Plant parasitic nematodes broadly divided into two groups above ground feeders and underground feeders. On the basis of feeding behaviour, underground feeders are falling under different sub groups like ecto-, endo- and semi-endoparasites. Further, they classified as sedentary endoparasites (Meloidogyne and Heterodera), migratory endoparasites (Pratylenchus and Radopholus), semi-endoparasites (Tylenchulus and Rotylenchulus), sedentary ectoparasites (Sheath nematode, Hemicriconemoides) and migratory ectoparasites (Longidorus, Xiphinema, Trichodorus). We dealt here only endo- and semi-endoparasites.
1. Root-Knot Nematode, Meloidogyne Spp:
The root-knot nematode, being sedentary endoparasites, a common name collectively given to Meloidogyne spp. It produces knots/ galls of various sizes and shapes on the roots of wide varieties of plants. They are considered as number one farmers’ enemy of agricultural crops in the most developing nations.
The crop diseases produced by root-knots was first reported on the greenhouse grown cucumbers in England by Berkley (1985). In India, Barber (1901) recorded this nematode for the first time on tea from Kerala. The four most common species in the world are M. incognita, M. javanica, M. arenaria and M. hapla. Patel et al., (1988) first time reported M. javanica pathotype 2 on peanut in Kapdwanj area of Kheda district of Gujarat State.
i. Host Range:
Meloidogyne spp. is noticed to attack over 2500 hosts. The more preferred hosts are dicot vegetables, pulses, fibres, fruit crops (papaya, grapes, pomegranate etc.), tobacco, peanut, tea, coffee, mulberry, flowers, spices, ornamentals and some monocots like sugarcane, sorghum, banana, rice as well as large number of weeds.
The root-knot nematodes basically parasitise plant roots or underground stem, pods, rhizomes, corms etc., and lowers ability of roots in its function of nutrient and water uptake and translocation. The nematode infected plants exhibit yellowing, dwarfing, stunting, and drying of leaf margins and such other symptoms as if it is suffering from nutritional deficiencies.
iii. Disease Complex with Other Pathogens:
Root-knot nematode interacts with a variety of fungal and bacterial pathogens leading to disease complexes. Galled roots are extensively colonised by rotting fungi Rhizoctonia solani and the damage is compounded. Nutrient rich giant cells provide substrates for the proliferation of wilt causing micro organisms like Fusarium, Verticillium and Pseudomonas solanacearum.
2. Cyst Nematode, Heterodera Spp:
The second most threat to farmers next to root-knot nematode is cyst nematode (sedentary endoparasites) because of its wide distribution and protection to eggs by cyst formulation. More than 81 Heterodera spp., 12 Globodera spp. and 9 Cactodera spp. are recorded in the world. The first report of Heterodera sp. from India was that of H. avenae which was then reported to be associated with molya disease of wheat and barley crop in Rajasthan. H. avenae, H. cajani and H. zeae were key cyst nematodes having wide distribution in various states of the country. Globodera rostochiensis and G. pallida are still major constraints to potato production in Tamil Nadu.
i. Host Range:
Heterodera spp. is known to attack over 1500 hosts. The most preferred hosts are cereals, pulses, tobacco, sugarcane, potato, sugar beet, rye, some vegetables as well as some weed species.
Nematode attacked plants initially exhibit patches of stunted plant growth and chlorosis appear when the crop is about 1-2 month old. With continuous cropping of hosts, such patches gradually increase in size. Tillering is greatly reduced, clums become thinner and weaker.
The affected plants may flower prematurely and earheads bear few grains. Roots become typically bushy with slight swelling marking the sites of nematode infection. On uprooting such plants, several whitish to brown females are seen on the roots.
3. Potato Cyst Nematode, Globodera Rostochiensis and G. Pallida:
It was first discovered by Kuhn in Germany, but he considered it as a form of H. schachtii. Potato cyst nematode is distinct from H. schachtii. It was later renamed as H. rostochiensis after the name of the place (Rostoch) from where it was recorded originally. In India this nematode first time recorded by F. G. W. Jones ins 1961. H. rostochiensis and H. pallida transferred to separate genus Globodera on the basis of round cysts in contrast to lemon-shaped cysts of Heterodera.
i. Host Range:
This nematode feeds on the roots of Solanaceae family plants, such as potatoes and tomatoes.
