Perspective Chapter: Integrated Root-Knot Nematodes (Meloidogyne) Management Approaches

Abstract

Meloidogyne genus contains the most prevalent and harmful worms formally known as root-knot nematode species. They attack a wide range of plants belonging to different plant families. The infective second stage juveniles (J-II) feed on the roots and as a result, the host plant roots become swollen/produce galls. The attack plant shows stunted growth and in extreme cases, the death of the plant occurs. An integrated pest management (IPM) approach is required to tackle these harmful nematodes spp. The integrated tactics include cultural/agronomic practices, biological and chemical control. A sole management method is not enough to deal with the root-knot nematode. Therefore, a proper IPM package is required for the farmer to gain good health for the crops.

1. Introduction

Root knot nematodes (RKN) are sedentary internal plant parasites and belong to the genus Meloidogyne. The word Meloidogyne is originating from Greek that means a cup-shaped female of RKN (Figure 1). They cause huge economic losses due to diverse host range and adaptation to vast climatic conditions. Reverend Miles Joseph Berkeley (clergyman) for the first time discovered galls on the cucumber roots in 1855 [1]. The finest work of Chitwood even remains accurate until now; he classified Meloidogyne from Heterodera. That is why the current name of Chitwood is used as intermingled for RKN [2]. Most of the species are pathogenic that may reproduce sexually mainly but in certain cases, they may reproduce through asexual means (facultative pathogenesis) [3]. The worm-like males are short-lived and die after matting with a cup-shaped female (Figure 1) that is long-lived and penetrates the root tissues to lay about 500 eggs in a sheet formally known as an egg sac (gelatinous sheet). The amateur stages are juvenile (J) I, II, III, and IV. The first two (I and II) are worm-like and only J-II (Figure 2) actively feed and move [4]. The biology of RKN is given in Figure 3.

Figure 1.

Adult female of root-knot nematode (microscope view).

Figure 2.

Juvenile-II female of root-knot nematode (microscope view).

Figure 3.

Lifecycle of root-knot nematode.

The RKN lacks any rigid skeletal form and thus utilizes the turgor-pressure (TP) for sustaining the bodily shape and locomotion [5]. They possess tiny stylet-like insects that are injected into the plant roots for taking nutrients. The adult female releases secretary proteins that induce the captured cells and cells to become multi-nucleated (with no cell wall formation). This process release protein that is ingested by the RKN through a feeding tube that filters the sap from plant roots. Because of this feeding behavior and cell divisions, the neighboring cells also grow bigger and causing swelling in the roots that ultimately leads to gall formation (Figure 4) in the roots [6].

Figure 4.

Galls on infected roots of root-knot nematode.

2. Diversity of root-knot nematode

RKN are among the most successful parasites because of the huge range of hosts and flexible behavior in adapting to a variety of environmental conditions [7]. The Meloidogyne genus has almost 100 species [8]. However, the four species are of prime significance including Meloidogyne javanica, Meloidogyne arenaria, Meloidogyne incognita, and M. hapla) [9].

3. Damages of root-knot nematode

RKN poses a severe danger to the quantity and quality of numerous economic crops around the world. Only the top 20 life-sustaining crops are predicted to suffer an annual crop loss of 12.6% (equivalent to $215.77 billion) due to these worms [10].

The RKN dislodges the vascular system of the host plant and the attacked plant tends to exhibit stunted growth and, death of the seedlings. The RKN infection leaves the plant vulnerable to the attack of other pathogens. The yield declined drastically and in certain cases, the losses may reach up to 90–100% if no management practices are initiated [4].

4. Host range of root-knot nematode

RKN attack a diverse range of plants belonging to different families. The host range is surpassing 5500 plant species. They attack shrubs, trees, ornamental plants, vegetables, and field crops [11].

5. The microbiome of RKN in soil with roots and other microorganisms

The bacterial genera, Bacillus micrococcus, Sphingomonas, Rhizobium, Methylobacterium, and Bosea exhibit a dominance against J-II of Meloidogyne hapla. The fungal genera Davidiella, Rhizophydium, Plectosphaerella, Lectera, Gibellulopsis, and Malassezia suppress J-II of Meloidogyne incognita. According to Topalovic et al. [12] that in soil, Bacillus thuringiensis and Plectosphaerella cucumerina were shown to be associated with Megalaima incognita J-II. B. thuringiensis produces proteinaceous protoxin crystals {also known as crystal protein or cry protein (CP)} that cause intestinal lysis that leads to nematode death.

