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Plant parasitic nematodes are microscopic, thread like, soil-born organism, found in every conceivable environment. Severity of nematodes as plant pathogen is more a problem in tropical than temperate climate. These organisms are basically root feeders causing injury to the plants, which adversely affect the function of the root to absorb and translocate nutrients and water efficiently. This result into nutrient deficiency like symptoms, and as such nematode problems go unnoticed. Such symptoms are not corrected even after a dose of fertilizers.

Based on feeding habits, they are classified as migratory ectoparasitic, sedentary semi-endoparasitic or endoparasitic nematodes. They feed silently but constantly without killing the host producing debilitating symptoms and in the process reproducing to great numbers at the end of the crop season. Normally, nematodes complete their life cycles in a span of 20-25 days at temperature of 25-30 °C. They have a great egg laying capacity, which varies from 40-1000 eggs.

The most important nematode is root-knot nematode, Meloidogyne sp. distributed worldwide and can infest more than 2000 species of plants. Cereal cyst nematode, Heterodera avenae - an important nematode pest of wheat, barley and oat, citrus nematode, Tylenchulus semipenetrans associated with citrus dieback and slow decline, burrowing nematode, Radopholus similis causing spreading decline in plantation crops, the reniform nematode, Rotylenchulus reniformis in vegetables and cotton and the rice root nematode, Hirschmanniella spp in lowland rice are few other damaging nematodes.

Crossandra

Crossandra (Crossandra undulaefolia) is one of the important commercially grown flower crop in Tamil Nadu. In recent years there is a marked decline in the cultivation of this crop in Madurai, Dindigul, Coimbatore, Salem, Thiruvannamalai and Tiruchirappalli districts. The common problem with the crop is death of plants in patches during the second and third year. This is due to the nematode-fungal complex disease in the crop.

Investigations revealed the association of root-knot nematode, Meloidogyne spp. and the root-lesion nematode, Pratylenchus delattrei with the crop along with fungal patheogen, Fusarium solani.

Root-knot nematode

Both M. incognita and M. javanica are reported to cause damage and yield loss to the crop. The affected plants exhibit stunted growth, yellowing of leaves and also have chlorotic symptoms. The nematode in association with R. solani causes serious damage to the crop. The infested plants show root galls.

Crossandra (Crossandra undulaefolia) is one of the important commercially grown flower crop in Tamil Nadu. In recent years there is a marked decline in the cultivation of this crop in Madurai, Dindigul, Coimbatore, Salem, Thiruvannamalai and Tiruchirappalli districts. The common problem with the crop is death of plants in patches during the second and third year. This is due to the nematode-fungal complex disease in the crop.

Rice

Plant parasitic nematodes cause serious damage to rice crop. Thirty two species belonging to 13 genera were observed in association with the crop. Among them few are considered to be important. They are the rice white - tip nematode (Aphelenchoides besseyi), rice stem nematode (Ditylenchus angustus), rice root nematode (Hirschmanniella oryzae), root-knot nematode (Meloidogyne graminicola) and the cyst nematode (Heterodera oryzicola).

The white-tip nematode (Aphelenchoides besseyi)

The yield loss due to the white-tip nematode is estimated to be as much as 17.4 to 54.1 per cent. The nematode is distributed in India, Bangladesh, Sri Lanka, Japan, Indonesia, Taiwan, Former USSR, Italy, Cuba and Madagascar.

Plant parasitic nematodes cause serious damage to rice crop. Thirty two species belonging to 13 genera were observed in association with the crop. Among them few are considered to be important. They are the rice white - tip nematode (Aphelenchoides besseyi), rice stem nematode (Ditylenchus angustus), rice root nematode (Hirschmanniella oryzae), root-knot nematode
(Meloidogyne graminicola) and the cyst nematode (Heterodera oryzicola).

Introduction

Plant parasitic nematodes occur wherever plants grow. These nematodes are referred to as plant pathogenic when they cause crop losses by direct injury to roots, stems, leaves and seeds. A large number of plant parasitic nematodes have been found associated with fruit crops in all over India but pathogenisity has not been proved for all. Most of commonly occurring nematodes belong to genera Meloidogyne spp., Pratylenchus spp., Rotylenchulus spp., Hoplolaimus spp., Tylenchulus spp., Xiphinema spp., and Radopholus spp. etc. The nematodes damage the crops not only by feeding on plants but also by interacting with various other organisms like fungi, bacteria and viruses etc. Estimated overall average annual yield loss of the world’s major crops due to damage by plant parasitic nematodes was 12.3%. Estimated annual crop losses in India have been accounted to Rs. 242.1 billion. The crop losses due to nematode pests are not only in the form of reduced plant growth and yield but are also in the marketable quality of the produce.

