1.2 Nematodes of maize
1.2.2 Genus Meloidogyne Goeldi, 1887
Root-knot nematodes are obligate plant parasites consisting of more than 50 species world- wide (Sasser, 1977). Within species, several races with differential host ranges occur (Sasser and Triantaphyllou, 1977; Kleynhans, 1991). They belong to the order Tylenchida and family Heteroderidae (Heyns, 1971; Dropkin, 1989). The young vermiform larvae penetrate the roots of plants near the growing points, migrate towards the stele and become sessile inside the root as adult females.
Meloidogyne spp. comprises of adult females, adult males and juveniles (Fig. 1.3). Adult females are flask-shaped, sedentary nematodes, embedded inside the root (Dropkin, 1989).
The adult females are about 0.5 mm long and 0.3-0.4 mm wide. They are characterized by a
7 distinctive pattern of striations surrounding the vulva and anus (perineal pattern), which is used for species identification (Heyns, 1971; Dropkin, 1989). The adult males are elongate and move about slowly in the soil/root. They vary in length, with a maximum of up to 2 mm, and with a length/width ratio close to 45. The head is not offset, and the stylet is almost twice as long as that of the female (Heyns, 1971; Dropkin, 1989). The male tail is short and rounded. Juveniles resemble those of Heterodera except that they are more delicate, with a shorter and thinner stylet (Heyns, 1971).
Figure 1.3: Meloidogyne spp. A-G. A. Entire female body. B. Perineal pattern. C. Second stage larva. D. Anterior end of larvae. E. Larval tail. F. Entire male body. G. Male head. H.
Male tail. (Williams, 1972).
According to Dropkin (1989), most Meloidogyne spp. reproduce without males though both sexes are necessary in other species. Eggs are deposited into a gelatinous egg sac that probably protects them from desiccation and perhaps from micro-organisms. In most host- parasite combinations, a gall is formed from which an egg sac usually protrudes. The embryo develops into a juvenile that moults once within the egg. Second-stage juveniles hatch under favourable temperature and moisture conditions and move through soil towards growing root tips. Once within a root, a juvenile moves between cells until it locates a site close to the stele, often in the area of a developing side root (Dropkin, 1989). Here, it becomes sedentary and causes the transformation of cells upon which it feeds. The juvenile swells, moults rapidly a second and third time without feeding, and matures into an adult male or female (Dropkin, 1989). Adult males, which may aggregate within a single egg sac, elongate within the fourth-stage cuticle and emerge from roots. The adult female remains attached to its feeding site within the stele and with its posterior at the root surface (Dropkin, 1989). It continues to produce eggs throughout its life, sometimes reaching a total of up to 1 000 eggs (Barker et al., 1985; Dropkin, 1989). The life cycle may be as short as 3 weeks and as long as several months depending on the host and temperature (Taylor and Sasser,
8 1978). The sex ratio is also influenced by the environment, with more males developing when roots are heavily attacked or nutrition is inadequate (Dropkin, 1989).
Histologically, M. javanica infection of maize roots shows typical multinucleated giant cell development in vascular tissue as well as embedded egg masses in inconspicuous galls, mostly close to root apexes (Asmus et al., 2000). Staining of maize root systems is advisable to be able to assess nematode penetration if root-knot nematodes are suspected or juveniles are detected in the soil (McDonald and Nicol, 2005). Root tip galls can, however, be confused with galls produced by ectoparasites like Xiphinema. Riekert (1995) modified the sodium hypochlorite (NaOCl) extraction technique specifically for root-knot nematode assessment on maize though gall indices and other staining methods can still be used.
Root-knot nematodes are widely distributed and are of economic importance on most crops in the world including weeds (Sasser, 1977; Meyer and Van Wyk, 1989). On maize, several species of Meloidogyne have been reported from around the world. Meloidogyne incognita and M. javanica have been reported to damage maize in almost all maize-growing regions of the world (McDonald and Nicol, 2005). Meloidogyne africana and M. arenaria have been recorded on maize in India (Krishnamurphy and Elias, 1967) and Pakistan (Maqbool, 1980).
Meloidogyne arenaria has been reported in the USA on maize (Ibrahim et al., 1993).
However, despite the occurrence of root-knot nematodes in maize fields, information on their importance and the economic losses they cause is scarce. According to Dickson and McSorley (1990), failure for maize to exhibit yield reduction due to nematode parasitism is a result of extensive root growth in this crop after the seedling stage. This results from the high fertilization and irrigation levels applied to this crop in commercial settings, hence obscuring measurable injury levels. Nonetheless, it is important to be alert to root-knot nematode infestation of maize, particularly in low input production conditions.
Above-ground symptoms of Meloidogyne spp. damage on maize include stunting, leaf chlorosis, wilting and patchy growth. Root galls may be small or large, terminal or sub- terminal or further back along the root (McDonald and Nicol, 2005). Typical gall symptoms may be totally absent (Idowu, 1981; Riekert, 1995; Asmus et al., 2000), leading to maize being mistakenly considered a poor host or even immune to root-knot nematodes. However, according to Koenning et al. (1999), lack of adequate nematode control measures on maize has been the major reason for ignorance of nematode damage on this crop. In Jamaica, greater root-knot damage occurred when maize was sown after sugarcane (Hutton, 1976).
Studies on interaction between M. incognita and mosaic virus showed that nematode
9 reproduction was greater when both pathogens were together than when alone (Goswami and Raychaudhuri, 1978).