Plants have devised a mechanism in which hostile microorganisms in the rhizosphere are selectively stimulated and enriched [13]. The ability of plants to recruit antagonists under various soil management approaches will improve the foundation for new profitable and sustainable microbiome-based crop production systems [14]. Most horticulture growers use harmful chemicals to combat soil-borne infections, whereas organic farmers use conservative practices that preserve soil biodiversity and encourage the RKN antagonistic microbiota. Researchers recently discovered differences in the rhizosphere microbiome under various crop techniques, including impressively low levels of plant pathogens under long-term organic-farming (OF) [15]. Furthermore, microbial shifts in rhizospheres after RKN inoculation suggest that soil can be managed to attract beneficial microorganisms. RKN performance on plant roots should be reduced by the recruitment of microorganisms.

6. Morphometric markers identification of root-knot nematode

Since proper identification of Meloidogyne spp. is critical for crop management, a more precise procedure of identification is required. To address this issue, a biochemical procedure of identification was quickly created as a supplement to the morphological way of identification [16]. body width, Body length, stylet length, anal body width, head end to excretory pore, dorsal gland opening, head end to metacorpus valve, esophagus length, hyaline tail length, and tail length are some of the most common morphometric markers used to identify nematodes [17]. Eisenback [18] developed a perennial pattern, head anatomy, and stylet of females to identify Meloidogyne hapla, M. javanica, Megalaima incognita, and M. arenaria by using light microscopy (LM) and scanning electron microscopy (SEM). Eisenback [18] distinguished M. hapla, M. javanica, M. incognita, and M. arenaria based on stylet morphology and head shape of the males using LM and SEM; produced a visual key based on morphological traits to distinguish M. hapla, M. javanica, M. incognita, and M. arenaria. Eisenback [19] successfully distinguished J-II of Myrmecina graminicola, M. nassi, and M. javanica using tail morphology. Furthermore, Eisenback [19] used more comprehensive morphology to identify numerous Meloidogyne spp. Identification of nematodes using morphometric data is very simple up to the genus level, but it becomes a difficult task once you get down to the species level. It’s not uncommon for major descriptive characteristics from different species to overlap, which can lead to misidentification [17].

7. Molecular techniques for the identification of root-knot nematode

Many fungi, bacteria, and plant-parasitic nematodes have been speciating via polymerase chain reaction (PCR). PCR is a technique that uses a set of primers to amplify a specific region of the genome. PCR can be used to compare genetic similarity or variability between and among species when combined with restriction fragment length polymorphism (RFLP) or sequencing [17]. Powers [20] used the example of a protein-coding 600 nucleotide piece of DNA being able to identify 10 million species based on the variability present on that segment to demonstrate the value of researching DNA that codes for genes. This example illustrates how beneficial PCR may be in differentiating specimens because it amplifies a section of the genome. Denaturation, annealing, and extension are the three processes that make up PCR. To allow the primers to attach to a specific region of single-stranded DNA, double-stranded DNA is de-naturated at a high temperature (90–95°C). Annealing, or the binding of oligonucleotide primers to the target area, takes place at a lower temperature (45–60°C). As the primers attach to the target site, the temperature is raised slightly (70–74°C) to allow the primers to extend on the template DNA with the help of DNA polymerase, a process called extension. To achieve a million-fold amplification of the target location, this technique is routinely performed 30–40 times [17].

8. Root-knot nematode integrated pest management

Integrated pest management (IPM) was defined by Prokopy [21] as “a decision-making procedure involving coordinated employment of numerous methods for maximizing the control of different types of pests (diseases, insects, vertebrates, and weeds) in a way that is both environmentally and economically beneficial. Nematode management is challenging. Preventive approaches, such as sanitation and plant variety selection, are the most reliable. Present infestations can be decreased by following, rotation of crop, and soil solarization. These approaches, however, only work for about a year since they diminish nematodes in the upper foot or soil. They’re best used for annual plants or to aid the establishment of young woody plants. If nematodes have infested a crop or an area, struggle to limit infection by shifting dates of the plant to cooler seasons when worms are less active and to make plants more resistant to nematode infection, try to create optimal circumstances for plant growth, such as adequate watering and soil additives [22]. In IPM of root-knot nematode, there are some methods used such as cultural control (crop rotation, sanitation, host plant resistance, solarization, planting, and harvesting dates and irrigation and soil amendments) biological control, and chemical control.

