Soybean germplasm collections with potential resistance to rust disease were screened under greenhouse conditions during the off-season by LSU AgCenter researchers in Dr. Zhi-Yuan Chen’s lab. Shown here, Chen and his student Sunjung Park evaluate different soybean lines for levels of resistance two weeks after rust inoculation.

LSU AgCenter researchers continue their quest for short-term and long-term solutions in the battle with soybean diseases – particularly Asian soybean rust.

Scientists are conducting a variety of what are called basic and applied research projects as part of that effort. Basic research lays the fundamental groundwork for beneficial results later on, while applied research can yield positive benefits in a relatively short time.

Four projects funded by the Louisiana Soybean and Grain Research and Promotion Board are led by LSU AgCenter scientists Dr. Ray Schneider, Dr. Zhi Yuan Chen, Dr. Svetlana Oard and Dr. Boyd Padgett. Those projects are looking at the biology and control of major diseases of soybeans, new strategies to control soybean rust, developing genetic resistance to Asian soybean rust and evaluation of soybean cultivars and fungicides in disease management.

Some of the projects involve multistate cooperation or similar projects being conducted by researchers across the soybean-growing regions of the country. For example, Schneider coordinates one of the nationwide screening nurseries for resistance to soybean rust.

The LSU AgCenter expert said that effort probably is "one of the most important" projects he’s involved with "because disease resistance will be the most efficacious means of disease control."

Oard, who is in the LSU AgCenter’s Department of Veterinary Science, is among those looking for novel genes that can be inserted into soybean varieties to increase disease resistance – particularly resistance to Asian soybean rust.

"We have identified a gene that encodes strong antifungal as well as antibacterial activity," Oard said. "This gene is an excellent candidate for developing soybean lines with stable resistance against Asian soybean rust pathogens."

Known as Purothionin, or PTH for short, the gene is an antimicrobial peptide from the endosperm of wheat seeds.

"PTH presents the potential for an attractive alternative to costly chemicals to develop Asian rust resistance in soybeans," she said, stressing, however, that the work is still in its early stages. "Learning how to make this very potent peptide work in soybeans will take some effort, but the outcome can exceed expectations."

While Oard’s work has focused on inserting genes to increase soybean resistance to diseases, the efforts in Chen’s lab have focused on understanding how diseases infect plants, determining how plants respond to a disease attack and developing detection methods for infections.

For example, to understand how rust infects soybeans and how hosts respond to the attack, Chen and the scientists working with him have compared protein differences between rust-infected and noninfected soybean leaves. They are studying proteins induced by rust infection to determine whether "increasing the production of these proteins may provide a new approach in disease control."

Another major area of research in Chen’s lab has been to develop a reliable method to detect soybean leaf blight disease caused by the fungus Cercospora kikuchii. He and Schneider are cooperating in a study to determine how long it takes from the time infection has occurred until symptoms appear.

Schneider’s comprehensive research program is focused on a variety of disease-management options and economic considerations – including evaluating fungicides for disease control and developing protocols for their use.

"As a result of such research, recommendations have been developed for Asian soybean rust, pod and stem blight and Cercospora leaf blight, although the latter disease has defied our attempts to find effective disease control measures," Schneider said.

Work on yield-loss and economic models for the major diseases also is progressing "so that producers can make science-based decisions when deciding whether or not to spray a fungicide," he said. Models have been completed for Cercospora leaf blight, frogeye leaf spot and pod and stem blight, but efforts are just beginning on models for Asian soybean rust, he said.

Schneider and one of his graduate students also completed a comprehensive research project recently in which they examined reasons for the widespread occurrence of Cercospora leaf blight lately – and their findings indicated a rapid genetic shift within the pathogen population. "That’s very significant, because it means we can expect new virulent strains of the pathogen to arise shortly after resistant varieties are released," he said.

Schneider also is pursuing a research thrust that involves evaluation of "prescription nutritional amendments." For example, during the past two years, Schneider discovered that preplant incorporation of potash delays disease onset and greatly reduces ultimate disease severity with Asian soybean rust and Cercospora leaf blight.

"These diseases are not completely controlled, but severities are reduced to the point that fungicide applications may not be necessary," he said, adding that further studies have looked at the particular elements that may be influential in reducing disease severity.

Padgett is directing his colleagues in applied research projects that primarily involve evaluating fungicides for managing soybean rust.

Some of the objectives involve determining whether triazoles are all-effective for managing rust and whether they are effective on other diseases affecting soybeans; specifically testing current chemistries such as Quadris, Headline and Topsin M on soybean rust; and determining the residual activity of the fungicides.

"Looking at residual activity not only tells us about the potential impact on the environment of these fungicides but also helps determine the number of applications needed," Padgett said.

The researchers also are evaluating the effects of application timing on disease epidemics and yield. "This will help producers time the fungicide applications based on the diseases present or likely to develop," he said.

Although developing more resistant varieties has been a major thrust of research, Padgett points out that current varieties in LSU AgCenter variety tests also are being studied for resistance to plant pathogens.–Tom Merrill

(This article was published in the 2008 edition of the Louisiana Soybean & Grain Research & Promotion Board Report.)