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)