Scientists Develop Better Way To Detect Presence Of Soybean Fungus

SDS is a mid- to late-season disease caused by a particular strain of fungus known as Fusarium solani forma specialis glycines. Symptoms of the disease can occur on soybean roots without being observed. Affected plants can suffer root rot, crown necrosis and vascular discoloration of roots and stems, eventually reducing crop yields by as much as 70 percent, previous UI research has found.

Much of Illinois -- the nation's largest soybean-producing state -- is hit hard by the disease each year. Data from 1998 and 1999 showed that Central Illinois counties had a high occurrence rate of SDS.

The new detection method -- designed using a DNA-amplification technique called polymerase chain reaction -- consistently detected even minute traces of the disease-causing strain of F. solani in soil and in plant tissues grown in both the laboratory and in fields across Illinois. Its presence was found even from samples that were thought to be free of disease. Researchers also were able to detect the SDS pathogen in the presence of 55 non-disease-causing strains of F. solani and 20 other soybean pathogens.

The PCR-based method uses two primers that were designed from the extraction and genetic alteration of material taken from various strains of F. solani.

"Our system is much more sensitive and accurate than the traditional methods that have been used to detect this pathogen," said Shuxian Li, a senior research specialist in the UI department of crop sciences and National Soybean Research Center. "We believe that as we refine our detection method we will be able to quantify the actual amount of pathogen that is present in a particular area."

Previous methods have relied on the placement of plant roots into a medium that lacked sensitivity and specificity. Even with trained eyes and proper equipment, scientists often could not positively identify the disease-causing fungus from other similar looking strains and pathogens.

The new tool's sensitivity is very precise, said Glen Hartman, professor of crop sciences and scientist with the USDA-Agriculture Research Service. "The detection of this specific fungus is important, because of its pathological implications compared to all the different fungi that occur in the soil and in plant roots," he said. "Having this tool is like going to the doctor and having him tell you specifically what flu virus you have. Diagnosis is often the first step toward treatment."

Li presented data on the method's success during the 92nd Annual Meeting of the American Phytopathological Society in New Orleans in August. Earlier this year in the journal Phytopathology, Li, Hartman and Yan Kit Tam, a researcher at Albert Einstein College of Medicine in New York, documented the method's ability to differentiate among various strains of F. solani.