Soybean cyst nematode (SCN; Heterodera glycines) is an important nematode pest of soybean in the U.S. and Canada that is responsible for over a billion U.S. dollars in losses annually. Since SCN was first identified in North Carolina in 1954, the nematode has continued to spread through soybean producing areas in North America. As of January 2020, SCN has been detected in 30 states in the continental U.S., three Canadian provinces, and Puerto Rico. Soil sampling for SCN is the most effective way to identify the presence of the nematode and its presence in a field. This information enables growers to make the best management decisions for their farms.
Symptoms and Signs
Soybean cyst nematode is a plant-parasitic nematode that feeds and reproduces on soybean roots. The nematode interferes with the uptake of water and nutrients by the soybean plant and may increase the severity of diseases caused by other root-infecting pathogens. In many cases, symptoms may not be visible in a field until SCN populations build to a high enough level that substantial yield loss is already occurring. When aboveground symptoms are apparent, they commonly appear as yellowing of the leaves, stunting of plants, and early maturity (Figure 1). Symptoms are most commonly observed in circular or lens-shaped patterns in the field, and in areas with lighter soils (e.g., sandy or loam), high pH, and under dry conditions. Aboveground symptoms are not diagnostic, and may be confused with nutrient deficiency, flooding, herbicide injury, compaction, drought, or root rot damage.
Management of SCN requires an integrated approach of host plant resistance, crop rotation, scouting/soil testing, and seed-applied nematode protectants or nematicides.
Host plant resistance can be an effective management tool, but careful selection and rotation of soybean varieties is necessary to maximize the effectiveness of the resistance. Several sources of resistance to SCN are known in the soybean germplasm, but the majority (over 90%) of SCN-resistant soybean varieties contain ‘PI 88788’. A minority of varieties to date contain the ‘Peking’ (aka PI 548402) source, and fewer yet contain ‘Hartwig’ (aka PI 437654) or ‘PI 89772’. As resistance to SCN is not conferred by a single resistance gene, soybean varieties developed from the same source of resistance do not all perform the same under similar SCN pressure. The most effective varieties may perform very well, while the least effective varieties may experience the same level of yield loss as a variety with no genetic resistance.
Additionally, the nematode populations are slowly but steadily overcoming the resistance genes from the PI 88788 source of resistance and to a lesser extent Peking. Thus, it is critical that soybean growers rotate the sources of resistance in varieties they select. Similarly, it is just as important to rotate varieties developed from the same source of resistance, as this is likely to result in rotation of the resistance genes that are deployed in a given field.
The use of a non-host or poor host crop such as barley, canola, corn, cotton, durum, flax, rice, grain sorghum, sugarbeet, sunflower, or wheat in a crop rotation sequence can help to reduce SCN population densities. Alternately, avoid SCN host crops like dry edible bean (such as pinto, navy, and kidney), snap bean, and edamame in a crop rotation sequence. Soybean volunteer and weed management is also an important consideration in fields where weed hosts or cover crops such as lespedeza, crimson clover, henbit, purple deadnettle, field pennycress, vetch species, and other legumes are present that can sustain an SCN population for the subsequent soybean crop.
Soil movement plays an important role in the introduction of SCN into new fields and new geographies. Cysts in soil particles can be moved via water, equipment, wind, as tare soil, or any other method by which soil can be moved. To reduce soil movement, it is important to ensure newly purchased or borrowed equipment is properly cleaned of soil and soil particles before introduction into a new field.
Other management options include chemical or biological seed-applied nematicides. Efficacy of these seed treatments have been highly variable in university field research trials conducted in multiple states, thus, they are often best used in combination with host plant resistance, especially in fields with multiple species of soybean nematodes.
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Kaitlyn Bissonnette, University of Missouri; Travis Faske, University of Arkansas; and Albert Tenuta, Ontario Ministry of Agriculture, Food, and Rural Affairs.
Tom Allen, Mississippi State University; Gary Bergstrom, Cornell University; George Bird, Michigan State University; Carl Bradley, University of Kentucky; Emmanuel Byamukama, South Dakota State University; Martin Chilvers, Michigan State University; Alyssa Collins, Pennsylvania State University; Anne Dorrance, The Ohio State University; Nicholas Dufault, University of Florida; Churamani Khanal, Clemson University; Nathan Kleczewski, University of Illinois; Alyssa Koehler, University of Delaware; David Langston, Virginia Tech; Sam Markell, North Dakota State University; Dean Malvick, University of Minnesota; Rodrigo Borba Onofre, Kansas State University; Paul “Trey” Price III, Louisiana State University; Marisol Quintanilla, Michigan State University; Edward Sikora, Auburn University; Damon Smith, University of Wisconsin-Madison; Darcy Telenko, Purdue University; Lindsey Thiessen, North Carolina State University; and Guiping Yan, North Dakota State University.
All photos were provided by and are the property of the authors and reviewers, except Figure 9 from Travis Legleiter, University of Kentucky
This project was funded in part through Canadian Agricultural Partnership (CAP), a federal-provincial territorial initiative. The Agricultural Adaptation Council assists in the delivery of CAP in Ontario. The authors thank the United Soybean Board, the United States Department of Agriculture - National Institute of Food and Agriculture and the Grain Farmers of Ontario for their support.