Soybean cyst nematode problem requires management

Corn has been king in the Midwest for many years, but with drought pressure and low prices, producers are expected to plant more soybeans in 2018. Growers need to be aware of diseases and pests affecting soybeans and how to manage them.

Soybean cyst nematodes are a growing problem for soybean producers. Iowa State University researchers continue to study this tiny worm that causes up to $1.2 billion worth of damage to soybeans each year in the United States, making it the most serious threat to sustainable U.S. soybean production.

Thomas Baum, an Iowa State plant pathologist, said the conditions must be just right for the soybean cyst nematode to survive and thrive and thus cause damage to the plant.

“This is the state-of-the-art of our understanding of the molecular mechanisms at work when the cyst nematode infects the plant, heavily emphasizing the molecular signals, so-called effector proteins, the worm sends to the plant,” said Baum, who has been studying the parasite for more than 20 years.

The lifecycle of the SCN has three main stages—the egg, juvenile and adult. SCN can complete its lifecycle in four weeks under ideal conditions and soil temperatures at 75 degrees Fahrenheit. Eggs develop into juveniles and then hatch and enter the soybean root to feed and grow. The adult females break through the surface but remain attached to the root and the adult males leave the root. Adult males mate with adult females on the root surface. Females lay 50 to 100 eggs in an egg mass on their posterior. The female fills up with 200 or more eggs and the female eventually dies. Her body wall hardens to form a tough cyst around the eggs.

“This is a worm that needs a living cell. It becomes sedentary and does not move from cell to cell. It takes one place and says this is where I’m going to live and eat, and I need to make sure my fridge is full all the time,” Baum said.

Greg Tylka, Iowa State University plant pathology and microbiology professor, said SCN is the most damaging soybean pathogen in North America, followed by sudden death syndrome, seedling diseases, Phytophthora stem and root rot, and charcoal rot.

Severe symptoms of SCN can be seen during the growing season when stunting and yellowing are visible. Root stunting, discoloration and fewer nodules are viewed as below-ground symptoms of SCN and SCN adult females can be seen with the naked eye; thus scouting should be taking place according to Tylka.

“In SCN susceptible varieties, there can be up to a 50 percent yield loss seen as compared to SCN-resistant varieties. That can be 35.8 bushels per acre as compared to 71 bushels per acre,” said Tylka.

Typical yield loss between SCN-susceptible varieties versus SCN-resistant varieties would be about 10 percent in a field with non-SCN symptoms.

“Continual research since 1996 shows SCN presence is not changing much. In 1996 to 1997 74 percent of fields tested positive for SCN. From 2007 to 2008, 72 percent of Iowa fields were positive for SCN presence. And for the 2017 growing season, our numbers show 63 to 86 percent of fields were SCN positive,” said Tylka.

Seed companies have continued to develop soybean cyst nematode-resistant varieties. Over 1,000 SCN-resistant soybean varieties are available for 2018. Tylka said the desired SCN-resistant varieties have vigorous, upright growth, are high yielding, in the appropriate maturity group and resistant to SCN.

However, that SCN resistance is losing steam. Data from 25 years of Iowa State University variety trial experiments shows that as SCN reproduction increases on varieties containing the PI 88788 gene, yields of resistant soybean varieties decrease by as much as 14 bushels per acre.

“We have to continually develop new soybean lines, because we want to avoid resistance on the SCN side, similar to what we have seen with herbicides,” said Tylka.

Planting SCN-resistant varieties and rotating crops are two ways to help prevent problems with SCN and seed treatments can be used as well. Research shows some of the treatments work best with the combination of SCN-resistant varieties.

Baum’s research shows the proteins connected with allowing the SCN to cause problem to the plant. Years of study have identified more than 80 confirmed effector proteins, but researchers suspect there will be hundreds more. Baum believes scientists will eventually solve the SCN problem, but it will take time and sustained research efforts.

“We can do things to make the plant a very poor host to the worm, but then it’s also not a good crop anymore,” Baum said. “This is an organism that is only alive because it has evolved mechanisms to parasitize a plant without killing it, and it’s been doing that for many millions of years. Over time, the mechanisms that didn’t work well didn’t make it, but those mechanisms that are at work today are very robust.”

Tylka said for now, management of soybean fields is the best way to avoid loss due to SCN. Scouting fields to assess the disease and pest situation is imperative. Growing non-host nematode crops like corn can help clear up the problem, as well as growing resistant soybean varieties or using nematode-protectant seed treatments.

“The use of cover crops may also help with problems of nematodes, but that answer is yet to come,” added Tylka.

Jennifer Carrico can be reached at 515-833-2120 or [email protected].