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2.3.1.2 Economic Importance and Distribution

Several species of root knot nematodes including M. incognita and M. javanica have been detected at damaging levels in almost all maize growing regions of the world (McDonald and Nicol 2005). M. africana and M. arenaria have been recorded on maize in India and in Pakistan, and M. arenaria has also been reported by several authors from the USA (McDonald and Nicol 2005). Above ground symptoms include stunting, leaf chlorosis and patchy growth. Root galls may be small or large, terminal or sub-terminal or in some cases totally absent. For this reason maize has often mistakenly been considered a poor host or even a non-host for root knot nematodes (McDonald and Nicol 2005). Although root knot nematodes occur frequently in maize fields, information on economic losses is lacking and requires further study. However, indirect observations when nematicides are applied in root knot nematode infected soils, suggest that these nematodes are economically important (Riekert 1996; Riekert and Henshaw 1998). It is important that growers are alert to the possibility of root knot nematode infestation of maize, particularly in low input production conditions (Table 2.2).

Many more reports exist for the lesion nematodes which have a cosmopolitan occurrence in maize fields with several of these species reported to be associated with poor growth and yield reduction (McDonald and Nicol 2005). The most commonly occuring species include Pratylenchus brachyurus, P. zeae and P penetrans in subtropical and tropical regions but many other species have been noted. Lesion nematodes have wide host ranges which can affect the selection of the crop used to control nematodes in rotations. In addition, the presence of weed hosts in a field can strongly influence lesion nematode (and indeed root knot nematode) densities. The nematode species, population density and environmental conditions affect symptom expression of root lesion nematodes and hence the aboveground symptoms are not specific. Nematode damage is associated with lesions to the root as a result of the destruction of cortical parenchyma and epidermis, which may cause sloughing-off of the tissue and severe necrosis (McDonald and Nicol 2005). In addition, severe root pruning as well as proliferation of lateral roots may occur. More definitive yield loss studies have been conducted for root lesion nematodes on maize. Smolik and Evenson (1987) found direct relationships between P. hexincisus and P. scrib-neri and maize yield loss, with P. hexincisus more damaging to dry land maize than P. scribneri to irrigated maize. In Nigeria, P. brachyurus has been reported to be responsible for a 28.5% yield reduction, with this reduction being correlated with a 50% increase in nematode density (Egunjobi 1974). Indirect evidence has been obtained with nematicides where detected yield increases suggest that lesion nematodes are important limiting factors in maize cultivation with yield increases of 33-128% in South Africa, 10-54% in the USA and a two-fold increase in Brazil reported (reviewed by McDonald and Nicol 2005).

The third group of nematodes of importance for maize are the cyst nematodes. Although more than nine species of cyst nematodes have been recorded as being associated with maize in subtropical and tropical countries, only three (Heterodera zeae, H. avenae and Punctodera chalcoensis) are considered economically important (Luc 1986). Heterodera cajani, H. delvii, H. gambiensis, H. graminis, H. ory-zae and H. sorghi have been recorded sporadically, but their role as parasites of maize remains uncertain (reviewed by McDonald and Nicol 2005). As with other nematodes the above ground symptoms are relatively non-specific. H. zeae infested plants exhibit poor growth and are stunted and pale green in colour (Koshy and

Swamp 1971). Punctodera chalcoensis is limited in distribution to Mexico where it has been given the local name of Mexican corn cyst nematode and is considered of extreme importance. The symptoms of P.chalcoensis are the same as those for H. zeae, with the chlorotic leaves also exhibiting pale colour stripes (McDonald and Nicol 2005).

Pathogenicity of the cyst nematode H. zeae, has been demonstrated on maize but data on economic damage to the crop is lacking (Koenning et al. 1999). Plant growth reductions are directly correlated with initial nematode population density and maize growth and yield are suppressed by 13-73% in the presence of H. zeae with this damage more profound under hot and dry conditions (reviewed by McDonald and Nicol 2005). It is also important to note the wide host range of H. zeae and the need to select crop rotations carefully in order to minimize population increase (Ismail 2009). There is limited published information about the importance of H. avenae on maize but this could be very important in wheat & maize production systems as this is a well acknowledged pathogen on wheat. Unlike the other two cyst species the host range of P. chalcoensis is highly restricted with only two plants, Z. mays and Z. mexicana (Teosinte), considered hosts (Stone et al. 1976). Damage by P. chalcoensis can be severe and is dependent on cultivar susceptibility, nematode density and adequate soil moisture levels in the later part of the growing season. Under glasshouse conditions, Sosa-Moss and Gonzales (1973) obtained a reduction of about 60% in yield in heavily infested soils. Although yield loss in the field is considered to be high, experimental data is lacking.

2.3.1.3 Major Methods of Control

Although there are limited groups working on controlling nematodes in maize production systems there have been many local reports identifying resistance against not only the 3 main groups described above but also many other genera and species (reviewed by McDonald and Nicol 2005). Some of these are in commercial varieties but most are within breeding lines and land races.

Cultural practices such as crop rotation, planting time, application of organic amendments and biological control have been tested and in many cases were demonstrated to be effective in reducing various nematode populations but these are genera and species specific (McDonald and Nicol 2005). For example in Mexico early sowing dates and adequate fertilization reduces damage caused to maize by P. chalcoensis (Sosa-Moss and Gonzalez 1973; Sosa-Moss 1987). In terms of rotation maize has been suggested as a good 'rotation crop' that can help reduce populations of some nematodes but little is actually known about the effects of rotation on root knot population densities in a maize crop. Crop rotations or sequences where maize was involved demonstrate the dangers of ineffective crop choices in rotations due to the susceptibility of maize to various species of nematodes (McDonald and Nicol 2005). Biological control has been investigated against several species of plant parasitic nematodes of importance to maize (McDonald and Nicol 2005), with many of these offering potential. However, none of these biocontrol agents can be used economically at the present time in extensive cereal crops.

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