Cercospora leaf blight is the No. 1 fungal disease in Louisiana’s No. 1 row crop – soybeans.

The emerging disease, which has appeared only in the past five to 10 years, has no resistance in any soybean lines, according to LSU AgCenter plant pathologist Zhi-Yuan Chen.

The Cercospora fungus survives in infected seeds and plant residue, and spores form on the residue surface during warm, humid weather, experts say. The spores are wind-blown or rain-splashed to new soybean tissue where infection occurs. Seeds also can carry the fungus.

The foliar disease can remain latent in growing soybean plants for several weeks before they show symptoms. In severe cases the disease can cause defoliation, and it is associated with the green stem disorder.

In an attempt to solve the lack of resistant plants, Chen has been using molecular tools to find ways to make soybeans resistant to the disease.

“There are no known resistant lines that we can use to learn how soybeans defend themselves against Cercospora kikiuchii infection using the available molecular tools,” Chen said.

Instead, Chen’s research team in the Department of Plant Pathology and Crop Physiology has been focusing on identifying genes that produce cercosporin, a toxin that can suppress a plant’s defense mechanism.

So far, he has identified two genes using proteomics – the branch of molecular biology that studies the full set of proteins encoded by a genome – and gene disruption studies. These genes appear to be important for cercosporin production by the pathogen.

The genes are currently being tested for their ability to suppress the toxin and the resulting fungal infections. If they actually do what the researchers believe, these genes will be introduced into new soybean lines to produce resistant varieties.

Chen’s laboratory team also has identified several genes that potentially can play an important role in soybean resistance to rust infection. They currently are testing these genes using a virus-induced gene silencing approach.

“Preliminary data show that soybeans with reduced expression of these genes are more susceptible to rust infection, indicating that enhancing the expression of these genes in the future could provide better protection against soybean rust diseases,” Chen said.