While it is still fresh in your mind, many of you probably noticed the great deal of variability this year in yields that occurred as you were driving the combine across the field. Part of the variability is due to the presence of soybean cyst nematode (SCN). Why is it important to know where it is and what the levels are? Here are a few reasons.
1. Yield losses can be huge when SCN populations are high and susceptible varieties are planted. This year in some of our trials, we have documented yield impacts of 100% when susceptible varieties were planted into fields with high levels of SCN. The presence of SCN and the drought really hammered some plants this year.
2. SCN tends to sit in pockets, especially in no-till production systems, so knowing where to sample is important. From our North Central Soybean Research Program funded project we have been able to grid sample numerous fields over the past five years. From this exercise we have also been able to show how variable SCN is in fields. Within a 25-ft. span, we can go from undetectable levels to over 2,000 eggs/cup of soil. This level is where we begin to see measurable yield losses. Monitoring a field and noting where yields are low is key and those are the places to target soil samples to begin with. Long-term monitoring of the location and the size of the infestation to be sure it doesn't grow and get out of hand.
3. SCN is adapting to some of the sources of SCN resistance. We have found, in Ohio, some populations that like to feed on the roots of plants that have the genes from the primary source of resistance, PI 88788. We don't know how widespread this is, but it does mean that if you have been planting SCN resistant varieties, then you all must be sure to monitor SCN as well. You are not off the hook from soil sampling. If the populations in your field have adapted, you will see an increase in SCN populations over time – not a decline.
4. Rotation might not be working as well as it should. We have visited some fields in Ohio over the past three years where SCN is a major problem. These are fields where SCN populations got above the 10,000 eggs/cup of soil or greater. The producers did the right thing and rotated out of soybeans for three or more years. Replanted again with a resistant soybean and got hammered again. When we took samples from these fields, the numbers were still too high. This phenomenon has been observed by both Greg Tylka at Iowa State University and Terry Niblack when she was in Missouri. We have some SCN populations that are really only interested in feeding on soybeans and they will wait until the field is planted.
5. Best management practice overall is to keep those populations low and don't let them get above the 5,000 eggs/cup of soil mark. It's a lot easier to keep low populations low than it is to reduce high populations. This can be done with rotation, rotation, rotation – keep the numbers low by planting non-hosts (corn, wheat and alfalfa). Keep weeds under control in the fall and spring that might add another cycle of reproduction. Plant varieties with the best SCN resistance package. If SCN populations stay below this level then hopefully we will not end up in a situation like No. 4.
6. Sample. Target the areas where you saw the yields drop, plants were stunted or they matured early than the rest of the field. They tend to be the hot spots. Note the size of these areas in the fields and continue to monitor them. But also remember that SCN can reduce yields significantly without causing any visible symptoms.
The following Ohio labs all process soil for SCN: