MATERIALS AND METHODS

In this study, a total of 7 factors were hypothesized to influence occurrence, abundance and distribution of plant parasitic nematodes. These factors included; historical background, soil texture, Agro-Ecological Zone (AEZ), altitude, variety, crop cycle and crop management practices. A total of 81 samples were collected from randomly selected farms in four sugarcane production zones namely Nzoia, Mumias, Busia and West Kenya. A total of 20 sub-samples were taken from each farm and thoroughly mixed before drawing a composite sub-sample of about 1 kg which was transported to the laboratory. Extraction was done following the modified Baermann funnel technique described by Hooper et al. (2005). The effect of three soil textural classes, clay, clay loam and sandy clay on nematodes was evaluated. The influence of climatic factors on nematodes in the region was assessed by sampling three Agro-Ecological Zones (AEZs) namely lower midlands 2 (LM2), upper midlands 2 and 3 (UM2 and UM3) as described by Jaetzold and Schmidt (1983). The effect of the crop cycle was evaluated by taking samples from the plant crop, first ratoon, second ratoon, third and more ratoon crops. Anthropogenic factors were evaluated by assessing two sugarcane management practices; the management practices employed by the factories in the nucleus estates and that of the out-grower farmers. Factory managed nucleus estate farms were characterized by incorporation of organic substrates in the form of sugarcane baggasse and filtermud as opposed to farmer managed out-grower farms where no organic substrates were applied. All data collected were log transformed and subjected to General Linear Model and means separated by Duncan`s Multiple Range Test using SAS Release 8.1.

RESULTS

Mean population densities of plant-parasitic nematodes associated with sugarcane were variable (p≤0.05) among the four sugarcane zones in Western Kenya (Table 1). Among all zones, Nzoia was the most heavily infested followed by Mumias, Busia and West Kenya in decreasing order of infestation. The most dominant nematodes detected were of the genera Pratylenchus, Scutellonema and Meloidogyne with overall mean numbers of 61, 54 and 39 in 200 cm³ soil, respectively. Nematodes of the genera Rotylenchus, Aphelenchoides, Tylenchus and Helicotylenchus, showed moderate occurrence as compared to Paratylenchus, Hoplolaimus, Ditylenchus and Xiphinema amongst others whose occurrence was low in the sugarcane growing zones.

Soil texture had a significant effect on the distribution of plant-parasitic nematodes in the sugarcane zones studied (Table 2). Generally, sandy soils harboured more plant-parasitic nematodes compared to clay loam and clay soils which had lower nematode populations. Nematodes of the genera Pratylenchus, Scutellonema and Tylenchus were predominantly higher in sandy soils while Hopololaimus spp. and Tylenchorhynchus spp. were significantly higher in clay-loamy soils. Clay soils were significantly dominated by Ditylenchus spp., Paratylenchus spp. and Rotylenchus spp.

Table 1:	Mean population densities of plant-parasitic nematodes associated with sugarcane crop in Nzoia, Mumias, West Kenya and Busia in western Kenya zones
*Significant at p≤0.05, **Significant at p≤0.01, NS: Not Significant, Means followed by the same letter(s) along rows are not significantly different

Table 2:	Effect of soil texture on the distribution of plant-parasitic nematodes associated with sugarcane in Western Kenya
*Significant at p≤0.05, **Significant at p≤0.01, NS: Not Significant, Means followed by the same letter(s) along rows are not significantly different

Table 3:	Effect of sugarcane crop cycle on the density of plant-parasitic nematodes in Western Kenya
*Significant at p≤0.05; **Significant at p≤0.01; NS: Not Significant, Means followed by the same letter(s) along rows are not significantly different

The abundance of plant-parasitic nematodes was significantly influenced by the crop cycle (Table 3). Densities of nematodes of the genera Pratylenchus, Ditylenchus and Tylenchorhynchus steadily increased in the first two cycles of the crop (ratoon 1 and 2) followed by a rapid decline in the third and subsequent crop cycles. The populations of all other plant-parasitic genera that are frequently associated with sugarcane remained constant throughout the cropping cycles. Sugarcane management practices led to significant differences in numbers of plant parasitic nematodes associated with sugarcane in the study area (Table 4). Populations of plant-parasitic nematodes were significantly higher in out-grower farms compared to factory managed nucleus estates. With the exception of nematodes of the genus Hoplolaimus, numbers of nematodes of the other genera were higher in out-grower farms compared to factory-managed farms.

Table 4:	Effect of management practices on the distribution of plant-parasitic nematodes in Western Kenya
*Significant at p≤0.05; **Significant at p≤0.01; NS: Not Significant, Data are means of 81 samples. Means followed by the same letter(s) along rows are not significantly different

DISCUSSION

This study has demonstrated that nematodes of the genera Pratylenchus, Scutellonema and Meloidogyne are widespread in sugarcane fields of Western Kenya. These findings are in agreement with previous studies elsewhere (Cadet and Spaull, 2003; Cadet et al., 2003). Sugarcane is normally grown in a continuous monoculture with usually no more than a few months break between removing the last ratoon crop and replanting the field. This practice favours the development of relatively large number of some species, especially those that are highly pathogenic to sugarcane crop like Pratylenchus spp and Meloidogyne spp. (Mazhar et al., 2002). Previous studies on different crops have shown that root-knot and lesion nematodes are widespread and occur in large numbers in western Kenya (Desaeger and Rao, 2000). Lesion nematodes are widely recognized as pests of cereals while root-knot nematodes are devastating on leguminous and vegetable crops (Arim et al., 2006).

Nzoia sugar zone was more heavily infested with nematodes compared to the other zones within the study site. Part of Nzoia lies in marginal sugarcane areas (UM2 and UM3) as opposed to the entire Mumias zone which lies within the ideal sugarcane zone in LM1 (Jaetzold and Schmidt, 1983). It is conceivable that the conditions experienced in marginal sugarcane areas are more favourable for multiplication of nematodes and hence the high density observed in Nzoia as compared to Mumias Scheme. In addition, unthrifty crops growing in marginal areas are more vulnerable to attack by nematodes (Walker, 2004). Busia zone is the most recently established sugarcane production zone so that the nematode numbers are yet to build up in the fields. In the West Kenya zone, sugarcane is established using cane tops, this practice combined with the long period the crop stays in the field, is thought to cause accelerated yield decline. Ploughing the sugarcane under after only one ratoon crop may be a viable option to manage nematodes (Agyarko and Asante, 2005). The frequent disruptions of the nematode cycles may have limited their multiplication leading to the less numbers observed (Desaeger and Rao, 2000). Sugarcane in West Kenya and Busia is produced wholly by out-grower farmers where extended fallow periods between subsequent crops are common. According to Agyarko and Asante (2005) periodic fallowing has contributed to the reduced numbers of nematodes.

This study has also shown that soil texture is a major factor that influences abundance and distribution of plant parasitic nematodes. The higher numbers of plant-parasitic nematodes associated with sandy soils could be attributed to high porosity and aeration that favour nematode mobility. The poor drainage associated with clay soils may explain the low numbers of nematodes recovered from these soils. Similar studies have revealed that the effect of soil texture on the distribution of some of the nematodes is more than that of climatic or topographic factors (Mateille et al., 2002; Cadet et al., 2003). According to Cadet and Spaull (2003), it is the quantity of sand and organic matter in soil that appears to affect the distribution of nematodes associated with sugarcane. It has been suggested that sandy soils enable easy movement of nematodes and provide optimum aeration which leads to increased pathogenicity. The negative effect of nematodes on plants is aggravated by the fact that sandy soils have a lower water and nutrient holding capacity (Avendaño et al., 2004).

Generally, higher nematode numbers were recorded in the out-grower farms as compared to the factory-managed nucleus estates. Bagasse and filter press mud are applied in nucleus estates on a routine basis and may account for nematode suppression as well as increase in sugarcane yields (Akhtar and Malik, 2000). According to Maqbool et al. (2001), the quality of planting material, especially the varieties, may be a contributing factor to the differences that occurred between the two management types. Indeed in Western Kenya, factory-managed farms use certified seed that is well treated and most likely free from nematodes as opposed to the individual farmers.

This study showed that numbers of plant parasitic nematodes increased steadily in the ratoon crops and then declined after the third ratoon. Increase in nematode numbers could be due to multiplication of the nematodes as a result of continuous presence of susceptible crops (Kratochvil et al., 2004). According to Quader et al. (2001) populations of plant parasitic nematodes were continuously building up in Australasian vineyard fields mainly due to the perennial nature of the crop. Loss of plant growth vigour coupled with competition among the nematodes, for limited reserves could explain the decline (Akhtar and Malik, 2000). In addition, the subsequent build-up of microorganisms that are antagonistic to nematodes partly results in their decline after the third ratoon (Kiewnick and Sikora, 2004; Tiyagi and Shamim, 2004).

CONCLUSION

Sugarcane fields in the western sugarcane zones of Kenya were heavily infested with plant parasitic nematodes with the predominant ones being in the genera Pratylenchus, Scutellonema and Meloidogyne. The marginal sugarcane zone in Nzoia is more heavily infested with nematodes than the other zones. Sandy soils are more suitable for plant parasitic nematodes and application of organic amendments in the form of sugarcane bagasse and filter mud could account for the lower numbers of plant parasitic nematodes in sugarcane fields under the management of sugarcane factories. More work is required to determine the economic importance of the nematodes associated with sugarcane in Kenya.

ACKNOWLEDGMENT

The Kenya Sugar Research Foundation (KESREF) is acknowledged for funding this study.