8.1 Cultural control

8.1.1 Management of plant parasite root-knot nematode through crop rotation

The herbivore behavior of RKN has given two good options that can be carried out with crop rotation and change farming methods [23]. In plasticulture systems, control measures such as rotations to non-host crops are restricted because two or more crops vegetables are frequently produced each year on the same land, limiting cycles of rotation. Furthermore, some commercially marketed vegetable types resist nematode [24]. Crop rotation, which involves cultivating non-host crops or resistant types, aims to keep nematode populations below the tolerance limit. By adding non-hosts between sensitive crops, the number of life cycles is reduced significantly, and the nematode population is reduced to a significant level. Crop rotation’s effectiveness in reducing the build-up of some plant-parasitic nematodes in cropping systems has been well reported [25]. There is some example such as the rotation of maize with alfalfa or oat it is a non-host crop that can reduce the populations of RKN. Because various species have distinct host ranges, identifying the specific species in the field before relying on crop rotation as a management method is always a good idea [26]. Green manure plants were also tested for their efficiency as crops rotation with beans to reduce RKN. They also explored as basic additions in the control of nematode [27]. Meloidogyne graminicola, the rice root-knot nematode, has gained widespread attention due to its ability to cause significant harm in rice-wheat cropping systems. It has emerged as an issue in nurseries and upland rice, as well as its widespread prevalence in deep water and irrigated rice in Southeast Asia’s various countries. So the crop rotation of rice with marigold (Tagetes sp.) plays an important role in lowering RKN populations because of its nematicidal properties [28].

8.1.2 Sanitation

Infested soil or plants are commonly used to transfer nematodes into new locations. Use only plants that are free from nematodes acquired from reputable nurseries to keep nematodes out of your garden. Prevent placing plants and soil from affected areas of the garden to control the spread of nematodes. Irrigation water from around infested plants should not be allowed to flow off, as this will propagate nematodes [29].

8.1.3 Resistant or tolerant varieties and rootstocks

Using nematode-resistant vegetable types and fruit tree rootstocks is one of the most effective strategies to manage nematodes. Tomato varieties resistant to nematode species with the code Fusarium, Verticillium, Nematodes (FVN) on the seed packet should be cultivated. Tomatoes that resist nematode produced about six times high tomatoes than a variety susceptible in recent vegetable garden-type studies on root-knot nematode soil [30].

8.1.4 Solarization

Solarization is used to reduce temporarily nematode populations in the upper 12 inches of soil, allowing for the shallow-rooted annual crops production and the establishment of early-stage plants before worm populations rise. Fruit trees, vines, and woody ornamental plants, on the other hand, will not benefit from solarization in the long run. For maximum solarization, moist the soil and cover it with a clear plastic sheet. During the warmest portion of the summer, the sheet should be left in place for 4–6 weeks. When the soil temperature reaches 125°F for 30 min or 130°F for 5 min, RKN, including eggs, die [31].

8.1.5 Planting and harvesting dates

Many nematode species are prevalent during the summer season, and an average temperature below 64°F prevents them from penetrating roots. As a result, farmers could avoid nematode damage to fall-planted crops like carrots, lettuce, spinach, and peas by waiting until soil temperatures drop below 64°F [29].

8.1.6 Irrigation and soil amendments

To lessen the impact of nematodes on crop plants, the soil can be treated using a variety of organic amendments. Manure, peat, and composts are among the amendments that can help increase the water and nutrient-holding capacity of the soil, particularly on sandy soils. Because nematodes are most likely to injure plants that are water-surface, boosting the capacity of soil to grasp water can reduce nematode damage. Similarly, more frequent irrigation can aid in the reduction of nematode damage. You’ll have the same number of nematodes in the soil in either situation, but they’ll do minimum injure [29].

8.2 Management of plant parasite root-knot nematode through biological control

Many approaches have been made to control plant-parasitic nematodes with varying degrees of success. This includes biological control via soil-borne microbes. Soil suppressiveness is the inability of pathogens to survive and establish in diverse soils, or the ability to establish but not cause disease to a significant amount. Soil biotic suppressiveness can be general, where multiple diseases are suppressed by complex ecological interactions, or specialized, where one or a few organisms fight a specific pathogen [12].