Yellowing of leaves and stunting growth of plants with patchy appearance of plant stand. Nematode cause early senescence and proliferation of lateral roots. Roots are covered by white and yellow coloured small immature females at flowering. Females can be observed on the tuber surface but with less frequency.
4. Lesion Nematode, Pratylenchus Sp:
The genus Pratylenchus was reported in 1936. It has a cosmopolitan distribution prevalent in temperate, sub-tropical and tropical regions. The species of Pratylenchus are popularly known as lesion nematode or meadow nematode. They are migratory endoparasites and various pulses crops are damaged by them. Walia and Seshadri (1986) reported the P. Thornei attacks chickpea, P. zeae on mungbean and P. mulchandani on urdbean.
i. Host Range:
The species of Pratylenchus infect pulse crops like chickpea, green gram, black gram, lentil, pigeon pea, cowpea, grass pea, french bean and faba bean besides wheat, rice, barley, maize, sorghum, pearl millet, cotton, peanut, sunflower, potato, ginger, turmeric, coffee, cabbage, cauliflower, carrot, radish, turnip, banana, mandarin, tobacco, apple, plum, grape, poppy, avocado, lima bean, common bean and alfalfa etc.
The infested plants exhibit various types of symptoms and results in heavy yield losses. At seedling stage infection is manifested by patchy appearance, stunted growth and pale green colour of the lower foliage. Root lesions vary with the cultivars but small lesions are common symptom of this nematode.
The lesions may appears on young feeder roots but may be formed anywhere on roots. Later on, the lesions enlarge, coalesce and may girdle the entire root. The secondary attack is by bacteria and fungi resulting in rotting. The severely infected plants are easily pulled out from the soil due to destruction of the root system. Generally the individual roots are discoloured, stubby and mutilated.
5. Citrus Nematode, Tylenchulus Semipenetrans:
The common name ‘citrus nematode’ signifies its host specificity. In India, it was first reported by Siddique (1961) from Aligarh (Uttar Pradesh). This nematode encountered throughout the world wherever citrus is grown. In India also it has been recorded from almost all the states.
i. Host Range:
All species of citrus and 11 other species of Rutaceae are hosts of T. semipenetrans. Besides, some populations (biotypes) attack olive, grapes, loquat etc.
By the attack of this nematode leaves become smaller and chlorotic and leaf drop is more pronounced producing exposed branch terminals. Early wilting occurs during water stress. Infected roots are dirty in appearance with poor development. Faster decay of feeder roots and in severe infestation of nematode root may die. Infested trees are smaller and less productive than normal ones.
6. Reniform Nematode, Rotylenchulus Reniformis:
This nematode first described from Hawaii, USA has widespread presence in the tropics and subtropics. Seshadri and Sivakumar (1963) recorded it for the first time on cotton in India. As the name indicates, reniform nematode female is characterized by typical kidney shaped female. There also exist two races i.e. A and B for Reniform nematode.
i. Host Range:
It has a wide range of host plants that includes fruit trees, lentil, cotton, pigeonpea, tea, tobacco, soybean, pineapple, banana, okra, coconut, cabbage, sweet potato, alfalfa, corn, asparagus, pahn, cucumber, tomato, squash, cassava, radish, eggplant, guava, melon, and ginger etc.
Above ground symptoms on host plants include dwarfing, shedding of leaves, formation of malformed fruit and seeds, and general symptoms of an impaired root system. Roots are discoloured and necrotic with areas of decay. Heavy infestations lead to plant mortality.
The burrowing nematode was first observed by Cobb in 1890-91 while investigating a banana disease in Fiji, but a complete description of this species was published by him in 1915 under the name Tylenchus similis. Thorne (1949) created the new genus Radopholus to include T. similis, as well as other species.
Banana, citrus, coconut, ginger, tea, black pepper, are the preferred plants but potato, eggplant, tomato, bird of paradise, some ornamentals, and some grasses and weeds may serve as the alternate hosts.
Infected plants remain stunted in growth with reduction in size and number of leaves and bunch weight also reduced. Reddish elongated lesions that first appear on roots gradually enlarge and coalesce leading to rotting. On banana, the damage starts when the nematode enters into the primary roots and attacks the corm that cause reddened spots around the feeding sites.
This infection is called as ‘Blackhead toppling disease’, wherein the entire feeding site is exposed, showing the blackened and broken primary roots. Nematode feeding and movement cause severe necrosis and cavity formation within the cortex of banana. R. similis interacts with Fusarium oxysporum f. sp. Cubense and cause Panama wilt in banana.