Root-gall nematode disease was first discovered by Berkley 1885 in England when he studied galls on the roots of cucumber plant grown in green house. Goeldi in 1887 first used the generic name meloidogyne for the nematode, causing root galling of coffee trees in Brazil. Root-gall nematode have however, been given different scientific names since then.
Before 1988, the root-knot nematode were considered as one species Heterodera marioni (Chit wood 1949) made a morphological study of the root-knot nematodes and removed them from the genus Heterodera and again placed them in the genus Meloidogyne with five (5) species and one sub-species.

Presently, over thirty-six (36) known species of the genus meloidogyne have been discovered (Esser et al, 1982) and three (3) sub-species. Root –knot nematodes are worldwide in distribution; although some may be restricted to certain climatic zones. It is reported that they could attack more than 2000 species of plant, some of which are field crops such as cotton, yam, groundnut, tobacco, sugarcane, rice, soybean, potatoes, maize, and cassava. etc, vegetable crops such as pepper, okra, pumpkin, carrot, cucumber, onion, tomato, cabbage. etc, while others can also attack tree crops and ornamental plants. Also many weeds have been shown to harbor and increase populations of root-knot nematodes (Ogbuji and Diarua, 1990). Three (3) species and one sub-species of the root-knot nematode which are known to occur in Nigeria are:
Meloidogyne incognita
Meloidogyne javanica
Meloidogyne incognita acrita(Caveness,1983)
These nematodes have been recovered from virgin forest soil and farm lands throughout Nigeria. In the Eastern part of Nigeria example Ebonyi State, root-knot nematodes are major problems for commercial crops (Ogbuji, 2001).
The plant parasitic nematodes appear under the microscope as tiny worms among the plant cells. The root-knot nematode exists as male and females and they are identified on the basis of shape, size and special structures. The adult female nematodes are vermin-form in shape, when examined under a microscope while the juvenile nematodes are worm-like. Reproduction is by means of eggs from which hatch tiny larvae, these process is then reported on the third and fourth stage larvae before it finally becomes and adult nematode. The life cycle requires 25-30 days and is repeated as long as the host plant grows. (Wilber, D. 1983).  Generally, root-knot nematodes are bilaterally symmetrical. They have stylet or mouth spear that are similar in structure and function to a hypodermic needle. They use the mouth to puncture the plant cells and then inject digestive juice and ingest plant fluid through it. As they feed they damage the root system and reduce the ability of the plant to obtain water and nutrients from the soil (Agrios, 1969).
The root-knot nematode, Meloidogyne spp, depends on successful formation of feeding sites (coenocytes) in susceptible plants for its development and reproduction. These coenocytes usually fail to develop in roots of non-host and resistant cultivars (Cootzee, 1996).
The ability of root-knot nematodes to infect their hosts is dependent on factors such as initial nematode population; the species involved, the race of the species, dull the plant cultivar (weather resistant or susceptible). The age of host plant at the time of infection and the existing biotic and abiotic environmental conditions are also factors. Certain soil borne organisms that have co-existed with nematodes, especially the root-knot nematodes have been found to influence the latter.(Albert,1985) observed that the co-existence of the nematodes with fungus: Fusarium oxysporum, Fusarium vasinfectum, resulted in a wilt-nematode complex. Also field survey in New Jersey showed that Pratylenchus spp and Meloidogyne spp  are often found co-inhabiting numerous host plants (Olowe 2001).
Nematophagus fungi have been tested for biological control of parasitic nematodes for almost 50 years (Barron, 1977 and Cooke, 1968). Endo-parasitic fungi which infect nematodes with their condia have been suggested as possible bio-control agents. However, it was observed that the endoparasitic nematophagus fungus; Meria cornospora reduced root –knot nematode galling on tomatoes in green house pot trials (Cooke, 1982).
