliar and soil applications may have also played a role.

Cercospora likely not solely seed borne

Proving one line of evidence that the pathogen may not be solely seed borne, LSU assistant professor of mycology Vinson Doyle has identified seven different plant hosts in addition to soybeans for Cercospora flagellaris, the most prevalent species associated with foliar symptoms of the Cercospora pathogen in Louisiana.

Regardless of the dominant species found on the seed, the pathogen species prevalent on blighted leaves in Louisiana is always Cercospora flagellaris and remains so into the harvested seed, he said.

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

By monitoring spore traps in the field, Nelomie Galagedara, a doctoral student advised by Doyle and Thomas-Sharma, is collecting samples and developing a diagnostic test to rapidly assess the diversity on each of the samples.

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

Most of the alternative hosts are found in field margins on trees or other weed species, such as pokeweed, giant ragweed and even Venus’ looking-glass. The study also looks at methods for determining how long the Cercospora pathogen will survive on debris.

Rapid lab protocol screening for Cercospora leaf blight progressing

Thomas-Sharma said work is progressing on the development of a rapid lab protocol for screening for Cercospora leaf blight resistance, which may soon supply breeders with a new tool to help reduce the number of soybean varieties to include in field trials.

By measuring the toxicity of cercosporin on smaller leaf discs in the lab, researchers are attempting to predict the 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 same seed varieties as those used in official variety trials conducted across the state to provide more data for each variety tested with disease ratings by field location.

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

Karol Osborne

LSU graduate assistant Maria Zivanovic is exploring new approaches for managing Cercospora leaf blight. Zivanovic observed that increases in sucrose levels in soybean leaf tissue seem to trigger toxin production, while ammonium phosphate added to soybean leaf extracts at reproductive growth stage R5 suppresses toxin production. Photo by Zhi-Yuan Chen