Illinois Natural History Survey - University of Illinois

New Form of an Old Soybean Pathogen

Soybean brown stem rot is a disease caused by the fungal pathogen Phialophora gregata. The disease was first discovered in Illinois in 1949. Now the disease is found in many soybean production areas including the midwestern U.S., Brazil, Egypt, and Japan. In Illinois, the disease is one of the major soybean diseases in the northern two-thirds of the state. The disease could cause up to 30% yield loss if conditions are right.

Soybean cultivars resistant to brown stem rot have been used to control the disease in the past 15 years. However, in recent years, more and more disease incidents of brown stem rot have been observed in the resistant soybean cultivars. Scientists at the Natural History Survey in cooperation with plant pathologists in Illinois and Wisconsin are investigating the emerging population of the pathogen that causes brown stem rot on the previously resistant soybean cultivars. A new form (type B) of the old soybean pathogen has been identified. This new form of the pathogen can be identified by several DNA markers that can unambiguously separate the new form from the traditional form (type A) of the pathogen. The scientists have developed a robust and quick test to identify and differentiate the two genotypes of the pathogen (see figure). This new form of the pathogen is widespread and has been observed in seven midwestern states.

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A robust test showing the genotypes of the soybean brown stem rot pathogen (Phialophora gregata) and separating the new form (B) from the traditional form (A).

Field studies have shown that the new form of the pathogen is mostly isolated from soybean cultivars like Bell and S282N, which are resistant to brown stem rot, whereas the old forms of the pathogen are primarily isolated from soybean cultivars like Sturdy and Pioneer 9305 that are susceptible to the disease. Discovering the new form of the pathogen is a significant breakthrough in our understanding of the widespread high-disease incidents among previously resistant soybean cultivars. The discovery provides a new target for breeding future resistant soybean cultivars. It also provides a means to study and understand the host-specific infection by the brown stem rot pathogen in order to develop efficient control strategies and to increase soybean yields for Illinois agriculture.

Weidong Chen, Center for Biodiversity



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