Cercospora may translate to big yield losses for Louisiana soybean producers, but for AgCenter researchers the problem is a small one — in fact, you can say it is a microscopic one.
The cercosporin produced by the microscopic pathogen Cercospora cf. flagellaris causes oxidation of the plant’s leaf tissues, explained LSU plant pathologist and crop physiologist Zhi-Yuan Chen. He and his colleagues have been working to prevent Cercospora from ever reaching the farmers’ fields by attacking the fungal ailment with in-depth genetic research. His two main projects in this area include reducing the production of the cercosporin toxin that causes the fungal symptoms to be present in the first place.
“Whenever the pathogen produces this toxin, it gets more virulent on the host,” he said. “So, what we’re trying to do is to reduce the production of this cercosporin toxin.”
There are two ways Chen is using his research to reduce the damage or losses to soybean by the pathogen. The first method is to pinpoint the genes that aid in the production of the cercosporin. He explained that a technology similar to what was used to combat the COVID-19 pandemic has been used to combat Cercospora leaf blight (CLB) disease.
“If you are familiar with the Pfizer or Moderna COVID vaccines, they are using rNA vaccines,” he said. “So, basically, we are using a similar idea. We are using rNA produced in bacterial cells that are targeting the genes involved in cercosporin production. We then apply that to the leaf.”
Using this line of research stops the production of the cercosporin that affects soybeans. There were two fields planted to study this vaccine-style approach — one at the Ben Hur Research Station and another at the H. Rouse Caffey Rice Research Station. Unfortunately, the field at the Rice Research Station was planted later than the ideal time and was hit with a dry spell causing the seedlings to die, Chen explained. In the Ben Hur trials, Chen and his associates plan to inoculate some of the fields with the rNA produced by bacteria while other fields will be allowed to develop the CLB naturally. From there the researchers will determine whether there are any symptom differences between the treated fields and the control fields.
The second method Chen explored used research from a previous study that involved a means of biocontrol. Chen said within soybean leaves there is a certain type of endophytic bacteria that proves inconducive to the proliferation of this Cercospora pathogen. Having identified and isolated these bacteria, researchers are able to apply them to stunt the growth or spread of the Cercospora.
“If you put these bacteria next to the Cercospora, the Cercospora will not grow,” Chen said. “This bacterium has an antagonistic effect on the Cercospora pathogen.”
He explained that this method includes applying the bacteria to the soil, then planting the soybeans and inoculating the plants with the Cercospora pathogen. Results using this biocontrol method have been promising, he said.
“Initial data is encouraging,” he said optimistically. “So far, we see there is much better growth in the ones that were treated with bacteria. The plants are much greener, much healthier and it seems to improve the iron uptake or other nutrient uptake.”
Now in the second year working on this project, Chen said there are trials planned to analyze the efficacy of different application methods. The three application methods being tested are drenching the soil with the bacteria, spraying the bacteria onto the plants and applying the bacteria as a seed treatment.
Elsewhere, Cercospora management is taking a different approach. AgCenter plant pathologist Vinson Doyle is working on characterizing the production and spread of inoculum and infection strategies for Cercospora leaf blight.
AgCenter plant pathologist Trey Price said, “The most important thing farmers can do is to plant a tolerant variety that was tested in their area.”
Fungicide resistance has also been a large research component in Cercospora management, Price said. He and his associates have identified fungicide resistance in the majority of that pathogen population, but there are still fungicidal treatments that do show efficacy.
“Some of the newer SDHI [fungicide] materials — the triazoles — are effective on Cercospora blight,” Price said. “So, fortunately, we do have some fungicides that have been working for us over the last several years.”
As part of research into individual fungal isolates, researchers have unlocked a new challenging discovery. Price said there have been at least three species of Cercospera that are involved with fungicidal resistance. He said the multiple species confounds management.
“We’re not sure if different species cause different symptoms,” Price said. “There are a lot of different symptoms attributed to this disease.”
Despite these new challenges, Price said, AgCenter pathologists will continue to work closely with plant breeders to identify varieties that are resistant to Cercospora. He said one such collaboration includes the University of Missouri.
LSU AgCenter plant pathologist Vinson Doyle is also working on characterizing the production and spread of inoculum and infection strategies for Cercospora.