Introduction

Several species of root-knot nematode (RKN; Meloidogyne spp.) can infect and reproduce on soybean in the U.S. The most common species are the southern RKN (M. incognita), peanut RKN (M. arenaria), Javanese RKN (M. javanica), northern RKN (M. hapla), and guava RKN (M. enterolobii, syn. M. mayaguensis). Of these species, the southern RKN is the most widespread and therefore causes the greatest total damage to U.S. soybean production. In general, the northern RKN is less damaging to soybean than the other species, which are capable of causing severe yield losses in individual fields.

Winter survival of the southern RKN occurs as far north as the 39°N latitude, but is more frequently found in the southern states. The peanut RKN is found in peanut-producing states (OK, TX, AL, GA, VA, NC, SC, and FL) and the Javanese RKN in TX and southeastern states (VA, NC, SC, GA, AL, and FL). The guava RKN has been found in the U.S. only recently and is only known to occur in SC, NC, and FL. The northern RKN is the most common species north of the 39°N latitude and is found in all soybean-producing states in the U.S.   

Post-harvest soil samples have proven to be reliable predictors of RKN induced symptoms and yield losses. The sooner samples are taken after harvest, the more reliable the prediction of damage since RKN will migrate out of the sample zone, the top 8 inches of soil, as soil temperatures drop. In sand to sandy loam soils, a threshold of 100 southern RKN per 100 cm³ soil is predicted to cause at least a 10% yield loss. The same level of yield loss can be observed with only 50 peanut RKN. These thresholds increase as percent sand content of the soil decreases. In a silt loam soil, the 10% threshold for southern RKN would be at least 150 per 100 cm³ soil, and for peanut RKN, it would be at least 75 per 100 cm³ soil. Thresholds for northern RKN and Javanese RKN would both be higher than for southern RKN. The threshold for guava RKN likely resembles the peanut RKN threshold.

Symptoms and Signs

Symptoms vary based on soybean susceptibility, nematode population density, RKN species, soil texture, and soybean growth stage. Plants may not exhibit foliar symptoms at low to moderate RKN population densities when grown in a silt loam soil, but express severe symptoms when grown in a sandy soil with the same nematode density.

Stunted seedlings may be observed in fields with high population densities, while stunted and chlorotic plants are common when soybeans are at mid- to late-reproductive growth stages (Figure 1). Symptoms on foliage of infected plants can range from slight chlorosis to interveinal chlorosis and necrosis, and can eventually progress to shriveled, dry, and dead leaves (Figures 2 and 3). Affected plants often senesce or die prematurely compared to non-infected soybean plants. Aboveground symptoms are most severe when additional fungal diseases are present such as southern blight (Athelia rolfsii), red crown rot (Calonectria ilicicola), Phytophthora root rot (Phytophthora sojae), or Fusarium root rot (Fusarium oxysporum), or during periods of drought stress.

Root galls are caused by nematode salivary secretions that result in excessive cell growth and division around the permanent feeding site established by a female RKN (Figure 4). Typically, root galls are visible 30 to 35 days after seedling emergence, but galls are small at early vegetative growth stages, often smaller than Rhizobium nodules (Figure 5). Root galls can be quite large once the plant has flowered, with some being larger than Rhizobium nodules. When multiple nematodes establish feedings sites, galling can be extensive, with multiple large galls on a single root. In general, galls caused by guava, peanut, and southern RKN are typically larger than those of northern RKN; however, the size and severity of galls are not a consistent nor highly reliable way to differentiate RKN species. As plants mature, galls induced by early RKN infections become necrotic because the females have died. As plants approach physiological maturity, infected roots are often discolored, may die, and become brittle (Figure 5).

