Nematodes are microscopic, nonsegmented roundworms. They are a highly diverse group of organisms with many different feeding habits, including “free-living” beneficial nematodes and nematodes that parasitize plants. Soilborne plant-parasitic nematodes feed on plant tissues to survive and reproduce. Their feeding can result in root damage, which may compromise root function and result in yield loss.

Although plant-parasitic nematodes are microscopic—typically only 0.01 to 0.04 in. (300 to 1000 μm) long—the damage these nematodes can inflict on a soybean crop can sometimes be visible to the eye as disease symptoms. Their impact can also be measured when yields do not make the expected mark. In addition to direct damage, feeding wounds on soybean roots create a point of entry for secondary diseases caused by fungi or bacteria. Because the life cycle of most types of plant-parasitic nematodes is three to four weeks, many generations of nematodes can occur in a growing season.

Plant-parasitic nematodes are often more problematic and have greater impact on yield in coarse, sandy soils, although they may occur in any soil type and any growing climate. Sandy, coarse soils have larger soil pores, which allow for easier nematode movement. Nematodes that affect soybean survive on or inside of root tissues and move through water films between soil particles to find a host plant. Because they are so tiny, nematodes do not move very far through the soil on their own, in some cases traveling only up to 5 feet per year. However, they may be dispersed longer distances via events that move the soil they inhabit, such as flooding and wind, or movement of planting or tillage equipment from field to field.

Because plant-parasitic nematodes feed on the roots of soybean, aboveground foliar symptoms are often similar to those caused by nutrient deficiencies or water stress. Nematodes may cause stunting, wilting, and chlorosis (yellowing) of aboveground portions of the plant. In severe cases, plant stands may be reduced, leaves turn brown, pods may not fill, and plants may die.

Symptoms of nematode feeding are often seen in patches or “hot spots” within a field. These patches are typically oval or irregular in shape and often stretch in the direction of cultivation.

Certain genera (types) and species of nematodes can produce characteristic symptoms in soybean. These specific symptoms are discussed in this chapter.

Proper management of nematodes requires adequate diagnosis of the genera of nematodes present in the field and sometimes the specific species causing damage. The first step in diagnosing a new nematode problem or managing existing nematodes is to collect soil samples for nematode analysis. A composite soil sample is collected from the field and then evaluated by a nematode assay laboratory to detect the presence of plant-parasitic nematodes and record their population counts. In North Carolina, soil samples may be submitted to the Nematode Assay Laboratory, a service of the NCDA&CS. Private nematode assay labs also offer analysis; check with your local Extension agent or crop consultant. The assay lab will evaluate total plant-parasitic nematodes, which includes those that affect soybean and those that may affect other rotational crops.

To obtain the most accurate picture of the problem, it is important to collect a good soil sample that is representative of the field. It is best to collect soil samples in the fall (late August through November) because nematode populations are likely to be at their highest during this time, making the diagnosis easier. Soil samples can be collected any time of the year, except when soil is excessively wet or dry. Sampling in the fall will guide nematode management decisions for the following year’s crop.

Use a soil probe or small hand trowel to collect about 20 soil cores or a shovelful of soil from an area of about 4 to 5 acres in size. If you are sampling an area larger than 5 acres, it is best to split the survey area into multiple sections and collect a separate sample from each section. Collect soil cores by walking in a zigzag pattern across the field.

Collect soil at a depth of at least 6 inches (scrape away the top 1 to 2 inches of soil before sampling). If the soybeans are still in the field, collect from the root zone, close to the stem. Place the individual soil cores in a clean bucket and gently mix together. Remove a subsample and place it into a clean plastic bag. Seal the bag and place it in the required sample submission box. Including small pieces of root or pods may also aid in the diagnostic process. Be sure to label each sample with an identifying code that will help you remember where the sample was collected from.

Submit soil samples to the laboratory as soon as possible after collecting, ideally within two days. If you must store the sample before submitting it to the laboratory, place it in a cool (50°F to 60°F), dark location. Do not freeze the soil sample. There will be fees for processing samples. Check with the lab or your county Extension agent for current costs and sample submission forms.

The assay lab will provide a report of the genera and population counts of each plant-parasitic nematode identified in the soil sample. These population counts are expressed per a volume of soil (typically 100 cc or 500 cc of soil, depending on the laboratory). The report will help you determine what genera of plant-parasitic nematodes are present and at what population counts. This information will help you assess the risk posed to your soybean crop.

Additional information about nematode analysis is available at the NCDA&CS websites for predictive samples and diagnostic samples.

Some of the most common problematic and yield-limiting nematodes in soybean are presented here. This section isn’t a comprehensive representation of all plant-parasitic nematodes that can affect soybean.

Soybean Cyst Nematode
(Heterodera glycines)

Soybean cyst nematode (SCN) has been the most serious nematode disease in North Carolina. It is found in all soybean-producing regions of the state, and severe losses are especially common in sandy, coastal plain soils. Cool-to-moderate conditions with adequate moisture tend to favor SCN reproduction and spread. At the end of the growing season, the female nematodes form cysts in which their egg masses overwinter until a susceptible host is available, contributing to inoculum in future growing seasons.

