Search continues for answers to soybean diseases

Frances Gould, Bogren, Richard C.  |  8/14/2015 8:10:45 PM

Rodrigo Valverde examines soybeans in a greenhouse on the Baton Rouge campus. He planted seeds harvested last year from plants affected with the green stem malady to see if the problem is transmitted by seeds. Photo by Rick Bogren

Many scientists look for genes that are expressed in a particular way, such as for yield or plant stem strength. But Zhi-Yuan Chen is looking for genes that can make soybeans more resistant to infection from the soybean rust pathogen.

"We have found some soybean lines that are resistant to rust, but they’re not commercial varieties," Chen said. "We’re looking for genes from these resistant lines by comparing differences between rust-resistant and rust-susceptible soybean lines."

These genes can be "turned off" so they don’t do what they’re supposed to. This process is called "virus-induced gene silencing."

Chen’s procedure is to insert a gene of interest into a modified soybean virus that does not cause disease on soybeans and apply the virus onto soybean seedlings by physically rubbing the plant tissue. After waiting three to four weeks for the modified virus to become established, he inoculates the plants with spores of soybean rust.

Chen is comparing the virus-treated soybean plants with control plants to see if the treated plant is more susceptible to rust.

"We have identified several genes that may be responsible for soybean resistance to this disease," Chen said. "The next step is to introduce the gene into soybean lines and breed a commercial variety."

Chen’s work is in the laboratory, verifying the strategy to be sure these genes are effective in controlling soybean rust disease. "We don’t want to waste time without knowing the results," he said.

Chen is using a similar strategy with Cercospora, another soybean disease problem in south Louisiana. "There are no soybean lines that are resistant to Cercospora leaf blight, so we can’t use the same strategy," he said.

Chen is looking at the pathogen to understand what makes it virulent. The goal is to make the pathogen less aggressive.

The scientists have discovered that Cercospora toxin production is light-induced.  

"We grow Cercospora in the lab in light and dark conditions and then compare for differences in gene expressions," he said. "If we can determine which genes are affected by light – which are active in light – then we can concentrate our efforts and disrupt those genes when the pathogen infects soybeans to reduce virulence."

The process, called host-induced gene silencing, is a new strategy with soybeans, Chen said. But it’s been successful in other crops, including wheat and barley.

Rodrigo Valverde is looking at the green stem syndrome in soybeans, which has been called a "malady" because no one is sure if it is a disease or insect problem.

"It seems random," Valverde said. "There’s no pattern. Some years are more severe, and it’s all over the country."

Valverde is taking two approaches to determine if the problem is a disease and if it is transmitted. The first is grafting affected plant material onto clean rootstock to see if the problem moves through the plant. The second approach is to plant seeds harvested from affected plants to see if the malady is transmitted through seeds.

Grafting is the first step in determining if the problem moves from the affected tissue to the rootstock, Valverde said. "If it moves, it’s a disease."

The grafting technique "is a universal way of proving a pathogen is involved," he said.

Valverde’s second test is to take seed collected from plants affected last year and plant those seeds next to healthy seed. Valverde’s hypothesis is that the problem is not caused by an infectious agent.

For now, the malady could be caused by plant genetics, a disease, an insect pest problem, chemical damage or other identified factor, he said.

"We have to find the cause," he said. "Then we have to find a way to eliminate, avoid or manage it."

Plant pathologist Clayton Hollier has added cultural practices into his mix of methods to see how they can be incorporated into a disease management strategy.

"This year we’re introducing soil preparation techniques – no-till, minimum-till and burying crop residue," Hollier said. "We’re looking at burying crop residue to reduce pathogens because if the host material is buried in the ground and decaying, it provides fewer places where the pathogen can survive. The pathogen is not eliminated, but it is reduced."

The first data will be analyzed this year.

The project continues to focus on yield loss caused by the pressure and actions of certain pathogens, Hollier said. "We’re looking at aerial blight, frogeye, anthracnose, Cercospora. The task is to go after whatever shows up rather than what you expect."

The initial process includes looking at fungicides and application timing.

Disease management starts with the planting environment, variety selection and cultural practices, Hollier said. The program goal remains to refine all tools to use in the best way possible.

Rick Bogren

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