Stop Soybean Cyst

October 1, 2010 11:19 AM

A silent killer stalks your soybean fields. Neither insect nor disease, the tiny, wormlike parasite called soybean cyst nematode (SCN) attacks soybean roots with such ferocity that yields can be slashed by up to 40% with nary a visual symptom. Although plants can sometimes appear stunted or yellowed, those symptoms are characteristic of many conditions. The universal symptom of SCN is yield loss.

During the past 25 years, SCN has moved northward and nearly every soybean-growing area is now infested. In some states, more than 80% of the fields have been confirmed to have SCN. A soybean checkoff–funded survey estimates that SCN costs U.S. soybean growers between 100 million and 150 million bushels annually.
Given these yield dings, University of Missouri soybean breeder Grover Shannon finds it hard to understand why soybean growers are so complacent about the problem.
“Every grower who complains that his soybean yields have flatlined or hit a plateau needs to take a good, hard look at the roots of his plants,” Shannon says. “Cyst nematodes can steal you blind. Yet a recent survey shows that two out of three farmers in the state of Missouri have never pulled a soil sample to test for SCN.”
Terry Niblack, University of Illinois plant pathologist, says many soybean farmers think they are taking care of SCN problems by growing a resistant variety. “Problem is, most SCN-resistant cultivars derive resistance from a single source called PI 88788,” she says. “In Illinois, we’ve found that 80% of the soybean nematode had adapted at some level to that source of resistance.”
Researchers and plant breeders are feverishly working to incorporate new sources of SCN resistance into the mix of varieties. Nematicides—while available for corn and cotton—aren’t really an option in the fight against SCN. But using the following management strategies can help you get a grip on cyst.
Take the test to beat the pest. Soil sampling is the first step in an SCN management program. The chances of finding SCN after a soybean crop are greater than sampling after a corn crop. Although you can sample for SCN almost any time of year, fall works best as you review harvest data and plan for the next crop.
Iowa State University nematologist Greg Tylka says SCN is often found in greater numbers in alkaline soils (soil pH greater than 7). “SCN is tiny, but is not uniformly distributed and doesn’t move far on its own,” he says.
SCN is spread by the movement of infested soil, so flood-prone areas, field entrances and sections along fence lines will often have higher concentrations of SCN-infected plants.
Tylka suggests pulling SCN soil samples from a 6" to 8" depth, using a zig-zag pattern, when collecting soil cores. You’ll need to take 10 to 20 cores in a 10-acre area. The more cores you take, the better the estimate of SCN population density across the field.
SCN cyst, egg and juvenile counts are typically reported per a certain volume of soil. Niblack suggests keeping things simple: “The object is not to see whether egg numbers are above or below some threshold, but to see whether the numbers are going up or down in a particular field.
“If numbers are going up, it’s time to change your management approach. If they are going down, keep on doing what you’re doing,” she adds.
Rotate to nonhost crops. SCN is an obligate parasite and there are many crops on which it is unable to feed. Rotation to nonhost crops like corn, oats, rye, wheat, sorghum and alfalfa helps control SCN population density.
“The greatest decrease in SCN population densities will occur during the first year a nonhost crop is grown following soybeans,” Tylka says. “Densities decline only slightly more if a second nonhost crop is grown.”
Plant resistant varieties. Resistance to SCN in soybean varieties is never 100% effective. Some adult female populations will reproduce on resistant varieties. “On the other hand, resistant varieties will always outyield nonresistant varieties, so we recommend them across the board,” Tylka says.
“SCN-resistant varieties pay dividends twice: good yields and control of SCN population densities. But not all resistant varieties provide the same level of nematode control—not even varieties that contain the same type of resistance.” Further complicating the picture is the fact that not all soybean seed suppliers rate their varieties for SCN resistance in the same manner.
Switch up resistance. To delay SCN populations developing the ability to reproduce on SCN-resistant soybean varieties, Tylka recommends growing varieties with different sources of resistance in different years. This is similar to using herbicides with differ­ent modes of action in different years to control weeds and avoid the development of herbicide-resistant weeds.
If it is not possible to obtain seed of an SCN-resistant variety with a source of SCN resis­tance different from what has previously been used, rotate among varieties with the common source of SCN resistance, PI 88788.
Don’t forget to consider other defensive traits, such as tolerance for iron deficiency chlorosis, resistance to sudden death syndrome and resistance to Phytophthora.
Control winter annuals. Common weeds such as purple deadnettle, henbit and field pennycress are moderate to good hosts for SCN. “If these winter annuals are growing in SCN-infested fields and soil temperatures are greater than 50°F, SCN reproduction and increases in population densities can occur,” Tylka says.
Be a good steward. Any management practice that promotes good soybean growth and plant health will lessen the yield loss caused by SCN. Avoiding compaction, drought stress, low fertility and other disease or insect pressure helps the soybean plant withstand the pressures of SCN and help you stop the stealth killer. 

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