August 13, 2018
What’s up with soybean cyst nematode in 2018? This tiny worm-like pest infests the soil in most soybean fields in Iowa, attacking roots and robbing yield. SCN populations vary greatly from field to field and areas within fields. In some years, the SCN population and yield reduction is greater than others. How much yield will SCN steal from your fields this year?
Soybean cyst nematode reproduction can be affected greatly by soil conditions, says Greg Tylka, Iowa State University Extension plant pathologist and nematologist. SCN numbers are positively correlated with soil temperature and negatively correlated with soil moisture. In short, the greatest SCN reproduction occurs in hot, dry growing seasons.
Temperatures in Iowa were unusually warm in May, and SCN got off to a fast start. In one field in central Iowa, adult SCN females were seen on roots on June 5, just 26 days after planting (read more here). Typically, SCN females do not appear on soybean roots in the spring until 35 or more days after planting, Tylka says.
SCN development in 2018
June and July continued to be warm, and parts of the state have remained dry going into August. USDA’s weekly crop progress and weather survey reported at the end of July that corn and soybean crop development in Iowa was seven to 14 days ahead of schedule.
“It’s likely SCN development has been accelerated this season, too,” Tylka says. “And hastened SCN development could result in greater-than-normal increases in SCN egg numbers.”
The SCN females produce eggs that hatch into nematode larva, which infest soybean roots.
Soon, patches of early-maturing plants will be appearing in Iowa soybean fields. There can be many different reasons for early senescence or early maturity of soybeans, he notes. A common cause of early-maturing soybeans that is not often discussed is SCN feeding. The results of a study conducted at ISU’s Northern Research and Demonstration Farm south of Kanawha, illustrate this.
A sampling grid pattern was established in the study area shown in the drawing below. Three soil cores were collected from each intersection of the gridlines in the spring. The three cores were combined and mixed into one composite soil sample, and the SCN egg population density present in each soil sample was determined. In mid-September, aerial images of the study area were collected.
A tell-tale symptom of SCN infestation
A distinct pattern of early-maturing soybean plants is seen near the top edge of the field in the aerial image. And grid cells with high SCN egg population densities are in a similarly shaped pattern in the map of SCN numbers. The soybean plants matured early where SCN population densities were the greatest.
TRACKING SCN: A map of initial soybean cyst nematode egg population densities (top) and an aerial image (bottom) taken in September of the sampled area in an SCN-infested field at ISU’s Northern Research and Demonstration Farm at Kanawha.
Actively manage SCN for the future
“It is not possible to eliminate SCN from a field once the field has been infested,” Tylka says. “Instead, an active, integrated management approach is needed to keep SCN population densities in check to preserve the productivity of infested fields for growing soybeans in the future.”
Integrated management of SCN includes growing non-host crops such as corn in rotation with SCN-resistant soybean varieties. Also, farmers should seek out soybean varieties with different sources of SCN resistance to grow in different years, he says. “And nematode-protectant seed treatments now are available to bolster the performance of SCN-resistant soybean varieties. Fields should be sampled in the fall prior to every second or third soybean crop to monitor SCN population densities.”
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