Life (Disease) Cycle

Female root-knot nematodes are endoparasites and, upon establishing a feeding site in the root, are sedentary (Figure 6). Eggs are deposited in a gelatinous matrix (egg mass) which extends from the female just above the root surface. The first-stage juvenile (J1) develops inside an egg, molts, and hatches as a second-stage juvenile (J2). The J2 is the only stage of RKN capable of infecting a host. The J2 moves through the soil in water films associated with soil particles and then penetrates host roots close to the root tip. The J2 typically moves through the root to the vascular system, where it establishes a feeding site. The feeding site is induced by nematode secreted enzymes, which cause root cells to become multinucleated and are referred to as giant cells. The resulting multinucleated cells are the permanent feeding site for the now sedentary female nematode. Excessive cell growth and division in response to nematode feeding leads to the formation of the typical gall, which is root tissue surrounding the swollen female RKN. The presence of multiple enlarged females and developing galls near the root vascular system restricts nutrient and water uptake by the plant, contributing to aboveground symptoms. Once feeding has been initiated, the J2 enlarges and molts three times to become an adult. Males, of some species, will develop into long, vermiform (worm-like) nematodes. Mating may or may not be required for all species, but after mating, males leave the root to die. Females continue to enlarge to a pear-like shape within the root. Each female has the potential to produce several hundred eggs deposited into an egg mass. In resistant soybean varieties, RKN are unable to initiate the creation of giant cells and the nematode starves to death before being capable of reproducing or egg-laying.

At the optimum soil temperatures from 77-86°F, a complete life cycle (freshly hatched J2 to egg deposition) can occur within 20 days, with multiple life cycles occurring during a single cropping season. Between cropping seasons, 85 to 90% of the hatched J2 die, but those that survive or over-wintered as eggs and hatch then infect soybean seedlings in the spring. Infection of soybean roots and hatching of J2 in the spring occurs once soil temperatures exceed 65°F.

Conditions that Favor Disease

Southern RKN is most commonly observed in coarse textured soils (sand, sandy loam, and loamy sand soils), which allow for better nematode motility. Coarse textured soils also predispose plants to early season drought stress, which is exacerbated by the vascular blockage caused by RKN galls. Damage thresholds are a sliding scale of nematode density, soil texture, and rainfall. Root-knot nematodes can be present in medium (e.g., loam) and medium-fine (e.g., silt loam) textured soils; however, their impact on soybean growth and development is less than that of the same nematode population density in a sandy soil. Very susceptible soybean cultivars may express foliar symptoms such as interveinal necrosis and galling on roots in these soils. High initial nematode populations can cause significant symptoms on moderately resistant and resistant soybean cultivars, especially if grown in coarse textured soils. Plants grown in compacted soils that result in stunting due to reduced root growth and simultaneous infection by southern RKN often suffer from increased southern RKN impacts and subsequent yield losses.

In many fields, RKN are not the only plant parasitic nematodes present. They can occur in combination with soybean cyst nematode, lance nematodes, lesion nematodes, reniform, and stubby root nematodes. When a RKN resistant soybean variety is planted, one of the other nematode species may still cause significant damage and yield loss.

Yield Losses and Impact

Yield losses can range from 10% to more than 50% when susceptible soybean cultivars are grown in fields with high densities of southern RKN. Severely infected plants produce fewer pods and smaller-sized grain. When grown in rotation with susceptible hosts such as corn or cotton, the population density of southern RKN can increase to damaging levels for the subsequent soybean crop. Root-knot nematodes consistently cause significant annual yield losses of soybean, especially in the Southern U.S.

Further data on yield and economic loss due to RKN is available on the Field Crop Disease and Insect Loss Calculator from the Crop Protection Network.

Diagnosis

Plants exhibiting foliar symptoms such as interveinal necrosis at mid-season typically have extensive root galling. In many cases, what appear to be individual galls are clusters of galls cause by numerous female nematodes (5 to 10) in the same location. Galls are easily distinguishable from Rhizobium nodules. Rhizobium nodules can be rubbed off the root surface, whereas a gall cannot. However, galled roots, especially secondary roots, can break where galls are present when uprooted by hand; therefore, uprooting plants with the aid of a shovel is preferred. It is important to note that although the presence of galls can confirm infection by RKN, it does not eliminate the possibility of other damaging nematodes being present.

Soil samples should be collected near stunted plants or edges of a “bad spot” in the field. Collect soil from the root zone of living, symptomatic plants to a general depth of 8 to 10 inches (20 to 25 cm) as the nematode population densities will be greater than in soil collected surrounding plants that have been dead for several weeks. 

These methods are useful in identifying a RKN issue, but identifying the species of RKN present requires additional testing, such as molecular assays. Furthermore, only a set of differential host plants can aid in the identification of specific races or biotypes within a RKN species. There are four races of southern RKN and all can reproduce on soybean. There are two races of the peanut RKN: race 1 primarily infects peanut causing severe damage but can also infect soybean; race 2 can infect and severely damage soybean, but not peanut. Neither race of peanut RKN can infect cotton. Cotton is an excellent rotation crop to control peanut RKN, but not southern RKN.

