If you don’t know whether soybean cyst nematode (SCN) lurks in your fields, this spring would be a good time to figure that out. This microscopic roundworm with a voracious appetite robs farmers of more than $1 billion each year. The high level of annual loss means SCN has the dubious honor of being the most serious soybean pathogen in North America.
To date, SCN has been identified as a significant pest in 31 states and Canada. Yield losses in an infected soybean field commonly top 30%, according to plant pathology scientists at the University of Minnesota (UM).
Their research shows that SCN stunts soybean root growth, impairs root translocation of water and nutrients, strips nutrients from soybean plants, reduces nitrogen fixation and makes soybean roots more susceptible to soil-borne plant pathogens.
A view through the microscope shows a cyst that broke open releasing eggs and juvenile nematodes. Photo by Amanda Anderson
Furthermore, SCN is commonly associated with the development and spread of the fungal disease, Sudden Death Syndrome (SDS), notes Missy Bauer, Farm Journal Associate Field Agronomist. University research shows SCN hastens the development of SDS symptoms and increases their severity, leading to greater yield loss.
The presence of SCN in fields is often ignored or misdiagnosed, as its above-ground symptoms are similar to other issues such as nutrient deficiency, drought stress, herbicide injury and disease. In addition, once established in a field, SCN is rarely eradicated, and its eggs can survive in the soil for many years.
SCN moves about very little under its own steam, but infested soil is readily spread by heavy rains and snowmelt. Flood-prone areas, river bottoms, field entrances and sections along fence lines often have high concentrations of SCN-infested plants.
"Moving tillage equipment from one field to the next is another common way SCN is spread," Bauer says.
What to look for. In-season, there are certain signs of SCN to watch for in your crop. Starting about four to six weeks after crop emergence and continuing through late season, Bauer says to look for odd-shaped areas in soybean fields that appear to have been affected by drought or standing water. Bauer uses Normalized Difference Vegetation Index (NDVI) aerial image technology to identify the hot spots and then goes to the field to ground-truth the NDVI map.
"You need an NDVI with a 1-meter or less resolution for affected areas to show up well enough for you to see them," she explains.
Bauer advises farmers to dig up samples of soybean plants along the edges of the areas in question. Do not pull up the plants, as the SCN can be easily dislodged from the plant roots.
"Gently shake off the soil and look for the female cysts," Bauer says. "Once in the root, a female cyst loses the ability to move, typically swells and becomes too large to remain embedded in the root, making her easy to identify. Do not confuse the cyst with a soybean nodule—cysts are much smaller."
Bruce MacKellar, Michigan State University Extension field crops educator, says other visual symptoms of SCN are often present in the infested fields later in the season. Heavy infestations often cause pockets of early leaf drop in soybean fields. Farmers should look for areas in fields that "turn" about one to two weeks early and then drop their leaves before the rest of the field.
Bauer adds that even in fields that appear healthy, farmers should still conduct visual inspections of roots and pull routine soil tests to evaluate whether SCN is present. Being proactive will help prevent SCN from becoming a severe problem.
Soil sampling is an especially effective way to detect and diagnose SCN, Bauer notes. She encourages farmers to conduct random samplings in fields to establish SCN baseline levels and monitor population densities.
The best time to pull a soil sample is immediately after harvest, when SCN numbers are typically at their highest, but spring sampling prior to planting soybeans can also be effective.
This past spring, Bauer selected 18 fields at random in south-central Michigan to test for SCN. Of those, 50% contained SCN. "Of the 43 samples we pulled, 40% had SCN with low to moderate pest numbers," she says.
Management. Bauer worked with farmers at the Farm Journal Soybean College in Coldwater, Mich., this past summer to help them identify SCN on roots and assess management practices.
While little can be done to address SCN in-season, Bauer encourages farmers to manage those populations via a variety of measures the following year. Implementing a corn-soybean rotation is a good first step.
Research by Greg Tylka, Iowa State University plant pathologist, shows that the greatest decrease in SCN population densities usually occur during the first year a non-host crop is grown after a soybean crop.
For example, in general, growing corn for one year after a soybean crop in Iowa will reduce SCN population density up to 50%. However, SCN population densities decline less in the second year of corn after a soybean crop, and they drop even less in the third year of corn.
Other options to address SCN infestations include planting resistant soybean varieties and non-host crops, using seed treatments, nematicides, biological controls and changing up tillage practices.
Bauer adds that any management practice that promotes good soybean growth and plant health will lessen the yield loss caused by SCN. Avoiding compaction, stress from drought, low fertility and other disease or insect burden helps the soybean plant withstand SCN pressure.
You can e-mail Rhonda Brooks at firstname.lastname@example.org.
Soil Sampling for SCN
Soybean cyst nematode (SCN) can be identified at most state university laboratories, as well as at independent laboratories. Some state universities even offer free sampling, thanks to state soybean checkoff programs, says Missy Bauer, Farm Journal Associate Field Agronomist.
To pull an SCN sample, follow these steps:
- Use a cylindrical soil probe to collect soil samples.
- Pull 10 to 20 soil cores by walking in a W pattern across the field or area to be sampled.
- Collect the soil cores at 6" to 8" deep.
- Collect soil cores only from areas of similar soil texture and cropping history. If different soil textures occur in the same field, sample them separately. "Don’t go to the middle of a hot spot to pull a soil sample," Bauer cautions. "In the worst part of the hot spot, there’s usually not a good root system present, so we want to pull samples from the outside fringes where the root system is still in pretty good shape."
- Use a zip-lock plastic bag to hold the samples you plan to submit for testing. "We want to keep these nematodes alive until they reach the laboratory, so put them in a plastic bag so they don’t dry out and die," Bauer says. "We carry coolers with us to put the samples in and then ship them to the lab via UPS that day; or, we put them in the fridge and overnight them the next day. Whatever you do, don’t put them on your truck dashboard and then drive around for two days before you send them to the lab."
- Understand the rating system that a specific laboratory uses to determine the level of severity of SCN in your field, Bauer says. You can then plan your agronomic practices accordingly for the following season.
Farm Journal Corn College and Farm Journal Soybean College
Head back to the classroom and out to the field with Farm Journal Field Agronomists and other experts at the 2014 summer training events. The hands-on sessions arm attendees with tips they can take home to improve yields and boost profitability, acre after acre. Details are being finalized for the annual Corn College and Soybean College events. Visit www.FarmJournalCornCollege.com for the full lineup.