Frances Gould, McClure, Olivia J. | 10/13/2016 6:58:05 PM
Eduardo Chagas Silva, a graduate student working under LSU AgCenter plant pathologist Ray Schneider, holds a soybean leaf in a solution used to expose the plant to cercosporin, the toxin associated with Cercospora leaf blight. Photo by Olivia McClure
Cercospora leaf blight has posed major problems for Louisiana soybean producers for the past several years. The disease, which is caused by a fungal pathogen called C. kikuchii, is nearly impossible to control, with no good fungicide options available and unsatisfactory levels of disease resistance.
But Ray Schneider, an LSU AgCenter plant pathologist, has been working to identify ways to at least suppress cercosporin production to levels that allow producers to still get good yields. Cercosporin is the toxin produced by C. kikuchii.
He has been studying how applying iron to the leaves of soybean plants can suppress Cercospora, which causes leaves to become dessicated and fall off during the critical R5 pod-filling stage of growth, resulting in yield losses.
When iron is present, production of the toxin as well as blight symptoms are suppressed, Schneider said. The toxin causes oxidative stress that can damage leaves, and iron seems to help plants better cope with the stress, he said.
Two of Schneider’s students, Brian Ward and Eduardo Chagas Silva, are studying how iron and the cercosporin toxin interact to affect soybean plant health.
Cercospora can cause massive, sudden defoliation of soybean plants. The fungus is an endophyte, meaning a plant may not show any symptoms for up to 90 days after being infected, Schneider said.
“The leaves may turn purple, or you may see some subtle signs of blight. But very quickly thereafter, the entire canopy becomes blighted and defoliate, which results in significant yield loss,” he said.
That is perhaps one reason why fungicides generally are not effective in controlling Cercospora leaf blight, he said.
Cercospora seems to be associated with green stem, a disorder that prevents the stems of plants from maturing, which makes them pliable and causes them to get tangled in harvesting equipment, Schneider said. It’s not clear, however, what causes green stem or how it’s connected to Cercospora leaf blight.
Schneider is also working with Zhijun Liu, a professor in the School of Renewable Natural Resources who studies medicinal plants, to find a way to solubilize cercosporin so that the solution can be used to screen varieties and breeding material for disease resistance.
By eliminating the pathogen and exposing the plants only to the toxin that causes symptoms, researchers can better assess resistance, Schneider said. The toxin is not water soluble, however, making it difficult to actually get it into the plant.
Liu has developed a proprietary compound that solubilizes cercosporin in water so it can enter plant tissue. In tests, soybean leaves with petioles are placed in culture tubes containing a bright red solution made from cercosporin and Liu’s compound. The leaves absorb the toxin and usually exhibit symptoms of Cercospora leaf blight within two days.
“We’re now able to reproduce symptoms of the disease, and that’s a major accomplishment for several reasons,” Schneider said. “We have a reproducible way to induce these disease symptoms in soybean leaves under controlled conditions. We’re still working to optimize important factors — what concentration of toxin to use, what light intensity to use — so that when we finally develop this protocol, others, especially breeders, can use it to screen for disease resistance.” Olivia McClure