INHS Reports January-February 1997

Soybean Disease Diagnosis Going High Tech

If you travel through the northern two-thirds of Illinois during late July and August, you may notice soybean leaves turning yellow prematurely. The yellowing of the leaves is due to interveinal necrosis of the leaf tissue. Longitudinally splitting these soybean stems will reveal a dark, reddish-brown discoloration. You are looking at an important soybean disease called brown stem rot. The disease was first reported in Illinois some 50 years ago, and it occurs in the north-central United States and Canada. Brown stem rot, which is caused by the fungus Phialophora gregata, is responsible for up to 25% yield loss in soybeans.


Soybean plot with healthy plants and plants infected with brown stem rot (lighter area at center right).

Brown stem rot is difficult to recognize because it often has no outward symptoms. The most common practice in identifying the disease is by looking at the browning of the soybean stem; however, stem browning is not exclusively associated with brown stem rot. Other fungi also cause browning of the stem but do not cause this disease. Therefore, conclusive identification of the disease involves isolation and identification of the pathogenic fungus from the diseased stem. The process of isolation and identification is time consuming, and a more efficient identification procedure is required for rapid diagnosis of the disease.

We are working to develop an alternative and efficient diagnosis technique of soybean brown stem rot. We collected more than 80 accessions of the pathogen from five north-central states and from Brazil and Japan. Accessions of the pathogen were compared among themselves and with other fungi that are also associated with soybean stems using molecular techniques, such as polymerase chain reaction (PCR) and DNA sequencing. PCR is a technique that allows amplification of a specific piece of DNA from a minute amount of DNA material. We have identified a piece of DNA that is uniquely shared by all the accessions of the pathogen and differentiates the pathogen from other fungi. Based on the sequence information of this unique piece of DNA, we designed oligonucleotides (primers) that allow sensitive detection of the genetic materials of the pathogen within infected soybean plants. By using the PCR technique with the specific DNA primers, we are able to unambiguously identify the brown stem rot disease within hours, whereas the traditional identification process of isolating the pathogen would take three weeks.


Cross sections of stems infected with brown stem rot.

The new diagnostic technique is not only fast but also accurate in identifying the brown stem rot disease. In addition, it provides a new tool for scientists to further study the disease in interactions of the pathogen with soybean plants of various cultivars. For example, the technique can be used to study the rate of movement of the pathogen within soybean plants to address fundamental questions concerning the mechanisms of soybean resistance to the disease. We are currently using this technique to study the mechanisms of soybean resistance and to identify new sources of resistance.

Weidong Chen, Center for Biodiversity, in cooperation with Lynn Gray, USDA-Agricultural Research Service at Urbana, and Craig Grau, University of Wisconsin at Madison

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