9 reproduction was greater when both pathogens were together than when alone (Goswami and Raychaudhuri, 1978).
10 Aldicarb has also been reported to be effective in control of Pratylenchus neglectus and P.
thornei when applied at a rate of ≥ 2.5 kg a.i ha-1 on wheat and barley (Taylor et al., 1999).
According to Rhoades (1979), soil fumigants and non-volatile nematicides increased growth and yield of field maize compared to untreated controls in Florida. Actual yield increases were 28% in the first year to 58% in the second year of nematicide application, and were related to the control of sting nematodes (Belonolaimus longicaudatus). Johnson and Dickson (1973) reported that B. longicaudatus could economically be controlled in maize with low rates of several organophosphates and carbamate nematicides incorporated in 38 cm band row treatments just prior to planting. Rhoades (1979) affirmed that application of fenamiphos, carbofuran, aldicarb, and oxamyl in a 38 cm band incorporated with rotary wheels at a rate of 2.2 a.i kg ha-1 just prior to planting controlled B. longicaudatus better than when the chemicals were applied in a 25 cm band in front of the press wheel or in the seed furrow with the planter. Increased plant growth and vigour were observed within a few days after plant emergence in plots treated with nematicides. The nematicide treated plots had dark green, vigorous plants with thick stalks, whereas check plots had small, stunted plants that wilted in the heat of the day (Rhoades, 1979). No evidence of phytotoxicity was observed as a result of applying these nematicides. Norton and Hinz (1976) reported a 26%
increase in corn yields after application of a carbofuran nematicide in sandy soils in Iowa.
Apple (1971) similarly reported 6.9% maize yield increase after carbofuran application.
According to Daynard et al. (1975), carbofuran is not inherently a stimulator of growth traits of corn but that it does control nematodes. Notably, some insecticides have been reported to have nematicidal properties hence indirectly additionally controlling parasitic nematodes in maize fields. For example, Rhoades (1979) reported that Terbufos, presently labelled only as a soil insecticide, provided good nematode control, and eventually resulted in higher yields when applied to maize. Effective chemical control of nematodes in maize could, however, be a useful production management tool when used in integrated nematode management systems (Johnson and Leonard, 1995). It is also recommended to inoculate soil planted to maize with effective nitrogen fixation agents, e.g., Azospirillum spp., to stimulate growth after treatment with nematicides (Fayez, 1990). As maize is a low value crop, it is important to take into consideration the environmental hazards and possible residual effects before any nematicides can be applied (McDonald and De Waele, 1987;
Johnson and Leonard, 1995).
1.3.2 Cultural control
Cultural practices like crop rotation, tillage, planting time, application of organic amendments and sanitation are effective in reducing nematode populations (McDonald and Nicol, 2005).
11 In most crop rotations, however, maize has been erroneously used as a non-host of nematodes yet it is susceptible to various nematode species (Riekert and Henshaw, 1998).
An effective crop rotation should target as many nematode species as possible (McSorley and Gallaher, 1992). Additionally, incorporating longer sequences of resistant crops before planting a susceptible crop is a highly effective crop rotation strategy (Johnson et al., 1999).
Knuth (2000) reported reduction of Pratylenchus spp. by radish as well as French and African marigold in maize-based rotations. However, rotation alone may not be sufficient to prevent subsequent susceptible crops from suffering nematode damage. Additional control strategies such as host plant resistance need to be integrated for effective management of plant-parasitic nematodes (Kinloch and Dunavin, 1993).
Tillage is important since several weed species have been reported to be hosts of a wide range of plant-parasitic nematodes (Salawu and Oyewo, 1999). Weeding of maize plots has been reported to reduce populations of Ditylenchus spp., Heterodera spp. and Tylenchorhynchus clarus (Youssef, 1998). It is highly recommended to combine tillage and rotation systems in nematode management strategies (Cabanillas et al., 1999). Removal of old maize roots during weeding has also been reported as an important contribution to the cultural control of nematodes, which survive on these residues during the dry season (Egunjobi and Bolaji, 1979).
Fertilizer application in combination with early planting has been reported to reduce Punctodera chalcoensis nematodes in Mexico (Sosa-Moss, 1987). Ivezic et al. (1996) obtained up to 60% reduction in nematode populations dominated by Pratylenchus thornei in maize fields after application of potassium fertilizers. Large amounts of yard waste compost were reported to reduce Paratrichodorus minor, Criconemella spp., and Pratylenchus spp. in maize in the USA with a subsequent increase in yield (McSorley and Gallaher, 1996).
However, the organic amendments had no effect on Meloidogyne incognita populations (McSorley and Gallaher, 1996).
1.3.3 Biological control
Parasitism and predation occur throughout nature. There is hence a natural biological control of nematodes wherever life processes are even and moderately sustained (Norton, 1978b).
However, none of these biocontrol agents can be used economically in cereals at present (McDonald and Nicol, 2005). Some of the biological control agents that reduce nematode densities and damage are: viruses, bacteria, fungi, amoeba, sporozoan parasites, mites, predacious nematodes and insects (Norton, 1978b). On maize, only a few of these predators have been tested with most emphasis on fungi (Norton, 1978b).
12 Nematode-trapping fungi exist and have been effective in significant reductions of nematodes in maize. Bourne and Kerry (1999) reported significant reductions in M.
incognita, M. javanica and M. arenaria in maize by the fungus Pochonia chlamydosporia.
Bourne (2001) obtained 50% reductions in M. incognita after application of Pochonia chlamydosporia in rotations with maize and susceptible crops. More than 50% control of Pratylenchus spp. was achieved with application of Paecilomyces lilacinus (Gapasin, 1995).
Other fungi such as Glomus mosseae, Trichoderma pseudokoningii, Pseudomonas fluorescens have been reported to improve maize grain yield by reducing P. zeae damage (Oyekanmi et al., 2007). For example, field plots to which G. mosseae was applied with P.
zeae had an increased root weight, leaf area and grain yield of 34.5%, 21.2% and 31.2%, respectively compared to plots with P. zeae only. Mycorrhizal fungi of the genus Glomus was reported to reduce Meloidogyne chitwoodi juveniles on maize (Estanol-Botello et al., 1999).
Strains of Pseudomonas spp. bacteria have been reported to inhibit invasion by Meloidogyne spp. and Radopholus similis in maize, tomato and banana roots (Aalten et al., 1998).
1.3.4 Host resistance
Host resistance, which is the focus of this study, will be examined closely henceforth. It is the best management option as it is cheap and offers no technical difficulties to the farmer. In maize, pedigree breeding without selecting for nematode resistance may result in highly susceptible and intolerant crops, which could be very costly in any kind of production system (McDonald and Nicol, 2005). However, when intending to select for resistance to nematodes in maize, it is important to ensure that it is not at the expense of other commercially desirable traits (Jordaan and De Waele, 1987; Williams et al., 1990).