An Overview of Reniform Nematode
Published: 11/13/2024
DOI: doi.org/10.31274/cpn-20241118-0
CPN-7002
The reniform nematode, Rotylenchulus reniformis, occurs primarily in the tropics and subtropics worldwide where it parasitizes a wide range of field crops (e.g., cotton, cowpea, soybean, sweetpotato, tobacco,). In the southern U.S., it is an important pathogen in cotton and soybean, with yield loss estimates being greater in cotton than soybean. This overview will focus primarily on cotton but includes information relevant to soybean production.
Winter survival occurs as far north as the 36°N latitude (southern MO and VA), but it is more frequently recovered in southern states (AL, AR, FL, GA, LA, MS, NC, SC, TN, and TX) with greater population densities in areas with a history of cotton production. A map that summarizes the distribution of the reniform nematode in the contiguous U.S. is available here.
The reniform nematode is easy to introduce into new fields because of its unique ability to survive in a dehydrated state in dry soils. Therefore, it can be transported long distances on field equipment. There have been reports of the reniform nematode being introduced into new fields in AL, AR, MS, and TX on infested implements or harvesting equipment from areas where the reniform nematode is known to occur.
Symptoms and Signs
Symptoms vary based on crop susceptibility, nematode population density, soil texture, and length of time a field has been infested. Infected plants may appear stunted, exhibit delayed flowering, produce fewer fruits, and have smaller fruit sizes, ultimately leading to reduced lint or pod yield (Figure 1). Unlike the southern root-knot nematode (Meloidogyne incognita), the reniform nematode does not cause galls on host roots, which makes diagnosis difficult based on symptoms alone. Instead, reniform-infected roots have a dirty appearance due to the presence of soil particles adhering to the nematode gelatinous egg masses on the root surface. Foliar symptoms are uncommon in most cotton-producing states; however, in AL, GA, and FL, chlorotic leaves, often in an interveinal pattern, referred to as “tiger striping” can be observed in the field (Figure 2). The “tiger striping” is a nutrient deficiency symptom, but can occur when reniform nematode densities are high, even when soil fertility is adequate. Notably, “tiger striping” is also strongly associated with infection by root-knot nematodes. Thus, soil samples for a nematode assay may be needed to accurately diagnose the cause of these symptoms. Stunted plants are more commonly observed after the initial introduction of the nematode into a field (Figure 1). Over time, a wavy pattern in plant height may change with different population densities in the field (Figure 3). Eventually, the distribution becomes more uniform making it more difficult to recognize in the field based on plant height or foliar symptoms.
Figure 1. Stunted cotton plants due to a high population density of the reniform nematode.
Travis Faske, University of Arkansas
Figure 2. Interveinal chlorotic pattern or “tiger stripping” on cotton leaves is a nutrient deficiency symptom that can be observed in fields infested with the reniform nematode, especially in Alabama, Georgia, and Florida
Zane Grabau, University of Florida
Figure 3. Hot spots of stunted cotton in a field with a moderate to high density of the reniform nematode.
Travis Faske, University of Arkansas
Disease Cycle (Life Cycle)
The reniform nematode overwinters as eggs and various vermiform (worm-like) life stages, but primarily as males and females in the soil. In the spring, the vermiform female infects the host by penetrating the root to establish a permanent feeding site called a syncytium (a curved sheet of numerous cells within a single cell wall) and become sedentary, remaining in one place. Only a portion (about one-third) of the female’s body enters the root, while the rear remains exposed on the root surface (Figure 4). Thus, the reniform nematode is a semi-endoparasitic nematode.
Males mate with females and return to the soil. The females enlarge into a swollen bean- or kidney-shape, hence the name reniform. Each female can lay about 50 to 75 eggs that are deposited into a gelatinous matrix (egg mass), which surrounds the portion of the female exposed on the root surface. The first-stage juvenile (J1) develops inside the egg and hatches as a second-stage juvenile (J2). The J2 molts three more times in the soil without feeding, eventually developing into either vermiform male or female life stages.
Figure 4. A reniform nematode female attached to a cotton root (50x).
Travis Faske, University of Arkansas
A complete life cycle (hatched J2 to egg deposition) can occur within 17 to 23 days at optimum soil temperatures, from 81 to 86°F (27 to 30°C), with multiple life cycles occurring in a single cropping season. In contrast, there is minimal nematode reproduction at temperatures below 59°F (15°C) and above 97°F (36°C). Between cropping seasons, the densities of the reniform nematode do not decline as dramatically as root-knot nematodes, so populations from fall to spring can remain at a similar density.
