UW Soybean Plant Health
The presence of SDS in Wisconsin fields has risen sharply and appears to coincide with the spread of SCN. The increased attention brought to a field by SDS frequently results in the detection of SCN.
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Figure 1. SDS has been confirmed in nine counties in Wisconsin, and likely to be found in at least one more |
Diseases of greatest importance are soybean cyst nematode (SCN), white mold, brown stem rot (BSR) and stem canker. Phytophthora root rot has appeared sporadically in recent years, but its severity is greatly diminished compared to the 1980’s. Soybean viruses have not been an issue for several years because of low soybean aphid and bean leaf beetle activity.
Soybean rust expanded its range north of the Gulf States and must be monitored during the 2007 growing season. Despite all the concern about soybean rust, Wisconsin soybean growers should be more concerned about sudden death syndrome (SDS). SDS was observed in many new areas in 2006 and appears to be on the rise.
Scouting for SDS
The first symptoms of SDS are usually observed on the foliage (Figure 2) . Leaves are characterized with interveinal necrosis and chlorosis, indistinguishable from the symptoms of brown stem rot. Despite the similarity of foliar symptoms, SDS symptoms can be separated from those of BSR by observing the timing of symptoms, absence of internal stem discoloration, and degree of root rot.
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Figure 2. Interveinal necrosis and chlorosis of foliage are typical of sudden death syndrome. Photo credit: Craig Grau |
Figure 3. Foliar symptoms of brown stem rot. Note the similarity between symptoms of brown stem rot and sudden death syndrome. Photo credit: Craig Grau |
Symptoms of SDS become apparent earlier in the reproductive phase, usually R3 or R4, in comparison to R5 or R6 for BSR symptoms. Often, petioles will remain attached to the stem in soybeans infected by the SDS pathogen. Internal discoloration in the stem is not observed beyond lower nodes with SDS symptoms; however roots may be brown and stunted due to root rot caused by the pathogen. Often, the location of infected plants may be limited to an area where water had been sitting for sometime, or along a wet portion of the field.
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Figure 4. A soybean variety susceptible to sudden death syndrome bordered by resistant varieties. Note the susceptible variety is very chlorotic, shorter and does not have canopy closure. Photo: Craig Grau
Click on image to view a larger version. |
Management of SDS can be achieved by use of resistant or moderately resistant cultivars. SDS resistance varies widely by variety. As prevalence of SDS has increased, companies are including SDS ratings for their product lines. An informal survey of seed company soybean product lines indicated 57% of the varieties were characterized for resistance. Individual companies have characterized 40 to 60% of their product lines for SDS reaction with most varieties having moderate resistance.
It is notable that not all seed companies report information on SDS, so it is important for growers and consultants to be aware of the potential risks. Because presence of SCN can alter a cultivars reaction to the SDS pathogen, it is desirable to utilize a cultivar with SCN resistance to limit yield loss. The presence of SDS usually means SCN is also present and SCN resistant varieties should be planted to manage this root pathogen. The majority of the seed companies have data on SCN resistance for their varieties so an effective management strategy may be implemented.
Cultural practices are an important component of an SDS management strategy.
Limiting soil compaction along with management of SCN will reduce the severity and yield impact of SDS. Compacted soils have less porosity and increased water-holding which favors infection and subsequent SDS development.
The severity of SDS is reported to increase with increasing sand content in soil and decreased as soil pH was lowered from 7.7 to 5.5. A lower soil pH is also less favorable to SCN. However, caution must be exercised if soil pH is allowed to drop to manage SDS and SCN because brown stem rot is favored by soil pH levels below 6.5.
Severity of SDS has been found to be greater in no-till systems, presumably because of cooler soil temperatures, increased moisture, and greater pathogen inoculum. Crop rotation appears to have limited value for management of SDS because the SDS pathogen is long-lived in soil.
Watch for SDS in 2007
The presence of SDS in Wisconsin fields has risen sharply and appears to coincide with the spread of SCN. The increased attention brought to a field by SDS frequently results in the detection of SCN. Because there is not one management option that will protect from yield losses, a multi-faceted approach is needed to manage SDS.
Both diseases can be managed by vigilant scouting for SDS and SCN, variety selection, and utilizing cultural practices that do not favor disease development. Information on SCN-resistant varieties can be found in 2006 Wisconsin Soybean Variety Test Results (UWEX A3654), but information on variety response to SDS must be obtained from seed companies.
Research and extension programs on SDS at the University of Wisconsin-Madison are funded by the Wisconsin Soybean Marketing Board and the North Central Soybean Research Program.
Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: University of Wisconsin-Madison, Departments of Agronomy, Entomology, and Plant Pathology at www.plantpath.wisc.edu/soyhealth.
3/07