There aren’t many polite things to say about a pest that can steal 30% of your yield before plants even show any symptoms. Soybean cyst nematode, or SCN, is the industry’s top yield thief, causing more than $1.2 billion in annual losses in the U.S. Still, SCN earns a grudging respect among the scientists who work with it.
“SCN is the ultimate pathogen,” marvels Greg Tylka, Iowa State University Extension nematologist and plant pathologist. “It causes yield loss directly and indirectly by making other things like soybean sudden death syndrome and soybean brown stem rot worse.
“SCN has a unique biology that makes it very difficult to control,” Tylka adds. Females mate with many different males, lay hundreds of eggs at a time, and cycle through four or five generations in a single growing season, he explains. That’s a recipe for creating exploding populations and remarkable genetic diversity — the reason cultural controls and resistance genes get overwhelmed by ever-changing populations of the nematode.
Subtle yield stealer
Symptoms of SCN damage can be easy to miss, so massive populations can build up in fields for years before farmers even notice there’s a problem. Yields can decline slowly, and stunting may be masked by environmental conditions that make patches of shorter plants less obvious, note Allen Wrather and Melissa Ketchum of the University of Missouri’s Division of Plant Sciences.
As infestations increase and crops encounter greater environmental stress, chlorosis, top die-back, stunting and plant death — including outbreaks of sudden death syndrome, or SDS — can become apparent. The trick is not to mistake them for nutrient deficiency, herbicide damage or drought stress, which SCN symptoms often appear to be.
To be sure, dig up chlorotic plants — particularly around the edges of areas you suspect are infested, where roots are likely to still be healthy enough to support the nematodes — and look for lemon-shaped white or yellow females on the roots. A soil sample may confirm the presence of SCN, too, though Wrather and Ketchum point out that if there are 2 million SCN eggs in an acre of soil, there is just a 63% chance
of detecting one egg in a pint-sized sample.
Can’t eradicate SCN
Once SCN is established in a field, it’s impossible to eradicate. So the first step is trying to prevent the spread of SCN into clean fields by avoiding the movement of soil from infested fields to uninfested ones. The most obvious approach is to work uninfested fields before moving equipment into infested ones.
If you suspect you have SCN, begin looking for it and managing it promptly.
“It is much easier to keep low numbers low than to bring high numbers down,” says Tylka.
A pest as tough as SCN requires an integrated strategy for management. Here are key steps to keeping SCN in check:
- Rotate crops. SCN cannot reproduce in non-host crops like corn, sorghum, sunflower, alfalfa, rye or wheat, and population densities decline every season the nematodes are denied a host.
- Control weeds. Weeds, including many legumes as well as clover, vetch and winged pigweed, are SCN hosts.
- Keep crops healthy. Plant stress from drought, nutrient deficiencies, insect damage, weed competition and other plant diseases is exacerbated by SCN. Protect crops from other threats to minimize SCN damage.
- Plant resistant varieties. Some resistant varieties can be effective on some populations of SCN, though the vast genetic diversity of the pest has overcome the most popular resistant genetics in many areas. It is important to rotate resistant genetics to keep them viable in the long run. Alternatives are scarce, but they’re worth seeking out.
- Protect roots. New nematicides — including a biological seed treatment — are on the market or on the way. Though they’re expensive tools, they can complement other management practices.
“An SCN infestation is not a death sentence,” Tylka says. “Finding it doesn’t mean you have to stop growing soybeans. You just have to manage it.”
Genetic solution closer for soybean cyst nematode
Tackling plant pests with genetic solutions is more fine-tuned than ever. The early days of inserting a protein into a plant to take care of a problem are still with us, but now plant genetics experts are tackling pests in new ways.
Earlier this year an international team of researchers looked at plant defense tools in one common genetic model — Arabidopsis, which isa mustard-like plant whose genome code scientists know well. They identified a gene called AtPAD4 that is responsible for the expression of a number of genes
for defense response in the plant.
The research team expressed a gene encoding AtPAD4 in soybean roots of composite plants to test the ability of the gene to stop nematode development.
The results were promising, with the gene cutting the presence of mature soybean cyst nematodes
by 68%. The impact on root knot nematodes was even greater, at a 77% reduction.
The work is young, but does show that this gene could be bred into soybeans to offer a new way to resist two strains of nematodes.
The AtPAD4 gene is involved with the plant’s own built-in defense mechanism. This method of genetic modification could be a more potent defense against key pests. Essentially, as the research report says, “this work provides a basis for unraveling the potential role of defense-signaling genes in quantitative disease resistance in this major crop species.”
For a pest that can cost the soybean industry more than $1 billion a year in losses, a dependable genetic solution will be welcome. Researchers on this project were with USDA Agricultural Research Service in Beltsville, Md.; the Genetics Division, National Institute of Agricultural Biotechnology, Rural Development Administration, South Korea; and the Plant Protection Department, Faculty of Agriculture, Fayoum University, Fayoum, Egypt.