Root-knot nematodes also are said to comprise of a group of endo-parasitic roundworms that cause major economic damage to crops around the world. (Williamson and Hassey, 1996). These microscopic organism penetrate the roots of thousands of plant species and migrate to the vascular cylinder, where they initiate a series of changes in the roots, resulting in the formation of galls which can be known as root-knots as well as the development of specialized feeding cells known as ‘’giant cell’’ in their host. Among the biological agents that may limit nematode population densities, mites are the least well known and studied. Though there are only few reports (Luc, M, Richard, A.S, and John,B.1990) of mites feeding on tomatoes. Evidence that soil mites of the sub-order Mesostigmata feed on the egg sac of Heterodera and on Meloidogyne spp has been shown (Manau et al, 1978) un-identified species have been also reported preying on root-knot nematodes. Several specimen were observe feeding on the egg masses of both  the cyst nematode and root-knot nematodes, indicating that Heterodera aculeifer is a non-specific biological agent, as are many other predators. Certain chemical control measures have however been used as nematocides while maintaining a low phyto-toxicity like the use of DBCP (1, 2-dibromo-3-chloropropane) while as a therapentant  DBCP reduced the incidence of galling on new roots (Linford,M.P and M.Oliveira,1989). But the folia spray of D-L-have been Phenylenine are more effective than soil drenches for inhibiting the development of Meloidogyne incognita in tomato plants (Selty, K.G.N.1977). Higher plants have been increasingly examined as source of novel compounds with activity against nematodes (Comley, 1990). Marigold (Targetes spp) was among the first plant to be examined for nematicidal compounds and found to suppress populations of soil nematodes. Results of the studies by Oosteribrink et al (1960) indicated that populations of four Pratylenchus species and of Tylenchorychus dubies were greatly reduced in soils after growing marigold for one season. Oostenbrink (2000) reported that tagetes mutual was also effective against Meloidogyne hapla. According to Taylor and Murrant (1986), population of Longidoruselon elongates and other plant parasitic nematodes were added into the soil. Other higher plants with naturally occurring phyto-chemicals with biological activity against nematodes includes; Physostigma venenosum (Chitwood, 1993), Canavalia enaiform (Marbon-Mendoza et al 1987), Mula helenium (Meher et al ,1983) and Eragrostic curvula(Osman and Vigherchio,1988). Bhali et al (1987) reported with Meloidogyne incognita and planted with tomato (Lycopersicon esculentum),eggplant (Solanum melongena) and okra (Abelmuschus esculentus) the nematode populations at harvest had increased by 69% under okra, 40% under tomato and 32% under eggplant, which implies that okra may be a good host, tomato a moderate host and eggplant a poor host of root-gall nematode. Similarly, Ogbuji (1979) reported that seven (7) cultivars of Bambara groundnut Voandzier subterranes tested for susceptibility were observed to be good host to Meloidogyne javanica and Meloidogyne incognita.
These alternations grossly affect nutrient partitioning and water uptake in the host. Mostly modern tomato varieties carry a single dominant gene called MI. This gene confers resistance to three of the most damaging species of the root-knot nematode (Meloidogyne spp). This gene has been a classic example of the use of host resistance to reduce the need for pesticide application (Medina-Filho and Tanksley, 1983). According to Roberts et al (1986). MI was introduced into cultivated tomato (Lycopersicon esculentum) from its wild relative L. peruvianum  in the early 1940s (Smith, 1944) with the assistance of linked markers, beginning with isozyme marker Aps-1 and more recently with DNA markes such as Rex-1,Mi has been incorporated into many modern tomato cultivars (Medina-Filho and Tanksley, 1983) (Williamson et al 1994a).
Microscopic studies have provided some information on the mechanism of resistance (Dropkin, 1969).(Paulson and Webster,1972, Ho et al 1992) nematodes are attracted and penetrate roots and they then migrate to the feeding site in a similar manner in resistant plants where there is no development in the feeding site. Instead, a localized tissue necrosis or hypersensitive response (HR) occurs at or near the feeding site where feeding normally would be initiated.
Nematodes that fail to establish feeding sites either dies or leave the roots. Resistance to diverse pathogens including viruses, bacteria, fungi and nematodes has been shown genetically to be mediated by single dominant resistance gene (R-genes) in the host that are effective only if an avirulence gene is present in the pathogen (Flor 1955, Keen 1990) such gene-for-gene interactions are characterized by commonalities including the presence of an (HR) (Hammond-Kosack and Jones,1996). Recently, R-genes have been cloned from several different plant species (Bent 1996, Dangl and Holub 1997, Hammond-Kosack and Jones 1997)
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