An Overview of Root-Knot Nematodes

Updated in 2026, this version replaces the 2021 An Overview of Root-Knot Nematodes.

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), and northern RKN (M. hapla). The guava RKN (M. enterolobii, syn. M. mayaguensis) is an invasive species present in some parts of the southeastern U.S. 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 it is more frequently found in the southern states. A map summarizing the distribution of the southern RKN in the contiguous U.S. is available. The peanut RKN is detected in several row crop fields in peanut-producing states (AL, FL, GA, NC, OK, SC, and TX), and the Javanese RKN is detected in TX and southeastern states (AL, FL, GA, NC, SC, and VA). The guava RKN was detected in the U.S. in 2002 and is only known to occur in AL, FL, GA, NC, SC, and TX. The northern RKN is the most common species north of 39°N latitude and is found in all U.S. soybean-producing states and Canada.   

Post-harvest soil samples have proven reliable predictors of RKN-induced symptoms and yield losses on the subsequent crop. The sooner they 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 percent yield loss. The same level of yield loss can be observed with only 50 peanut RKN. These thresholds increase as the percent sand content of the soil decreases. In a silt loam soil, the 10 percent 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 is likely to resemble that for peanut RKN.

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 RKN population densities, while stunted and chlorotic plants are common when soybean plants 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 in conjunction with the nematode such as southern blight (Agroathelia 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-secreted enzymes 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 (beyond R1), with some being larger than Rhizobium nodules. When multiple nematodes establish feeding 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, size and severity of galls are not a consistent or highly reliable way to differentiate RKN species. As plants mature, galls induced by early nematode infections become necrotic because the female RKN have died. As plants approach physiological maturity, infected roots are often discolored, may die and become brittle (Figure 5).

Life (Disease) Cycle

Female RKN are endoparasites (parasites internal to the plant) and upon establishing a feeding site in the root are sedentary (do not move) (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 life 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 lead to the formation of a 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, which contributes 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 can 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 optimal soil temperatures, generally from 77 to 86°F (approximately 25°C to 30°C), 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 percent of the hatched J2s die, but those that survive or overwinter as eggs and hatch then infect soybean seedlings in the spring. Infection of soybean roots and hatching of J2 in the spring occur once soil temperatures exceed 65°F (18°C).

Conditions that Favor Disease

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 then exacerbated by the vascular blockage caused by RKN galls. Damage thresholds are a sliding scale of nematode density, soil texture, and rainfall. RKN 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 exhibit foliar symptoms such as interveinal necrosis and root galling 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 RKN, often experience greater 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 nematodes, lance nematodes, lesion nematodes, reniform, and stubby root nematodes. When a RKN-resistant variety is planted, other species of nematodes present may cause significant damage and yield loss.

Yield Losses and Impact

Yield losses can range from 10 percent to more than 50 percent when susceptible soybean cultivars are grown in fields with high densities of southern RKN. Severely infected plants produce fewer pods and smaller grain. When grown in rotation with susceptible hosts such as corn or many cotton varieties, 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.

Additional data on yield and economic losses from RKN are 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 (R4) typically have extensive root galling. In many cases, what appear to be individual galls are clusters of galls caused 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. In addition, when cutting into nodules the inside of an active nodule will be pink and the nodule itself soft. However, galled roots, especially secondary roots, can break where galls are present when uprooted by hand; therefore, uprooting plants with 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.

To diagnose a nematode issue, soil samples should be collected near stunted plants or on the edges of a “bad spot” in the field. Collect soil from the root zone of living, symptomatic plants to a depth of 8 to 10 inches (20 to 25 cm), as the nematode population densities will be greater than in soil collected from 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 (DNA assays). Furthermore, only a set of differential host plants can aid in identifying 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, and causes 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. Moreover, cotton is an excellent rotation crop for controlling peanut RKN, but not southern RKN. 

Diseases, Disorders and Injury with Similar Symptoms

Diseases

Soybean Cyst Nematode (SCN; Heterodera glycines)

Both RKN and soybean cyst nematode (SCN) are problematic on susceptible cultivars.

How to distinguish SCN from RKN: Living SCN females are white, while dead females are yellow to tan or brown. When viewed using a good quality hand lens (>10 x), an SCN can be seen outside the root, whereas a RKN is primarily within root (gall) tissues and not observable.

Taproot decline (Xylaria necrophora)

Taproot decline (TRD) has been determined to be common in southern U.S. soybean producing regions. However, the range of the disease throughout this region outside of AL, AR, LA, MO, MS, and TN is yet to be determined. TRD symptoms consist of interveinal chlorosis and necrosis on leaves throughout the canopy and are typically distributed sporadically in a field (Figure 8).