Symptoms. Symptoms of SCN can be easily confused with herbicide injury or nutritional disorders. SCN is characterized by irregular patches of stunted soybeans, yellow soybeans, or both. A decline of yields can be observed over several years in fields affected with SCN. Failures in weed control may also be indicative of SCN infections. It may be possible to observe cysts on roots of affected plants, but they are not always visible. Young cysts are white to yellow in color and about the size of the tip of a ballpoint pen. Older cysts are dark brown in color, making them more difficult to see.

Management. Correcting other production limitations, such as infertile and hardpan soils, may curb the impact of nematode infections. Planting fields with susceptible soybean varieties for several years will increase field populations of SCN; rotation with nonhost crops (corn, cotton, tobacco, wheat, peanut) is recommended. SCN damage tends to be greater in early-planted soybeans, but yields of later-planted soybeans tend to be somewhat lower than those planted early. No-till practices can reduce SCN populations, as long as weeds can be adequately controlled. Several nematicides are labeled for management of SCN in soybean, yet their use is often not economical. Planting resistant varieties, where available, is the most economical means of managing SCN; however, you must select a variety with resistance to the specific race or HG Type of nematode in the affected field to gain maximum benefit.

Additional information about SCN may be found in the NC State Extension factsheet, Soybean Cyst Nematode.

Root-Knot Nematodes
(Meloidogyne spp.)

Root-knot nematodes (RKNs) can cause significant damage in fields with high populations. Several different RKN species are found in North Carolina, though their distribution is varied throughout the state. The highly aggressive species M. enterolobii is also present in several counties. RKNs have a broad host range and affect numerous field crops and weed species common to North Carolina.

Symptoms. Reduced vigor and stunting of plants are early signs of RKN pressure. Under hot, dry conditions, infected plants wilt more rapidly than unaffected plants. Severely affected plants show stunting and chlorosis due to nutrient and water deficiency. Diseased plants may die before maturity and fail to produce seed. Roots of affected plants have irregular deformations (galls, knots, or bumps along the root) that contain the adult female nematodes. As opposed to beneficial nitrogen-fixing nodules, RKN galls cannot be removed from the roots without damaging the roots extensively. Nitrogen-fixing nodules are pink colored inside, whereas RKN galls are white to gray colored inside.

Management. Correcting other production limitations, such as infertile and hardpan soils, may curb the impact of RKN infections. Because RKNs have a broad range of hosts—including cotton, peanut, sweetpotato, and corn—crop rotations are generally not a good management technique to limit RKN populations. Destroying crop debris, especially roots, and weed species helps limit the survival of RKNs in the soil. Several nematicides are labeled for management of RKNs in soybean, but are often not economical. Planting resistant varieties, where available, is the most economical means of managing RKNs.

Additional information about RKNs may be found in the NC State Extension factsheet, Root Knot Nematode of Soybean.

Lesion Nematode
(Pratylenchus spp.)

Lesion nematodes can cause significant damage in fields where high populations occur. There are several species of lesion nematode, many of which impact soybean production. Unlike SCNs and RKNs, lesion nematodes remain motile at the adult stage.

Symptoms. Reduced vigor, stunting, and chlorosis (yellowing) are common symptoms of lesion nematodes. Feeding by this nematode produces small black and brown necrotic lesions (spots of dead tissue) along the root. In severe cases, the outer layer of the root may slough off, exposing the inner part.

Management. Correcting other production limitations, such as infertile and hardpan soils, may curb the impact of lesion nematode infestations. Because lesion nematodes have a broad host range, crop rotations have limitations. Several nematicides are labeled for management of lesion nematodes in soybean, but are often not economical. Performing regular soil sampling will help to determine the level of risk that lesion nematodes pose to a soybean crop. No genetic resistance to lesion nematodes in soybean is known.

Additional information about lesion nematodes may be found in the NC State Extension factsheet, Lesion Nematode in Soybean.

Stubby-Root Nematodes
(Paratrichodorus spp.)

Several species of stubby-root nematodes are found in North Carolina. Stubby-root nematodes have a broad host range, including soybean, cotton, corn, peanut, small grains, potato, and other vegetables. Although these nematodes can occur in a wide range of soil types, damage is frequently more severe on sandy soils.

Symptoms. Reduced vigor and stunting are common symptoms of stubby-root nematodes. Feeding damage by this nematode may result in “stubby” or stunted roots, often accompanied by brown lesions.

Management. Correcting other production limitations, such as infertile and hardpan soils, may curb the impact of stubby-root nematode infestations. Tobacco and rye are poor hosts for this nematode and may be a viable rotational crop where other pathogens and production factors allow. Several nematicides are labeled for management of stubby-root nematodes in soybean, but are often not economical. Fumigant treatments are not recommended because stubby-root nematodes frequently occur at depths below which fumigants are placed. Performing regular soil sampling will help to determine the level of risk that stubby-root nematodes pose to a soybean crop. No genetic resistance to stubby-root nematodes in soybean is known.

