Fighting nematodes remains top concern of soy growers

At a time when soybean prices are likely to remain high for some time, and demand for sustainable fuels from sources including soybean oil is skyrocketing, ongoing efforts to combat soybean cyst nematodes are more important than ever.

Soybean cyst nematodes are microscopic roundworms, or Heterodera glycines, that infect the roots of soybeans and other plants, especially legumes and some weed species. Their egg packets, or cysts, can contain hundreds of eggs that are released into the soil. They can remain viable for years until a suitable host plant is found and can be assumed to be present at some level throughout the Great Plains and in soybean-growing regions in general, except in the deep South where other nematodes take their place.

They are the most economically damaging pest by far for U.S. soybean farmers. They are widespread across most Midwest and upper Midwest states, causing annual yield losses of up to $1.2 billion. Yield losses per field can approach 30%, or even 50% in hot and dry conditions.

Invisible symptoms

SCN is not a new problem, nor is it unknown to farmers. But it can be an invisible problem nevertheless, according to numerous sources involved in the fight against nematodes. Nematodes can cause yield losses long before any symptoms are visible. The conditions that favor SCN growth are the same ones that favor maximum soybean yields.

Even when above-ground symptoms of SCN become visible, they are both subtle and also easily mistaken for symptoms of other conditions. Leaf yellowing, for example, could be a sign of potassium deficiency—or nematodes. Infection by SCN can also potentially increase other soil-borne soybean diseases such as brown stem rot, sudden death syndrome, and fusarium root rot.

In the 1990s, a Soybean Cyst Nematode Coalition was formed to raise farmer awareness, promote soil sampling and quantification of SCN field populations and pressures, provide information on integrated management solutions, and champion further research and development into short- and long-term SCN control strategies.

This first SCN coalition was funded by the Soybean Checkoff. A partnership between soybean farmers, public and private researchers, and seed companies, it provided great value to soybean farmers and to the soybean industry, especially by championing the development and adoption—by conventional breeding—of soybean varieties with an SCN-resistant trait, PI 88788.

Ed Anderson, senior director of research at the Iowa Soybean Association and executive director of the North Central Soybean Research Program, said the awareness-raising campaign was successful in spreading the use of the PI 88788 trait.

This trait worked well, for about 20 years—until its effectiveness began to tail off. According to Greg Tylka, Morrill Professor in the Department of Plant Pathology and Microbiology at Iowa State University, who has spent his career at the center of the nematode fight, natural selection is the culprit. In the evolutionary arms race, nematodes evolved to resist the resistant gene trait that formerly worked against them. The trait that used to contain SCN is now only effective about 15% of the time.

In addition to PI 88788 resistance, soybean breeders are developing new soybean varieties with a breeding line named Peking, which effectively controls the nematodes that have overcome PI 88788 resistance. BASF is also developing a transgenic resistance trait that should be rolled out in a few years.

There are twice as many Peking varieties available this planting season as last year’s, according to Iowa State University. But 95% of all SCN resistant varieties still rely on PI 88788.

One reason for the slow adoption of breeding lines, including the Peking trait, is that it initially lowered yields. But that is being rectified, said Anderson. Yields from varieties with Peking SCN resistance have been going up. Peking breeding lines and commercial cultivars containing the Peking SCN resistant trait have higher yield potentials than previous genetics and can outyield soybeans with the PI 88788 trait under SCN conditions where the PI 88788 resistance has been overcome.

“We started to see higher yields with Peking about 10 to 15 years ago,” said Tylka. “We are seeing a consistent increase in yield potential.”

No silver bullet

All the experts caution against relying on any one silver bullet, however. Sooner or later, they say, populations of the soybean cyst nematode will overcome genetic resistance due to selection pressure and through evolutionary processes.

“Even with resistant traits, the best you can hope for with SCN is control and management,” said Mark Licht, an associate professor and Extension cropping systems specialist with Iowa State University Extension and Outreach. “You can never get rid of them 100%. They may not hatch every year.” Echoing him, Anderson said, “Nature finds a way.”

Keeping the pests off-balance and controlled is achievable, though. Soy-on-soy cropping can dramatically increase the pressure of this pest, Licht said. The best approach is an integrated suite of practices, beginning with constant and intensive soil sampling, which several universities involved in the SCN fight offer as a free service. “Take the test, beat the pest” was a slogan of the 1990s SCN Coalition efforts. Many of the techniques that combat SCN are also beneficial generally, increasing soil health and proving effective against many types of persistent pests. Balancing and rotating acreage is important for keeping the nematodes off-balance.

In 2015, the Soybean Cyst Nematode Coalition was revived and its members, including researchers, seed companies, growers have been holding regular conferences and events, the latest in December in Savannah, Georgia. The North Central Soybean Research Program continues to provide sustained investments in ongoing, multi-year SCN research that delivers progress and results, through the Soybean Checkoff. Some of this research is targeted toward learning more about the nematodes themselves and what future modes of action may be effective against them.

Anderson said the first two questions growers have to answer are, “What is my risk?” and “Where are the parasitic nematodes in my fields?” He is enthusiastic about how precision agriculture and “prescriptive agriculture” can better target the fight this time around. With vastly more, and more precise, real-time data being collected, analyzed and turned into practical decision tools and equipment controls, he said, farmers can place chemical, biological and genetic solutions where they are needed. For example, planting SCN-resistant “workhorse” seeds in one part of their fields that soil sampling has revealed to be problematic, while planting higher-yielding “racehorses” in less vulnerable parts, to maximize overall yields while still containing the SCN threat is one possible option in an overall integrated solutions approach.

According to the SCN Coalition site, a survey given in 2015 was repeated in 2020 to assess how effective the fight against SCN has been this time around. “The survey … revealed the projected economic impact of the coalition is staggering: 6% to 18% more soybean growers are actively managing SCN than in 2015.” In 2020, 77% of growers rotated soybeans with non-host crops like corn or wheat, 66% planted SCN-resistant soybean varieties, 49% rotated sources of SCN resistance, and 40% used nematode-protected seed treatments.

In the end, fighting SCN is “is not any one thing,” said Anderson. “It’s going to be a mix of prescriptive agriculture, biologicals, crop rotations and other agronomic strategies, genetics and chemicals. It’s still a $1.2 billion problem.”

For more information, visit the SCN Coalition site at thescncoalition.com/.

David Murray can be reached at [email protected].