Favorable Conditions and Yield Losses
The reniform nematode can be found in a wide range of soil textures, from fine (e.g., clay) to coarse (e.g., sand). The greatest densities are often in medium-fine (e.g., silt loam) textured soils. Yield losses are often greater in coarse textured soils when plants are stressed by drought and the reniform nematode; however, with the reniform nematode yield losses are more dependent on nematode densities than soil textures. Post-harvest soil samples can be reliable predictors of reniform yield losses. A threshold of 100 to 1,000 per 100 cm3 (or 473 to 4,730/pint) of soil for cotton and 1,000 to 5,000 per 100 cm3 (or 4,730 to 23,650/pint) of soil for soybean has been associated with yield losses of 10 percent or greater in coarse and medium-fine textured soils.
Yield losses can range from 10 to 80 percent when susceptible cotton cultivars are grown in fields with high densities of the reniform nematode. In drier regions of Texas, yield losses are often greater and range between 50 to 100 percent on susceptible cultivars, whereas in areas with more frequent rainfall such as Alabama, yield losses average 50 percent between fields with and without the reniform nematode. Based on nematicide field studies in the Mid-southern U.S., 15 to 30 percent is more common. In soybean, yield losses of more than 50 percent have been documented in greenhouse trials, but only 5 to 10 percent based on nematicide field studies. The reniform nematode can increase the severity of Fusarium wilt (caused by Fusarium oxysporum f.sp. vasinfectum) on wilt-susceptible cotton cultivars but not on wilt-resistant cultivars.
Diagnosis
It is difficult to diagnose a nematode issue in the field based on aboveground symptoms. Root symptoms can be helpful, but only for a few nematodes, like root-knot nematodes and the soybean cyst nematode. Using a hand lens, small clumps of dirt can be seen on the surface of the affected roots (i.e., “dirty roots”). These clumps consist of soil attached to the egg masses of the female reniform nematode. However, soil samples are the most reliable method for diagnosis and making management decisions. To determine which nematode species may be contributing to plant stunting, diagnostic soil samples should be collected from both non-symptomatic (healthy) and symptomatic (stunted) plants. Collect soil from the root zone to a depth of 8 to 10 inches (20 to 25 cm). To prevent cross-contamination, sample non-symptomatic plants first.
Diseases, Disorders, and Injury with Similar Symptoms
Southern root-knot nematode (Meloidogyne incognita)
The southern root-knot nematode is the most widespread and economically important nematode affecting cotton in the U.S. Infected plants may appear stunted, with discolored leaves that are associated with nutrient deficiencies.
How to distinguish reniform nematode from southern root-knot nematode
The southern root-knot nematode causes spindle-shaped to rounded galls on the infected cotton root systems (Figure 5). To examine galls, dig roots with a shovel rather than pulling as galls can detach and remain in the soil.
Figure 5. Spindle shaped galls on cotton roots caused by the southern root-knot nematode.
Travis Faske, University of Arkansas
Columbia lance nematode (Hoplolaimus columbus)
The Columbia lance nematode is an important yield-limiting cotton nematode that is widespread in GA, NC, and SC. Infected cotton plants may appear stunted with slight chlorosis with more severe stunting when nematode densities are high (Figure 6).
How to distinguish reniform nematode from Columbia lance nematode
Columbia lance nematode damage to roots when densities are moderate result in a high incidence of root forking due to damage to the growing points (Figure 7). Soil and root samples processed by a nematode diagnostic lab are the best way to confirm which nematode species is present.
Figure 6. Stunted cotton due to Columbia lance nematode.
John Mueller, Clemson University
Figure 7. Forked cotton root system due to Columbia lance nematode.
John Mueller, Clemson University
Sting nematode (Hoplolaimus columbus)
The sting nematode, Beloloniamus longicaudatus, can cause significant stunting to cotton at very low population densities (Figure 8).
How to distinguish the reniform nematode from sting nematode
The sting nematode uses its long stylet to pierce emerging roots resulting in severely stunted root growth. Excessive root damage often kills cotton seedlings. Sting nematode is limited to very sandy soils whereas reniform nematode is adapted to a wide range of soil textures. Sting is a rather large nematode and is limited to soil with a minimum of 80 percent sand. Soil samples processed by a nematode diagnostic lab are the best way to confirm which nematode species is present.
Figure 8. Stunted cotton plants due to the sting nematode.