Additional information about stubby-root nematodes may be found in the NC State Extension factsheet, Stubby-Root Nematode of Soybean.

Sting Nematodes
(Belonolaimus spp.)

Sting nematodes can cause severe losses when present. This nematode has a low threshold, meaning that even when population counts are low, impacts can be substantial. Sting nematodes are generally limited in distribution to sandy and coarse-textured soils (80% or greater sand content), so soil sampling and monitoring of these soils are important. Host crops for this nematode include soybean, cotton, corn, peanut, small grains, and numerous vegetables.

Symptoms. Stunted plants and poor stands are common symptoms of sting nematodes. Feeding damage produces stubby or stunted roots with small black spots or lesions. Roots may try to outgrow the feeding damage, resulting in an overly branched root system.

Management. Correcting other production limitations, such as infertile and hardpan soils, may curb the impact of sting nematode infestations. Tobacco is a nonhost for this nematode and may be a viable rotational crop where other pathogens and production factors allow. Several nematicides are labeled for management of sting nematodes in soybean, but are often not economical. Performing regular soil sampling will help to determine the level of risk that sting nematodes pose to a soybean crop. No genetic resistance to sting nematodes in soybean is known.

Additional information about sting nematodes may be found in the NC State Extension factsheet, Sting Nematode of Soybean.

The goal of plant-parasitic nematode management is to reduce the population count below impactful levels. This is best achieved through an integrated pest management (IPM) approach, using multiple tools from the nematode management toolbox. Strategies include crop rotation, cultural practices, host resistance, and chemical nematicides. A nematode management plan should begin with a soil sample to determine what nematode genera are present and if their population counts are above threshold.

Decisions about nematode management options must often be made before planting, such as selecting a soybean variety with resistance to RKNs or SCNs, applying a seed treatment or in-furrow nematicide, selecting rotational crops, or developing a sanitation plan that reduces movement of potentially infected soil on the farm. During the season, monitor the soybean crop for potential symptoms of nematodes, and collect diagnostic soil and root samples as necessary.

Avoidance

For nematode management, prevention is better than cure. Preventing the introduction of plant-parasitic nematodes into a field may save untold costs in future management. Clean off any soil attached to farm equipment, truck tires, and shoes before moving to a new field or location. Working in known infected fields last can save time spent cleaning equipment. Nematodes do not infect soybean seed, but they can be moved through soil or plant-root debris if these are present with the seed. If rotating the field with vegetables, use certified disease-free transplants. Regularly sample fields to maintain a history of nematode infection.

Cultural Practices

Cultural practices for nematode management are actions that modify certain production practices or the environment in which the crop is grown to reduce the chance that the crop becomes infected, or to reduce the rate at which the nematode infection advances. The use of nonhost cover crops or fallowing can reduce nematode populations by withholding a suitable host crop. The host susceptibility of some rotational and cover crops to the plant-parasitic nematodes affecting soybeans is listed in Table 8-1.

Because nematodes can infect and reproduce on volunteer plants, it is important to control volunteers. A large number of volunteer plants can diminish the benefits of a rotation. In addition, many weeds can serve as alternative hosts or reservoirs for plant-parasitic nematodes; therefore, managing weeds can eliminate these reservoirs from the field.

¹Examples of host and nonhost crops are given for the southern root-knot nematode (M. incognita), which is the most common RKN species in North Carolina. Other RKN species—such as peanut root-knot nematode (M. arenaria), northern root-knot nematode (M. hapla), Javanese root-knot nematode (M. javanica), and guava root-knot nematode (M. enterolobii)—have different host and nonhost crops. Consult your local crop consultant, Extension agent, or additional nematode-management resources for more information. ↲

Host Resistance

Host resistance is the most effective, economical way to control nematode problems in soybean. Resistance to RKNs and SCNs is available in a range of maturity groups and varieties with different agronomic characteristics. More information on host resistance can be obtained from seed companies or the Official Variety Testing data from NC State.

Chemical Control

Nematicides are chemical compounds that are toxic to plant-parasitic nematodes and can reduce their populations. Nematicides may be either fumigant or nonfumigants (which include seed treatment formulations). Fumigants are applied to the soil as a gas or volatile liquid, and they move upward through the soil as a gas. Use of fumigant nematicides is often cost prohibitive in soybean. Nonfumigant nematicides are liquid or granular products that move downward through the soil, or they may be absorbed by the plant systemically. Fumigant nematicides and several nonfumigant formulations must be applied before the crop is planted; always refer to the product label for application timing and other instructions. Nematicides should be part of an IPM approach, meaning they should be used in combination with host resistance, rotation, and cultural practices, with the goal of reducing disease to levels below economic thresholds. Nematicide products for soybean are listed in Table 8-2. Additional information may be found in the North Carolina Agricultural Chemicals Manual.

Table 8-2. Chemical Nematicide Options for the Management of Nematodes in Soybean

¹Always refer to the product label for precautionary statements, application timing, rates, and other instructions. ↲

²Seed treatments may not be effective in high-pressure environments. To determine if seed treatments may be effective in managing nematode populations, take soil samples in the fall, when nematode populations are highest. ↲