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Soybean Diseases: Brown Stem Rot And SDS

In some regions during the mid- to late- pod filling stages, soybeans show stem and root rot symptoms. Brown stem rot (BSR) and sudden death syndrome (SDS) have similar leaf symptoms (interveinal necrosis), but stem symptoms can differentiate the 2 diseases. Both diseases are caused by soilborne fungi, which infect the plant early in the growing season. However, Purdue University notes symptoms usually show up during the reproductive growth stages, specifically R3 to R4.

BSR is caused by the fungus Phialophora gregata and enters the plant through the roots and grows inside the vascular tissue, colonizing stems and leaves. Foliar symptoms are suppressed when temperatures are high, often making it difficult to determine the cause of dying plants. The BSR fungus survives in infected soybean residue, but unlike SDS, it doesn’t form any long-term survival structures.

According to Iowa State University, there are 2 genotypes of the fungus that cause BSR. Type A causes stem and leaf symptoms and is considered more damaging. Type B typically causes only stem symptoms. Foliar symptoms look similar to those caused by SDS, so splitting the stems of infected plants length-wise will help differentiate each disease. 

Usually BSR exhibits within the stem as a dark, brown, discolored pith that extends from the lower stem upward, whereas SDS-infected stems have a normal white pith. In comparison, SDS infections discolor the cortex of the lower stem and upper root, turning it tan to brown.

​​​​​​​As shown in the left photo above, BSR causes the stem pith to turn brown, whereas with SDS it remains white. Pictured right, SDS causes the lower stem and upper root cortex to turn brownish-tan in color. Photos by University of Minnesota Extension pathologist, Dean Malvick.

The most important management strategy for both diseases is to select a soybean variety with disease resistance and to be extra diligent for it in wet conditions, which can trigger disease infections. Fungicide applications will not help control BSR or SDS. Soybean cyst nematode is known to contribute to BSR and SDS infections; thus, it should be managed. Other strategies like crop rotation and tillage will help reduce BSR where present, while tillage and delayed planting may help reduce SDS.  
Monitoring fields for these diseases will help plan for the next growing season. Talk to your Golden Harvest Seed Advisor or agronomist about disease-resistant soybean varieties and additional agronomic insights.

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