Lindsey Thiessen
Management
Resistance
A rotation sequence with a non-nematode host crop or a resistant cultivar for two consecutive years can be beneficial in reducing reniform nematode densities. Non-reniform nematode host crops include corn, grain sorghum, rice, or peanut. However, all these crops, except peanut, are a host to the southern root-knot nematode (M. incognita), which can be found in the same fields as the reniform nematode. Over time the reniform nematode will become the dominant species in continous cotton production or in rotation with a poor host against the southern root-knot nematode. Use of a weed-free fallow period can also reduce reniform nematode density, but is generally less desirable than a non-nematode host crop.
Reniform nematode-resistance is available in cotton, albeit in a limited number of cultivars and further limited among tolerance traits to herbicides (Figure 9). Currently, cotton cultivars with reniform nematode resistance are also resistant to the southern root-knot nematode; however, the reverse is not always true. Although resistance is present in some soybean cultivars the resistance rating (e.g., resistant, moderately resistant) is often unknown. Furthermore, resistance to the reniform nematode in soybean does not confer resistance to other soybean nematodes such as root-knot nematodes and the soybean cyst nematode.
Figure 9. Healthy, reniform-resistant cotton cultivar (left) compared a stunted, reniform-susceptible cultivar (right) in a field with a high reniform nematode population density.
Zane Grabau, University of Florida
An important annual management practice is the timely management of weeds, regardless of whether a resistant or non-nematode host crop is used to manage the reniform nematode. The reniform nematode has a wide host range including many weeds found in cotton, soybean, peanut, and corn fields. Generally, grasses are poor hosts while many broadleaf weeds vary in host suitability from poor to good. Consequently, weeds can support the survival or even increase of reniform populations even when a non-nematode host or resistant crop is used in a rotation sequence.
Nematicides
Nonfumigant nematicides applied in-furrow or as a seed treatment provide some early season root protection against the reniform nematode. 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 [CPN-4006]). In general, in-furrow nematicides offer better yield protection than seed treatment nematicides, although neither last the entire season, and both rarely reduce final nematode densities on a susceptible host. 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 application methods should be paired with host plant resistance. In these situations, seed treatment nematicides may also help suppress other nematode species, such as southern root-knot, lance, or SCN. Preplant fumigant nematicides, such as 1,3-dichloropropene (Telone II), provide enhanced root protection, particularly when reniform nematode densities are high (Figure 10). However, the high cost and need for specialized application equipment limit the use of fumigants in cotton production in the U.S.
Figure 10. Stunted cotton plants (center picture and to right) caused by the reniform nematode compared to Telone II (1,3 dichloropropene) a fumigant nematicide (far-left).
Tristan Watson, Louisiana State University
Cultural Practices
Cultural practices that can reduce the impact and reproduction of reniform nematodes are limited. Adequate fertility and irrigation can be beneficial in reducing nematode-induced stress. Tillage practices that allow for deep root development can enable plants to recover from drought and nematode-induced stress better than those grown in compacted soils. Tillage to adequately kill cotton plants after harvest can eliminate cotton regrowth (Figure 11) and subsequent support for nematode reproduction beyond harvest.
Figure 11. Cotton leaf regrowth several weeks after defoliating and recently harvested. Cotton nematodes can continue to reproduce on an actively growing host until a freeze or tillage terminates the crop.
Travis Faske, University of Arkansas
Future Planning
Predictive soil samples (post-harvest or fall soil samples) collected after harvest (up to 6-wk after harvest) or after the last irrigation event in drier regions can be useful to monitor changes in reniform nematode population densities or to make management decisions for the subsequent cropping season. These 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 seasonal changes in nematode densities. The fall damage threshold densities for the reniform nematode varies somewhat between states but levels from 100 to 1,000 per 100 cm3 (or 473 to 4,730/pint) of soil for cotton and 1,000 to 5,000 per 100 cm3 (or 4,730 to 23,650/pint) has been associated with a 10 percent or greater yield loss. The volume of soil varies among nematode diagnostic labs across the U.S. Therefore, it is important that the volume of soil reported from the diagnostic lab is equal to those of the nematode damage threshold density.
Acknowledgements
Authors
Travis Faske, University of Arkansas; Tristan Watson, Louisiana State University; Terry Wheeler, Texas A&M University, and Zane Grabau, University of Florida.
Reviewers
Tom Allen, Mississippi State University; Gary Bergstrom, Cornell University; Adrienne Gorny, North Carolina State University; Heather Kelly, University of Tennessee; Maira Duffeck, Oklahoma State University; Ian Small, University of Florida; Damon Smith, University of Wisconsin-Madison; Darcy Telenko, Purdue University; John Mueller, Clemson University; Kathy Lawrence, Auburn University; Horacio Lopez-Nicora, The Ohio State University; and Mandy Bish, University of Missouri.
Sponsors
The authors thank Cotton Incorporated for their support on this project.
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