Plant parasitic nematodes comprise one of the major limiting factors in vegetable production. The extent of damage, however, depends mainly on the farming system employed. In general, nematodes play a less significant role under multiple intercrop farming systems in subsistence agriculture as well as in widely spaced rotations of commercial farming systems. They are highly damaging in intensive production systems like protected cultivation where mono-cropping is practiced. Extensive damage by nematodes occurs where nematode-infested planting material in the form of seedlings is used for planting. Though, more than 70 genera have been reported to be associated with vegetables in India, only three viz., Meloidogyne spp., Rotylenchulus reniformis and Pratylenchus spp. are of utmost significance. Besides, Tylenchorhynchus brassicae causes economic damage to crops like cauliflower, cabbage, knol-khol, radish etc.
(I) Root- knot nematode (Meloidogyne spp.)
Etymology: Meloidogyne: a greek word; melo= apple, oid = resembling; gyne= female meaning apple-like female; the shape the female of this genus acquires on maturity. Its common name root-knot nematode is given for the characteristic symptom of knots or galls produced by this nematode on the roots of host plants.
Distribution and host range
- Root knot nematodes are cosmopolitan in distribution, attacking almost all the major crops including vegetables, fruits, ornamental plants, pulses, cereals, plantation crops, cash crops and even weeds.
- Although over 90 species of Meloidogyne have been described to date, four species viz., M. incognita, M. javanica, M. arenaria and M. hapla with their cosmopolitan distribution and widest host range are of particular economic importance to vegetable production.
- Of these, whereas, M. incognita, M. javanica and M. arenaria commonly inhabit the tropics with an average temperature of 36°C or lower in the warmest months, M. hapla is common in the temperate regions with occasional occurrence in the cooler upland tropics.
- M. incognita and M. javanica are highly pathogenic to vegetables like tomato, egg plant, okra, chillies, cucurbits, carrot, potato and beans.
- M. hapla is encountered in Jammu and Kashmir, Himachal Pradesh and hilly areas of Assam where it is a problem on potato and mentha.
- Severity of nematode damage increases manifold under protected cultivation.
Biology and life cycle
- Root-knot nematode females are sedentary endoparasites.
- Pyriform females remain embedded inside the root galls, with heads located near the vascular tissue and terminal portions near the root epidermis.
- Reproduction is through parthenogenesis as vermiform males remain in soil and are free living.
- Female lays 200-400 eggs in the gelatinous matrix secreted by their rectal glands. Embryogenesis takes about 10-15 days as a result of which first stage juvenile (J1) is produced within the egg..
- First stage juveniles (J1) moult within the egg shell to become J2s.
- J2s hatch out and move about in the soil in search of a suitable host and on getting the one, penetrate the roots just behind the root tip with the help of repeated thrusts of their protrusible stylet.
- J2s fix their head near the vascular tissue with rest of the body in cortical region and initiate the development of feeding site known as ‘giant cells’. Giant cells (6-8 in number) are multinucleate cells produced due to karyokinesis without cytokinesis(nuclear division without the division of cytoplasm).
- As feeding commences, J2s destined to be females assume swollen shape with V-shaped genital primordium.
- Second moult occurs in about seven days.
- J3 and J4 retain their old cuticle; the pointed tail of J2 still visible as a spike and hence these stages are also referred as ‘spike tailed stages’. These stages lack stylet and thus are non-feeding. The third moult occurs quickly and the juvenile changes to J4.
- At the last moult, adult female becomes sac-like with fully developed reproductive system.
- Adult males remain vermiform, come out of the soil and are short-lived.
- The whole life cycle takes about 25 days at 25-30o C.
- M. incognita may have eight or more generations per year on tomato.
 
Figure 10.1 Longitudenal section of giant cells caused by root-knot nematode Courtesy: Photo by Victor Dropkin , Nemapix Vol.1
Figure 10.2 Root-knot nematode juvenile inside a galled root, Courtsey: Photo by Jonathan Eisenback Nemapix Vol.1
Symptoms
- The presence of galls on the root system is the primary and specific symptom associated with Meloidogyne infection. The size and form of the galls depend on the species involved, the number of nematodes in the tissue, host and plant age. In cucurbits, extremely massive, fleshy galls are formed whereas in crops like capsicum, galls are small. Other vegetables like tomato, eggplant, radish, carrots etc. may have small to large, firm galls.
- Symptoms of root knot on monocotyledonous crops such as onion and leek are very discrete, the main symptom being the presence of the protruding egg masses on the root surface.
- In case of severe infestation, the normal root system is reduced to a limited number of severely galled roots with a completely disorganized vascular system. Rootlets are almost completely absent. The roots are seriously hampered in their main functions of uptake and transport of water and nutrients.
- Plants wilt rapidly, especially under dry growing conditions from which they recover during cooler part of the day. Broad leaved plants show severe wilting.
- Plant growth is retarded and leaves may be chlorotic.
- In case of seedling infection, numerous plants die in the seedbed and seedlings do not survive transplanting. Even the plants that do survive transplanting show reduced ?lowering and fruit production in the field.
- The symptoms caused by M. hapla on vegetables differ from those produced by most other species in that only small, more or less spherical galls are produced with profuse root branching originating from the galled tissue causing a ‘bearded root’ system.
- Infection of the taproots (carrot and radish etc.) results in hooking and forking and tuber deformation. Tuber infection also makes long-term storage impossible as these tap roots begin to rot due to fungal infection associated with nematode gall degradation.
- In leguminous vegetables, nematode induced galls differ from rhizobium nodules in the following respects:
- Bacterial nodules are the side appendages whereas nematode induced galls are swellings of the roots itself.
- The rhizobium nodules can be easily separated from the root but not the root gall.
- Nodules are spongy in nature but galls are hard in texture.
  Patchy growth in okra field due to nematode infestation and Root galls formed on okra roots by Meloidogyne sp.
Survival and dissemination
- Being obligate parasites, the absence of suitable host plants for prolonged periods leads to the disappearance of Root-knot nematodes.
- In the absence of susceptible crops, they often survive on weed hosts.
- During stress, when nematode populations rapidly decline, a proportion of the eggs in the egg mass goes in diapause and ensures perpetuation of the species.
- Under adverse environmental conditions, emergence and juvenile activity are reduced, thus increasing the chances of survival. Survival is influenced mainly by moisture content of the soil and to a lesser extent by temperature.
Economic importance
- Vegetable crop losses in the tropics ranged from 17 to 20% on egg plant and 24 to 38% on tomato.
- In intensive commercial production, where sequential cropping of one susceptible crop after another is practiced with up to four crops per year, the lack of effective root-knot control would lead to total crop failure.
- Crop loss due to M. incognita under intensive cultivation ranged from 30 to 60% on brinjal.
- Nematode damage in protected cultivation, where susceptible crops are repeatedly planted in the same soil is very severe.
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