By Emmanuel Byamukama and Krishna Acharya
Soybeans are at or past the R1 growth stage in most of the state. By this growth stage, the soybean cyst nematode (SCN) will have completed at least one life cycle and therefore female cysts can be seen on soybean roots (Figure 1). Assessing soybeans for SCN can help determine if SCN is present in your field, and also give an indication of the extent of the infestation.
Figure 1. Soybean cyst nematode female cysts on soybean roots. Note the clustering of the cysts on just a few roots.
One of the challenges in managing SCN is the lack of obvious above ground symptoms, yet yield losses can still occur even when no above ground symptoms are being seen. By the time above ground symptoms are seen, heavy yield loss is already occurring. Checking for below ground SCN symptoms on the roots is one way of confirming the presence of SCN.
Assessment
Assessing SCN on soybean roots is relatively easy. Use a shovel to carefully dig out soybean plants. Carefully remove the soil around the roots without stripping the female cysts from the small fine roots. Examine the roots for the cysts, which are small, about the size of a period at the end of a sentence. The young cysts are cream-white to light yellow. When scouting for SCN, examine soybean plants in the entry areas, low spot areas and low yielding areas. Absence of female on cysts on roots may not mean absence of SCN in the field. A soil test may still be needed to confirm presence and to determine the population of SCN in the soil. Presence of numerous female cysts on roots of a resistant cultivar may indicate that the resistance may no longer be effective.
Populations in South Dakota
The SDSU Dairy Science Department is studying the diversity of SCN populations in South Dakota. This study is funded by the soybean check-off funds under the South Dakota Soybean Research and Promotion Council. Preliminary results indicate that of 73 SCN populations tested, 25% of them could reproduce ( >10% reproduction relative to a susceptible cultivar) on PI 88788. While a small number of SCN populations have been tested, these results suggest that most SCN populations can be managed by the resistance gene from PI 88788. However, cultivar resistance should be integrated with other management practices to avoid having a resistant SCN population develop.
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