SCN: Know the Numbers

As farmers prepare for another growing season, it’s not too early to think about managing soybean cyst nematodes (SCN), the leading cause of soybean yield loss in North America.

One thing that can be done in the spring is pulling soil samples to test for SCN. Farmers who have never checked for SCN or who haven’t checked for a number of years may still have time to take appropriate measures to address the problem if test results suggest it would be beneficial.

“More than 60 percent of Illinois farmers say SCN is the biggest problem in soybean production, but there is a huge disconnect between awareness of SCN as a problem and a willingness to sample for it,” says University of Illinois Nematologist Terry Niblack. “Less than 15 percent say they sample on a ‘regular basis,’ or every six years, as recommended for fields in a corn/soybean rotation.”

About 75 percent of Illinois farmers say they plant SCN-resistant varieties and seem to indicate that, by doing so, they believe they are doing what they can to address the problem. In fact, there is an advantage to be gained in knowing the level of SCN (the egg population density) present in a field and strategically managing it.

In fact, Iowa State University Nematologist Greg Tylka says 73 percent of the Iowa fields sampled in 2007-2008 had SCN.  Niblack reports that a 2005 survey showed 83 percent of the soybean fields in Illinois are infected with SCN, with average population densities high enough to cause significant yield suppression.

Tylka emphasizes that SCN may not show visible symptoms while impacting yield. Visible symptoms of SCN damage are rarely seen in high-yielding fields (those producing greater than 40 bushels per acre) or during years when soil moisture is plentiful.  Yet, SCN can cause 15 to 30 percent reduced yield (in susceptible varieties) without showing any other visible symptoms.

When aboveground symptoms do appear, they are often mistaken for symptoms of damage from soil compaction, iron deficiency chlorosis and other nutrient deficiencies, drought distress, herbicide injury or other plant diseases. Thus, SCN injury often remains undetected for several years because the symptoms are attributed to other causes.

Soil testing is important to identify fields where SCN may be impacting yield and to monitor fields where SCN is a known problem.

“Farmers who ordered SCN-resistant soybean varieties made a good choice,” Tylka says. “SCN-resistant soybean varieties pay dividends twice, first by producing good soybean yields on SCN-infested fields and secondly by preventing increases in SCN populations.” (The 2009 results of the Iowa State University SCN-resistant Soybean Variety Trial are available online at www.isuscntrials.info.)

Where SCN is present, research funded by soybean checkoff funds from the North Central Soybean Research Program (NCSRP) shows resistant varieties can produce a 40 to 50 percent yield increase over susceptible varieties. Even in areas where SCN is not present, most SCN-resistant varieties are competitive in yield to non-SCN-resistant varieties.

While resistant varieties are beneficial, monitoring the SCN population densities (numbers) is still needed to determine whether the resistant varieties are effective against the SCN in a given field. SCN populations can become resistant to varieties or resistance sources. The only way to get a reliable diagnosis of SCN is through soil sample analysis.

Though SCN can never be completely eliminated from an infested field, when it has been identified, soybean production can remain profitable with proper SCN management. The goals of SCN management are to improve soybean health and yield, keep SCN numbers low and preserve the yield potential of resistant varieties.

“It’s better to keep low numbers low than wait until it becomes a matter of driving high numbers down,” Tylka says.  This means continuing to monitor nematode numbers to make sure they are being kept in check.

Effective SCN management requires an integrated approach. Niblack says the most effective and typical means of managing soybeans in SCN-infested fields involves using resistant varieties rotated with nonhost crops. The nonhost rotation, which in Illinois and Iowa is usually corn, is effective in decreasing SCN population densities. The use of resistant varieties is more complicated because complete resistance to SCN has not been found and SCN-resistant soybean varieties vary in the level of nematode control they provide; moreover, nematodes readily adapt to resistance.

If SCN numbers are rising even while a resistant variety is being used, it is time to first switch to a nonhost crop and then use different, more resistant soybean varieties.

Growers who are committed to planting soybeans but learn that the SCN in their fields may have adapted to the resistance they are using might want to look into switching their seed order to a variety with a different source of resistance.

Because many resistant varieties have the same resistant parent, or “source of resistance,” the rotation of resistant varieties alone may not be sufficient to avoid the adaptation problem. That is part of the reason nonhosts must be included in the rotation to decrease the numbers of SCN and slow down the adaptation to resistant varieties.

“By 2005, more than 90 percent of the SCN resistant cultivars available in Illinois (and surrounding states), were derived from a single source, PI88788. We have found that more than 80 percent of the SCN populations in Illinois were adapted to PI88788 or varieties derived from it,” Niblack says. “This demonstrated that adaptation to this source of resistance has occurred. This is most likely due to the widespread planting of SCN-resistant cultivars derived from PI88788. Rotation with alternative sources of SCN resistance is recommended as a means to slow the adaptation.

“Our studies showed that most populations of SCN in Illinois have not adapted to PI548402 (also known as Peking) or PI437654 (also sometimes known as Hartwig and the branded CystX® resistance), so we encourage using these sources of resistance, especially when SCN population densities are high,” Niblack continues.

Another practice some growers consider is the use of nematicides. New seed treatments are being developed to help with SCN management.

While the use of existing nematicides increases cost of production and generally does not give season-long control, the effect may last long enough to provide an economic yield benefit. “By the end of the growing season, SCN numbers may be as high as or higher than they were at planting,” Tylka says. “Unpredictable yield responses and inconsistent decreases in SCN population densities are the main reasons nematicides are not more commonly used at the moment. Farmers should check with their local sources of information for recommendations regarding the use of nematicides, including new seed treatments that may become available in the near future.”

As the season progresses, anything that can be done to relieve stress on soybeans will help to avoid compounding the impact of SCN. Use of no-till and late planting may be beneficial. In addition, good management of weeds, insects (like soybean aphid) and soil fertility will also help.

Iowa Soybean Association Director of Production Research David Wright offers tips for collecting samples. It is important, he says, that the core be from the upper eight inches of soil. The more samples taken in smaller areas, the more accurate the results. Generally, 15 to 20 soil cores should be taken from an area of no more than 20 acres and be combined into one sample.

In fields that have never been checked for SCN, samples should be taken from areas where SCN is likely to establish first. With all the snow that’s been on the ground this winter, flooding and standing water are likely this spring. In fields where water has stood or has run across, even if they didn’t have SCN before, it may have been brought in with flooding or snow melt. Other areas where SCN is more likely to first be discovered include: near a field entrance, along fence lines, areas where weed control isn’t as good, areas of high soil pH (greater than 7) or areas where yield seemed to be low the last time soybeans were grown.

Spring samples should be collected before soybeans are planted. If soybeans were last grown in the field, samples should be taken from under the old crop rows.

To have a reference for the future, it is helpful to record specific details about the sampling. These details include the areas sampled, number of cores, sampling time, whether it was before or after a soybean crop and the lab that processed the samples.

-Funded through the Soybean Checkoff