Biological control is a non-lethal method of eliminating pests and pathogens. Antagonists and nematophagous microorganisms are the highest potentials than chemical nematicides. Various forms of nematicides are used to control nematodes, which can be harmful to the environment. As a result, finding new techniques to reduce RKN that aren’t hazardous chemical nematicides could be beneficial [32]. Therefore bio-agents can use against different pathogens. In RKN management, only a few nematophagous bacteria and fungi are commercially accessible [10].

Among biocontrol agents, fungi have different suitable strategies for controlling root-knot nematode. They may grab nematodes via constricting and non-constricting rings, adhesive tendrils, and colonies their body parts or produce toxic compounds to destroy them [33]. Many soil-dwelling fungi have been proven to be efficient biological control agents, especially Paecilomyces lilacinus, Trichoderma harzianum, Fusarium spp., Pochonia spp., Chlamydosporium, and Penicillium spp. These fungi have been discovered to be effective at killing worm eggs, juveniles, and female nematodes, as well as reducing parasitic root-knot nematode concentrations in soil [34].

Fungi that belong to the genera Penicillium, Pochonia, and Aspergillus have been identified as nematophagous or antagonistic to RKNs that operate directly by parasitizing eggs, as well as indirectly by stimulating plant defense mechanisms or to produce nematicidal metabolites [35].

Cultural filtrates of fungi were tested for their nematicidal action toward RKN in various plants. For example Aspergillus spp. decreased M. incognita egg production and were very toxic to juveniles. Soil drench action of Aspergillus spp. culture filtrates gave significant seedling growth of Vigna radiate and high rate of reduction in nematode population [36]. Sikandar et al. [37] discovered a major reduction in M. incognita invading after seed treatment with Penicillium chrysogenum cultural filtrates, suggesting P. chrysogenum as a possible biological control agent in the case of M. incognita in cucumber.

The importance of biological control of pests is growing, As such nematicides represent living systems, several difficulties exist to develop commercial bio-nematicidal products. Problems with their culture and formulation, variable gap between laboratory and field performance, potentially negative effects on non-target or beneficial organisms, and expectations of broad-spectrum activity and quick efficacy based on practice with synthetic chemical nematicides have been addressed in detail by some workers [10]. Bio-products containing antagonists of fungi and bacteria rank high among other bio-nematicides [38]. Rapid progress has been made during the past two decades in different aspects of bio-nematicidal production and use.

8.3 Management of plant parasite root-knot nematode through chemical control

Significant management of plant-parasitic root-knot nematodes in such production systems has relied on the use of chemical nematicides (any substance that is utilized to manage nematode infection in vegetables) as a brief-term control measure, reducing nematode rates in the soil to levels under recognized commercial harm thresholds. Nematode rate must be reduced to under threshold rate to decrease root damage and increase yield in affected fields [39]. Nematicides are chemically manufactured compounds that kill or harm nematodes. The first chemical nematode control trials against M. incognita on cantaloupe and tobacco were conducted in southern Italy in 1998. The global market for nematicides is worth around $1 billion per year, with RKN management accounting for 48% of this market. In the case of nematodes, nematicides might have a nemastatic or nematicidal effect. Nematicidal chemicals are extremely poisonous and harm exposed worms, but nemastatic compounds do not kill nematodes but prevent or postpone the hatching of nematode eggs [40].

8.3.1 Types of nematicides

Nematicides are classified as fumigant or non-fumigant based on their soil volatility.

8.3.1.1 Fumigant nematicides

Fumigant nematicides are hazardous chemicals that have a wide range of effects. They may be helpful against a variety of soilborne pests and pathogens in addition to killing plant-parasitic nematodes. In high-value crops like vegetables, fumigants are utilized to clean soil and decrease the risk of yield loss due to soilborne pests. When fumigant compounds are sprayed into the soil, they reach target organisms as a gaseous that passes among soil particles or as a liquid that dissolves into the water film that covers soil particles [41]. Plant-parasitic nematodes can be controlled by fumigant nematicides in a variety of soil, however, they are most successful in rough soils as compared to clay soils. Throughout the United States, including Georgia, soil fumigation has shown better performance in managing root-knot nematodes in vegetable crops production for decades. In Georgia, controlling the species of root-knot nematode such as Meloidogyne is critical to the production of vegetables (Table 1) [41].

