Issue No. 22, Article 5/September 2, 2004
Brown Stem Rot and Sudden Death Syndrome: Similar Yet Different Damaging Soybean Diseases
Both brown stem rot (BSR) and sudden death syndrome (SDS) may now be common and causing yield losses in Illinois soybean fields. Plants infected with these diseases can appear very similar both from a distance and close-up. Although SDS may be more commonly thought of, BSR is still a significant disease and should be looked for in fields. Plants thought from a distance to be affected with SDS may be affected with BSR in some fields.
Both BSR and SDS can cause browning and chlorosis of leaves, defoliation, and premature maturation of soybean plants. The diseases can be most readily distinguished by splitting stems; plants with BSR develop a chocolate-brown discoloration in the pith, especially at and between nodes near the soil line. BSR can cause severe internal stem browning in the absence of clear foliar symptoms. Symptoms of BSR generally become more common after the pods begin to form (after growth stage R4) and night temperatures begin to cool. Live plants with SDS maintain a white pith but develop a gray-brown discoloration under the epidermis of the lower stem. The leaves of plants with SDS often detach at the petioles. Reports of individual plants that appear to be infected by both BSR and SDS have been received.
Symptoms of brown stem rot (BSR) on soybean that appear very similar to SDS symptoms.
Symptoms of sudden death syndrome (SDS).
Stem-browning symptoms of BSR.
BSR and SDS can cause significant yield losses that exceed 25%, although potential and real yield losses due to SDS often exceed those caused by BSR. This difference in part is because of the more effective resistance available for BSR and the likely shorter survival of the pathogen (Phialophora gregata) that causes BSR in soil. Yield losses from both diseases, especially BSR, can occur in the absence of foliar symptoms. BSR is distributed throughout Illinois, but it is more common and generally more severe in the northern part of the state, whereas SDS can be found at severe levels from the most southern to the most northern counties in Illinois.
BSR is managed by planting resistant varieties and by crop rotation. The BSR pathogen survives in soybean residue buried and on the soil surface between soybean crops. Thus, agronomic practices that effectively reduce the amount of infested soybean residue remaining in a field can reduce BSR. The BSR pathogen may survive in soybean residue for 2 years or more, so rotation longer than 1 year out of soybean may help reduce BSR in fields where it is common. Planting a soybean variety that is resistant to the soybean cyst nematode (SCN) may also help to reduce BSR, because SCN can enhance the severity of BSR, even in varieties with resistance to BSR. For additional information on SDS and its management, see issue no. 18 of the Bulletin (July 23, 2004).
In summary, BSR and SDS are common in Illinois, and correct diagnosis is critical for optimal management of these damaging soybean diseases. In addition, scouting is important because both diseases may cause plant damage and yield losses in the absence of clear foliar symptoms. --Dean Malvick