 How to distinguish TRD from RKN: TRD-affected plants typically have black fungal growth (stroma) that can be observed on the main taproot or lateral roots at any plant developmental growth stage (Figure 9). However, the stroma is most readily observed on plants at more advanced growth stages (>R6).

Southern blight (Agroathelia rolfsii, syn: Sclerotium rolfsii)

Southern blight is a common soilborne disease in southern soybean-producing states. Affected plants often have yellow leaves with mild interveinal chlorosis beyond R4 in the upper canopy that ultimately shrivel and die, but stay attached to wilted, dead plants.

How to distinguish Southern blight from RKN: Southern blight affected plants will typically have a white, fanlike growth of coarse fungal mycelia on the base of plant stems and detached leaves present on the soil surface (Figure 10). Numerous small, round cream-colored to orange sclerotia (1/8 inch [or 2-4 mm]) can be present on the fungal mats, but are often more abundant on dead plants. These two diseases, the root-knot nematode and southern blight, are often observed occurring on the same plants.

Sudden Death Syndrome (Fusarium virguliforme)

Sudden death syndrome (SDS) typically occurs in wet, compacted soils. Leaf symptoms range from yellow spots to interveinal yellowing and necrosis (Figure 11). Symptoms are often observed after the flowering growth stages (> R1).

How to distinguish SDS from RKN: On SDS-infected plants, leaves shrivel and drop off but petioles remain attached (Figure 12). Blue fungal growth may be seen on the root surface of symptomatic plants. Galls will not be present on roots of SDS-affected plants.

Stem Canker (Diaporthe spp.)

Southern stem canker is more common in continuous soybean production and in no-till regimes. Affected plants have leaves with severe interveinal chlorosis and necrosis, but the midveins remain green on plants (Figure 13). Symptoms are often observed after beginning pod development (R3). 

How to distinguish stem canker from RKN: Stem canker affected plants have a characteristic reddish to brown lesion along the length of the stem starting at the soil line and will not have galling on roots (Figure 14).

Red crown rot (Calonectria ilicicola)

Red crown rot (RCR) occurs mainly in the southeastern U.S., especially in fields rotated with peanut, but recently has expanded to soybean in the Midwest. It occurs across a range of soil textures and is favored by high moisture levels. Symptomatic leaves are often observed first in the upper canopy. Symptoms can include yellow spots between veins that become chlorotic, while midveins and major lateral veins remain green (Figure 15). These leaves and petioles eventually drop off the plant.

How to distinguish RCR from RKN: RCR affected plants have a reddish discoloration to roots and lower stem. Reddish-orange reproductive structures (perithecia) are visible on the lower stem surface near the soil line, especially on plants that have died prematurely due to the disease (Figure 16). Perithecia can be easily overlooked when conditions are dry and are not as visible when conditions are wet.

Management

Avoid growing RKN susceptible crops and varieties for more than two consecutive years. A soybean-corn or RKN susceptible 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 and RKN-resistant cotton with southern RKN. Grain sorghum hybrids vary in susceptibility, but are considered susceptible hosts along with corn and cotton. The host ranges 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 U.S. (+ = 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 timely management of weeds. Many weeds and some cover crops are hosts for various species of RKN including southern RKN. Some weed hosts may support reproduction even though galling may not be evident. RKN densities can increase on weeds or cover crops once temperatures are above 65°F for 30 days. Therefore, weedy fallow fields may allow for survival or even the buildup of RKN populations if specific weed hosts are present.

Seed- and soil-applied nematicides provide some early-season root protection against the southern RKN. However, there is limited redistribution of the nematicide from the application point or seed coat throughout the developing root system (see How seed-applied nematicides work). In general, in-furrow or soil-applied nematicides offer better root protection than seed treatment nematicides, although neither provides season-long control or consistent grain yield protection. The in-furrow nematicide, aldicarb (AgLogic 15GG) is registered for use in soybean only in a few states, so it is important to consult product labels for state-specific approvals. Seed treatment nematicides are suggested in fields with low nematode pressure, while in-furrow nematicides are suggested for fields with low to moderate nematode densities. In cases of moderate to high population densities, both seed- and soil-applied nematicides should be paired with host plant resistance. In these situations, seed treatment nematicides may also help suppress other nematode species, 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 (i.e., predictive sampling). 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 mid-season 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.

Acknowledgements

Authors

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

Reviewers

Tom Allen, Mississippi State University; Alyssa Betts, University of Delaware; Mandy Bish, University of Missouri; Horacio D. Lopez-Nicora, The Ohio State University; Daren Mueller, Iowa State University; Trey Price, LSU AgCenter; Darcy Telenko, Purdue University; Richard Wade Webster, North Dakota 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.

How to cite: Faske, T. R., Mueller, J., Gorny, A., 2026. An Overview of Root-Knot Nematodes. Crop Protection Network. CPN-1027 (revision). doi.org/10.31274/cpn-20210820-2.