Most of the plant parasitic-nematodes (PPNs)are worm-shaped microscopic animals. They are virtually invisible through naked eyes when present in soil or plant materials. It has been estimated that of the known nematode species, approximately 50% are free-living found in soil and fresh water, 25% are marine (found in sea water), 15% are animal parasites, and 10% are plant-parasites (Walia and Bajaj, 2004; Sasser and Freckman, 1987). Free living nematodes contribute to maintain the soil bio-dynamic system, by influencing microbial colonization of substrates and mineralization of nutrients (Neher, 2001), whereas PPNs feed on plants and reduce crop growth and yield efficiency. Among the PPNs, a few nematode species feed on aerial plant parts (leaves, stems, flowers and seeds), but the majority of feed on underground plant parts (roots, bulbs, and tubers).

Citrus (Citrus spp.)

Citrus is an important fruit crop and ranks third in area which is followed by mango and banana. They are grown in 105,396 hectares in India. Among the oranges, mandarin oranges, limes and sweet oranges are very popular while lemons, grape fruits, pummelos, sour and bitter oranges are of minor importance. The Citrus spp. are grown in larger extent in the states of Maharashtra, Karnataka, Kerala, Tamil Nadu, Andhra Pradesh, Punjab, Haryana, Rajasthan, Assam, Orissa and Bihar.

Nematode menace in citrus gardens are very much realised. In 1912 the citrus nematode, Tylenchulus semipenetrans was reported from California and found to cause ‘slow decline’ disease. More than 30 genera of plant parasitic nematodes are associated with the Citrus spp. Among them, Tylenchulus semipenetrans, Radopholus citrophilus, Meloidogyne spp. Pratylenchus spp. Xiphinema spp. and Hoplolaimus spp. are often associated with the crop.

Citrus is an important fruit crop and ranks third in area which is followed by mango and banana. They are grown in 105,396 hectares in India. Among the oranges, mandarin oranges, limes and sweet oranges are very popular while lemons, grape fruits, pummelos, sour and bitter oranges are of minor importance. The Citrus spp. are grown in larger extent in the states
of Maharashtra, Karnataka, Kerala, Tamil Nadu, Andhra Pradesh, Punjab, Haryana, Rajasthan, Assam, Orissa and Bihar.

Crop productivity is largely affected by the several factors; however, biotic stress factor is one of the major concerns. Horticultural crop productivity is low in India as compared to other countries for many reasons including pest problems. Plant parasitic nematodes is one of the potential threat for cultivation of some important crops- root knot and reniform nematodes in vegetables; burrowing nematodes in banana, spices and plantation crops; citrus nematode in citrus; cyst nematodes in potato, foliar and lesion nematodes in flowers etc.

Black Pepper

Black pepper (Piper nigrum) is indigenous to the tropical forests of Western Ghats of South India. The crop is grown in Kerala, Karnataka, Tamil Nadu, Andra Pradesh and Assam.

The root-knot nematode, Meloidogyne incognita and the burrowing nematode, Radopholus similis are important pests of this crop. The root-knot nematode infested vines show yellowing of lower leaves and the leaves loose natural luster. The yellowing of leaves gradually progresses upwards. The affected vine leaves become flaccid and wither. The infested plant root show root galls of varying sizes.

The burrowing nematode also causes serious damage to the vine. Var der Vecht (1950) first reported slow wilt in this crop due to R.similis infestation in Indonesia. The nematode in association with Fusarium and Rhizoctonia causes serious damage to the crop.

Black pepper (Piper nigrum) is indigenous to the tropical forests of Western Ghats of South India. The crop is grown in Kerala, Karnataka, Tamil Nadu, Andra Pradesh and Assam.

Potato

Potato is one of the most important vegetable crop grown in India. It thrives best in cool climate. Therefore it is a summer crop in the hills and a winter crop in plains.

Twenty seven species of nematodes have been reported on potato among them the cyst nematodes (Globodera rostochiensis and G. pallida) and the root-knot nematodes (Meloidogyne incognita, M. javanica and M. hapla) are important in India.

Cyst nematodes (G.rostochiensis and G. pallida)

In India, the cyst nematode menace on potato was first reported by Dr. Jones during 1961 from Nilgiri Hills.

Geographical distribution

The potato cyst nematode is distributed in Central, South and North America, Europe, Iceland, Western USSR and South Africa.

In India it is distributed in Nilgiri and Kodai Hills and in New Delhi.

Potato is one of the most important vegetable crop grown in India. It thrives best in cool climate. Therefore it is a summer crop in the hills and a winter crop in plains.

Introduction

Agricultural soils harbour a rich variety of nematode fauna, where they coexist with a myriad of other microorganisms. It has been estimated that one cubic meter soil of highly fertile field may contain as many as 3×10¹⁴ bacterial cells (300 g), 5×10⁸ protozoa (39 g), 1×10⁷ nematodes (12 g) and fungi (400 g), besides other organisms. The nemic fauna comprises of free-living (saprophagous and microbivorous), predatory forms and plant parasites (phytophagous). In addition, certain animal and insect parasitic forms may spend a part of their life cycle in soil. Plant parasitic nematodes (PPNs) exist in poly-specific communities and attack all types of plants- cultivated or wild. They possess a needle-like stylet or spear at their anterior end, which is used to puncture the plant cells and ingest the cell contents. Majority of PPNs dwell in soil and draw their nutrition from plant roots. Only a few forms attack the above ground plant parts such as stem, leaves, inflorescence and seeds. PPNs may be ectoparasites (feed from outside and do not enter plant tissues) or endoparasites (enter inside the plant tissues). The endoparasites spend a part of their life cycle in soil or may come out into the soil to re-infect the other roots. These may be further categorized into migratory or sedentary types. The migratory forms keep on moving, feeding and laying eggs that are scattered in soil or plant tissues. The sedentary nematodes are more harmful and a particular stage of their life cycle infects the plant and establishes a feeding site by modifying plant tissues by their enzymatic secretions. The females of sedentary nematodes usually attain swollen shapes upon maturity and lay hundreds of eggs in clusters.

