Issue No. 24, Article 5/November 11, 2005
Now Is the Time to Sample for Soybean Cyst Nematode
The answer to one of the most frequently asked questions about soybean cyst nematode (SCN) is that the best time to sample is in the fall. This is true for two main reasons: the sampling should be done in cornfields that will be planted to soybean next year, and overwinter survival is high. The rationale for these reasons follows.
Soil sampling in cornfields is of course easiest to do when there are no corn plants present, but the recommendation to sample in the fall is not just for convenience. The population density of SCN in a cornfield at corn harvest is highly predictive of the numbers of SCN that will be present in the spring, because overwinter survival is very high (Figure 2). In southern Illinois, SCN numbers will decrease slightly over winter, depending on how warm the soil gets during the winter and before soybeans are planted. In northern Illinois, overwinter survival approaches 100%.
Figure 2. Mean population densities of SCN (number of eggs per 100 cm3 soil) in the same plot in a multiyear rotation study (latitude 40.19 N). Plots were sampled at harvest and every 30 days thereafter. The soybean cultivar was susceptible to SCN.
Another reason for sampling at corn harvest is that SCN populations decrease under corn, usually from 15% to 50% (mean of 35% in research plots). By harvest time in the fall, the SCN populations have stabilized. Sampling in the fall will also allow plenty of time to make decisions about whether to plant soybeans and which SCN-resistant soybean variety to use the following spring. The yield of SCN-susceptible soybean in SCN-infested fields is highly correlated with the population density of the nematode present at planting (Figure 3).
Figure 3. Linear function describing the relationship between SCN population density at planting and soybean yield. Data were collected from field microplots artificially infested with SCN at planting (latitude 39.0 N).
Keep in mind that over 80% of the fields in Illinois are infested with SCN, so the likelihood of any particular field being infested is high. Keep track of the numbers over time by sampling fields in the fall following corn, when SCN numbers are likely to be lowest.
A soil sample should be a composite of 20 or more cores taken in a zigzag pattern across a field. Most nematologists agree that one sample can adequately represent a 5-acre area; some say as high as 20 acres. So what do you do if the field is 300 acres? Collect samples from two or more arbitrarily selected 5-acre sections that represent similar soil types and crop histories. There's no need to sample the entire field unless you're planning to plant different varieties in different sections of the field. Excellent instructions for sampling are available at http://www.planthealth.info/scn_scout.htm.--Terry Niblack