Diseases, Disorders, and Injury with Similar Symptoms: Diseases

Management

Avoid growing RKN susceptible crops and varieties more than two consecutive years. A soybean-corn or cotton-soybean rotation is not much different than three consecutive years of soybean in terms of nematode population development.

Rotation with non-nematode host crops is limited to peanut with southern RKN. Grain sorghum hybrids vary in susceptibility, but are considered susceptible hosts along with corn and cotton. The host range of other species of RKN overlap with southern RKN with some variation among species (Table 1).   

Table 1. Host range of most common and emerging root-knot nematodes in soybean producing states in the United States. (+ = susceptible [i.e., good reproduction], - = resistant [i.e., poor reproduction])

Soybean resistance to the southern RKN is available, albeit in a limited number of varieties in early maturity groups (MG III-IV), compared to later maturity groups (MG V-VII), where many resistant varieties are available. Resistance is also limited among soybean herbicide-tolerant trait technologies. Resistance to one species of RKN does not guarantee resistance to other RKN species or other plant-parasitic nematodes that infect soybean, such as SCN and reniform nematode. Resistance to southern RKN does not confer resistance to peanut RKN nor is the inverse true. Furthermore, resistance to southern RKN or peanut RKN does not confer resistance to guava RKN. Some of the new southern RKN resistant cotton varieties do an excellent job of reducing southern RKN reproduction and residual populations at the end of a growing season and could be used as a rotation crop. 

An additional important management practice to consider on an annual basis is the timely management of weeds. Many weeds are hosts for various species of RKN, including southern RKN. Some weed hosts may support reproduction even though galling may not be evident. Therefore, weedy fallow fields may allow for survival or even the buildup of RKN populations if specific weed hosts are present.

Seed-applied nematicides provide some early season root protection from southern RKN, but there is limited nematicide distribution from the seed coat throughout the developing root system (see feature article). Seed-applied nematicides are often recommended in field situations where low nematode population densities are present. However, seed-applied nematicides should be paired with host plant resistance when moderate to high population densities are present. In this situation, the seed-applied nematicide may help control other nematode species present in the field, such as SCN, lance, or reniform nematodes.   

Fall soil samples collected after harvest (up to 6 weeks after the R8 growth stage) can be useful to monitor changes in RKN population densities or to make management decisions for the subsequent cropping season. Soil samples should be collected at an 8 to 10-inch (20 to 25-cm) soil depth within the root zone of the previously existing crop. Use the same nematode diagnostic laboratory for soil sample assays to monitor changes in nematode counts if used for midseason samples to diagnose a nematode problem. The southern RKN damage threshold is one individual per cm³ of soil (or 473/pt), but it is even lower in sandy soils.

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Acknowledgments

Authors

Travis R. Faske, University of Arkansas; John Mueller, Clemson University; and Lindsey Thiessen, North Carolina State University.

Reviewers

Tom Allen, Mississippi State University; Emmanuel Byamukama, South Dakota State University; Gary Bergstrom, Cornell University; Carl Bradley, University of Kentucky; Martin Chilvers, Michigan State University; Alyssa Collins, Pennsylvania State University; Nicholas Dufault, University of Florida; Adrienne Gorny, North Carolina State University; Alyssa Koehler, University of Delaware; David Langston, Virginia Tech; Dean Malvick, University of Minnesota; Daren Mueller, Iowa State University; Rodrigo Onofre, Kansas State University; Paul “Trey” Price III, Louisiana State University; Marisol Quintanilla, Michigan State University; Edward Sikora, Auburn University; Darcy Telenko, Purdue University; Albert Tenuta, Ontario Ministry of Agriculture, Food, and Rural Affairs; Nathan Walker, Oklahoma State University; and Tessie Wilkerson, Mississippi State University.

Sponsors

The authors thank the United Soybean Board and the United States Department of Agriculture - National Institute of Food and Agriculture for their support, as well as the Grain Farmers of Ontario who obtained partial funding through the Canadian Agricultural Partnership (CAP), a federal-provincial territorial initiative.