The soybean cyst nematode (SCN) is capable of causing serious yield loss on soybeans every year and is widespread throughout Iowa and much of the Midwest. In the “good ole days,” a farmer was “good to go” with SCN management if he or she knew what fields were infested with the nematode and then grew SCN-resistant soybean varieties in rotation with the non-host crop corn. Nowadays, though, things are a little more complicated.
The fact is, your SCN numbers may be building up unbeknownst to you. This buildup can go unrecognized because a soybean crop often does not appear stressed above- ground, even though SCN numbers are building up belowground and yield loss is occurring. Iowa State University Extension plant pathologist and nematologist Greg Tylka provides answers to the following questions.
What causes buildup in SCN numbers in a field?
The buildup in numbers can occur by growing soybean varieties that are described as SCN resistant but do not provide full protection against the nematode. Some varieties are described as moderately resistant instead of resistant to SCN. Farmers are advised to grow soybean varieties that have the strongest SCN control.
Figure 1. Sampling for SCN can be done after harvest in corn or soybean fields. The soil samples can be collected up until the ground freezes or snowfall occurs.
Also, full SCN resistance in soybean requires multiple genes from the breeding line that is used to develop the variety (such as PI 88788 or Peking). Not all varieties on the market described as SCN resistant possess all of the available resistance genes, a condition which leads to less than full nematode control. ISU tests the nematode control and agronomic performance of many SCN-resistant soybean varieties in experiments conducted throughout Iowa each year.
Where can you find more information about SCN-resistant varieties?
Results of the work, funded by the soybean checkoff through Iowa Soybean Association, are posted online at isuscntrials.info. A hard copy report of the results of 2016 experiments will be released in early January.
An inadvertent buildup in SCN numbers also can occur by growing SCN-resistant soybean varieties with the same source of resistance repeatedly in the same field, causing a shift in (or selection for) the SCN population with increased ability to reproduce on the source of resistance. The increased reproduction of SCN populations in Iowa fields is a real problem because almost all SCN-resistant soybean varieties available to Iowa farmers contain resistance genes from one breeding line, PI 88788.
How would you know if your SCN numbers are building up?
The only way to know the status of the SCN infestation in a field is to collect a soil sample and have it tested. Specifically, farmers should collect soil samples to determine their SCN numbers before every third soybean crop.
Figure 2. Example sampling pattern for collecting soil samples for SCN in different management zones in a field.
Collecting soil samples to check for SCN and to determine its numbers is not difficult. And this fall is a great time to sample fields in which soybeans will be grown in 2017. A few simple guidelines to follow are:
* It is best to use a soil probe, not a spade, to collect soil cores.
* Samples can be collected following a soybean or corn crop (see figure 1).
* Collect 8-inch-deep soil cores.
* The more soil cores collected from the smaller area, the more accurate the results will be. Collecting 15 to 20 soil cores from every 20 acres often is recommended.
* Collect samples from different management zones in a field, if such zones exist (see figure 2).
* Combine all soil cores in a bucket and mix them well before placing the mixed soil into a soil sample bag.