Plant parasitic nematodes are ubiquitous, and so far, more than 1800 species have been recorded. They are obligate parasites and must feed upon plant hosts, thus, debilitating the plants to some extent. The extent of direct damage by the nematodes to plants depends on several factors viz., nematode density in soil, nature of parasitism (ectoparasite or endoparasite), host susceptibility, cropping pattern, edaphic factors (mainly soil texture and moisture) and ambient climatic conditions (like temperature and relative humidity). Nematodes, by themselves, rarely kill the plants to ensure their own survival. Nematodes also inflict considerable indirect damage to crops by interacting with other plant pathogens (fungi, bacteria and viruses) to cause disease complexes in which nematodes act as incitants, aggravators, or vectors.

Introduction

Seed spices have prominent place in India’s national economy because of their very high domestic consumption and growing demand for export. This group of spices, being consisting of mostly annual crops, is grown extensively in rotation with food and cash crops and also as inter crops under rain fed as well as irrigated conditions.

There are about 20 seed spice crops grown in India. Out of which nine are important in terms of production and export. The most important ones are coriander, cumin, fennel and fenugreek (major seed spices) and ajowain, aniseed, black cumin, celery and dill (minor seed spice crops). Although these crops are grown all over India but Haryana, Rajasthan, Madhya Pradesh, Uttar Pradesh, Orissa, Tamil Nadu and Andhra Pradesh are the main seed spice growing states.

Nematodes are invertebrate roundworms that inhabit marine, freshwater and terrestrial environments. They comprise the phylum Nematoda or Nemata, which includes parasites of plants and of animals, including humans, as well as species that feed on bacteria, fungi, algae, and on other nematodes. Four out of every five multicellular animals on the planet are nematodes. All nematodes may look alike, the smallest being scaled down versions of the largest. The phrase tube-within-a-tube is a convenient way to think of nematode body structure and also a term used to refer to a major trend in the evolution of triploblastic metazoa. It refers to the development of a fluid-filled cavity between the outer body wall and the digestive tube. The nature of this body cavity has led to the grouping of metazoa into three grades, acoelomate, pseudocoelomate and eucoelomate. Nematodes together with Rotifera, Gastrotricha, Kinorhyncha, Nematomorpha, Acanthocephala and Entoprocta are traditionally grouped together as pseudocoelomates on the basis of possessing a body cavity that is not formed from the mesoderm or fully lined by peritoneum.

Introduction

A. Common species of nematodes encountered in poultry are:

1. Syngamous trachea (in the respiratory tract) large 2.00cm size live in the trachea and larger bronchi. Many species of domestic and wild birds are affected. Dyspnea, gasping and head shaking are main clinical signs. Treatment is difficult.
2. Ascaridia galli (in the intestine) They parasitize chicken, turkey, pigeon, quail and guineas. In younger birds parasites cause enteritis. In heavily infected birds clinical signs are emaciation and diarrhea.

2.1. Rice

2.1.1. Rice root knot nematode (Meloidogyne graminicola)

1. Symptoms

6.1 Nematodes as Biological Models
A model organism can be a non-human species (e.g., microbe, plant, or animal) with extensive genomic research data. These organisms are used to study and understand specific biological phenomena, with the expectation that discoveries made in these models will provide new insights into “big biological questions” (Semprucci et al., 2025). Model organisms are widely employed to explore potential causes and treatments for human diseases when direct experimentation on humans is unfeasible or ethically problematic. This approach is supported by the common descent of all living organisms, as well as the conservation of metabolic and developmental pathways and genetic material over the course of evolution. The use of model organisms is not confined to biomedical research. Significant advancements in developmental biology, eco-toxicology, fertilization processes, speciation, symbiosis, and sexual selection have been achieved using these models. Furthermore, emerging animal models—species with less neurological complexity than those protected by legislation—are increasingly recognized as a valid application of the Three Rs principle (Replacement, Reduction, and Refinement). To support this approach, databases of invertebrate species currently used or with potential applications in biomedical research are continually being developed and updated.
Nematodes, particularly Caenorhabditis elegans, are widely used as model organisms in biological research due to their simplicity, ease of study and shared biological processes with other animals, including humans. C. elegans, a small, free-living nematode, has a short life cycle, a known number of cells and a well-mapped genome, making it an ideal system for studying developmental biology, genetics and other aspects of animal biology.