Trade name	Toxicity	Main ingredient
Chlor-O-Pic	Nematicide/fungicide	96.5–99% chloropicrin
Telone I	Nematicide	1,3 dichloropropene (1,3-D)
Telone C-35	Nematicide/fungicide	65% 1,3-D, 35% chloropicrin
Telone C-17	Nematicide/fungicide	73% 1,3-D, 17% chloropicrin
Telone EC	Nematicide	1,3-D
Dominus	Broad-spectrum	Allyl isothiocyanate
Paladin	Broad-spectrum	Dimethyl disulfide
K-Pam	Broad-spectrum	Metam potassium
Vapam	Broad-spectrum	Metam sodium
PicClor-60	Nematicide/fungicide	40% 1,3-D, 60% chloropicrin
InLine	Nematicide/fungicide	61% 1,3-D, 33% chloropicrin

Table 1.

Currently available chemical fumigant nematicides for use in the production of vegetables.

8.3.1.2 Non-fumigant nematicides

Non-fumigant nematicides are non-volatile dangerous substances that can be used before, during, and after planting to lower nematode population densities and protect crops from injury via drenching, drip irrigation, or spraying into crop foliage [41]. These nematicides are divided into two types: contact (which kills nematodes in the soil by direct touch) and systemic (which kills nematodes as they feed on plant roots). Non-fumigant chemicals are distributed by soil water movement after being applied to the soil. Non-fumigants’ efficacy is not affected by soil temperature, unlike fumigant nematicides. Due to toxicity and environmental concerns, the many previous non-fumigant nematicides have been taken off the market. Prompting the creation of a new class of chemical molecules that address these issues while still providing effective plant-parasitic nematode management [41]. For usage in vegetable crops, some commercially non-fumigant nematicides are available (Table 2).

Trade name	Toxicity	Main ingredient
Salibro	Nematicide	Fluazaindolizine
Counter 20G	Nematicide/insecticide	Terbufos
Movento	Nematicide/insecticide	Spirotetramat
Mocap EC	Nematicide/insecticide	Ethoprop
Mocap 15G	Nematicide/insecticide	Ethoprop
Velum Prime	Nematicide/fungicide	Fluopyram
Nimitz	Nematicide	Fluensulfone
Vydate	Nematicide/insecticide	Oxamy

Table 2.

Currently available non-fumigant nematicides use in vegetable production.

8.4 Plants that inhibit the growth of nematodes

Lesion and root-knot nematodes are suppressed by Targets species of marigolds. The most effective marigolds are Petite Blanc, Queen Sophia, Nemagold, and Tangerine (varieties include Petite Blanc, Queen Sophia, Nemagold, and Tangerine). Nematodes will nourish on and generate on T. signata or T. tenuifolia, Signet marigolds. Marigolds are ineffective against the northern root-knot nematode, M. hapla, which is found in colder climates. Marigolds provide the best impact when grown as a continuous planting for the full season [42].

The plant extracts effect from Artemisia absinthium, Thymus vulgaris, Ricinus communis, Citrullus colocynthis, and Punica granatum on the motility of Meloidogyne incognita, and Helicotylenchus dihystera, as well as the reversibility of the movement inhibition, were investigated by Korayem et al. [43]. Megalaima incognita’s egg-hatching inhibition and H. dihystera’s acetylcholinesterase (ACHEs) inhibition surprisingly, extracts of P. granatum, A. absinthium, and T. vulgaris and inhibited AChE more than oxamyl, which was previously thought to be a potent AChE inhibitor [44]. Similarly, detailed knowledge on the modes of action of various biological nematicides in terms of nematode acetylcholinesterase inhibition is required [10].

9. Conclusion

Root-knot nematodes are potent silent killers of many plant species belonging to a wide range of plant families. They lower the yield of many economically important crops and decline the quality as well. Many farmers are unaware of their presence due to its concealed behavior under the soil and roots. The second stage juveniles (J-II) feed and reside in the roots that creates galls/knots on the roots which ultimately lead toward the death of the plant. Integrated approaches are advised to the growers to tackle these parasitic worms. The use of resistant varieties, crop rotation, chemical control and utilization of microbiota is necessary to keep their damages below economic threshold level.

Conflict of interest

The authors declare no conflict of interest.