An Overview of Soybean Cyst Nematode

Introduction

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.

During the cropping season, SCN is relatively easy to detect without magnification as small, white or yellow, lemon-shaped females (immature cysts) on the root surface (Figure 2). Each SCN female is approximately one-tenth the size of an average Rhizobium root nodule. As the females age, they turn brown and die, becoming egg filled cysts for which the nematode is named (Figure 3). The females and cysts are signs of this nematode and can be observed on susceptible soybean roots as early as 30 to 45 days after planting.

Disease Cycle

SCN overwinters as eggs encased in cysts, which allows them to survive adverse conditions for 10 or more years in the absence of a host. When eggs hatch, the juvenile nematode migrates toward and infects soybean roots (Figure 4).

Once in the root, the juvenile establishes a permanent feeding site called a syncytium where it remains through two molts before differentiating into either a male or female nematode. Adult males return to the soil and mate with one or more adult females that remain attached to the root. Females begin to reproduce within a few days, laying eggs that are retained inside their body or deposited in a gelatinous matrix attached to the posterior end of the immature female. The average adult female can lay approximately 200 eggs and it takes 24 to 30 days from hatching until reaching mature female development. Nematode feeding and reproduction inhibit nutrient and water uptake within the plant, which can reduce plant development and grain yield.  

Conditions that Favor Disease

Soil pH is one of the most important factors influencing SCN reproduction; higher egg levels and greater soybean yield losses are often observed in high pH soils (pH greater than 6.5). Soil class is also an important factor which influences SCN reproduction as SCN is most problematic in lighter soils. Lighter soils favor nematode reproduction and movement and are also more prone to water-stressed conditions, exacerbating the yield reduction caused by damaged and colonized roots. Drought, excessive soil moisture, and the timing of these events impact both SCN and associated yield responses.

Yield Losses and Impact

SCN is considered the most yield limiting disease of soybean in the U.S. and Canada with estimated yield losses exceeding 125 million bushels annually (see the CPN Yield Loss Calculator for more details). Yield losses from an SCN infestation can range from minimal when adequately managed, to near complete loss. Aboveground symptoms of an SCN infestation may not be observed until irreversible yield losses of up to 30% already have occurred.  

Diseases, Disorders, and Injury with Similar Symptoms

Management

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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Acknowledgments

Authors

Kaitlyn Bissonnette, University of Missouri; Travis Faske, University of Arkansas; and Albert Tenuta, Ontario Ministry of Agriculture, Food, and Rural Affairs.

Reviewers

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. 

Photographers

All photos were provided by and are the property of the authors and reviewers, except Figure 9 from Travis Legleiter, University of Kentucky

Sponsors

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.