ABSTRACT
These studies on evaluation of cassava cultivars and intercropping with legumes as an integrated nematode control strategy in cassava production were carried out as greenhouse/microplot and field experiments. Of the 200 cultivars screened for resistance to Meloidogyne spp., only 76 survived and from this lot only 3 lines 77/227, TMS30572 and 73/222, showed any resistance while 7 others namely NR8082, 75.668, 73/238 (Egbenegbe), 73/295, 73/118, 75740(Panya), 82/00661 and 73295B were tolerant. All the other cultivars tested were susceptible. The second experiment, to determine the economic threshold level of the root-knot nematode M. javanica populations on two cassava varieties, showed that, with the exception of the control plants, the root-knot nematode damage was obtained at all levels of the inoculum used and the root-knot nematode populations had no significant effect on the aerial growth parameters like leaf and stake weights. The two cassava varieties differed significantly in the final nematode population in their roots. The economic threshold level was identified as an initial density of 500eggs/plant. The lower pest values obtained for TMS30572 with increase in inoculum density further confirmed it to be resistant to M. javanica. Intercropping four cassava varieties with four legume types in microplots revealed that nematode at both levels used, (0 and 1,000 eggs/plant) had no significant effect on growth and yield of the cassava but amongst sole and legume-intercropped cassava cultivars, significant differences were obtained. The feeder roots and stake/stem weights of the sole cassava varieties were significantly higher (p>0.05) than their intercropped mixtures with legumes. The highest tuber yield values were observed for NR8083 planted sole (630g/plant) followed by TMS30572 intercropped with bambaranut (537g/plant). Nematode-treated plants were significantly affected when they were destructively sampled for damage symptoms on the cassava. Though no significant effect was observed on yield, the significant nematode damage effects on the cassava indicate that
the root-knot nematodes affect the outward appearance, the shelf life and storability of the cassava products. The productivity of the legumes under cassava was significantly different (p<0.05) especially on the aerial plant growth. Generally, the legume leaf, stake and feeder root weights (with a few exceptions) had higher sole values than their intercropped components. The same pattern was evidenced for the number of root nodules, where the sole legume number of nodules was higher than that of the intercrops. Significant legume yield differences (p<0.05) were obtained between NR8082-groundnut (60.62g/plant) and NR8082-bambarranut (17.2g/plant). The observed trend was that the intercropped legumes were lower in seed yield than their corresponding sole components. The number of root nodules had a direct linear relation with legume seed yield. However nematode treatment had no significant effect on the legume growth values. Legumes in intercrop mixtures had lower damage symptoms than their corresponding sole crops and sole groundnut was higher in infection scores right through than its intercropped legume cassava mixture. Notably, significant interaction was obtained in number of nematodes in the roots between the legumes in the mixtures and nematode treatment. The legumes in the mixtures significantly (p<0.05) reduced the damage effect of nematode treatment. The results from the two field sites followed the same trend as those obtained in the microplots. However, for bambaranut its intercrop variety for NR8083, TMS50395 and TMS30572 yielded more than the sole component. NR8083 cassava intercropped groundnut had the highest Land Equivalent Ratio (LER) of 1.811 in Site 1, followed by TMS50395 intercropped groundnut had highest LER (1.626) in Site 2.
In line with the goal of Integrated Pest Management (IPM), integrating use of resistant cassava cultivars and intercropping with legumes has been identified as a successful IPM package for our farmers.
INTRODUCTION
Recognition of the importance of cassava as a vital food staple across Africa from results of collaborative studies, conducted over several years have also brought with it concerns over the declining yields of cassava due to pests and diseases; and have necessitated increasing efforts to eliminate the constraints (Okogbenin, et al., 2007; Njoku, et al., 2009; Egesi, et al., 2009; 2010). However, such efforts have hitherto been embarked upon without proper and accurate assessments of pest and diseases, crop potentials, symptoms of pest damage and feasible and sustainable pest control strategies. And this is where, according to Onyenobi (2000), integrated pest management (IPM) has
come into play as the most effective and safest strategy in pest and disease control in maximizing crop protection.
Cassava, Manihot esculenta Crantz is a major tropical root crop found throughout the tropics (Asia, Africa, Oceania and Latin America ) and is vital to the subsistence economies and food security of many less developed nations (Hillocks and Wydra, 2002). It is one of the most important crops in tropical Africa and provides over 50% of the energy requirement for over 300 million people in the continent (Njoku et al., 2009). It has extremely high efficiency for caloric production over a wide range of ecological conditions, particularly on poor soils and with few inputs. It can be stored up to 3 years in the ground, serving as starvation insurance for the small farmer when other crops fail (Luc et al., 1990; Dahinya, 1994; Hillocks, 2002).
More cassava is now being produced in Africa than in South America where the crop originated. In many parts of Africa, several tons of cassava leaves and tender shoots are harvested and consumed as vegetables providing protein (26-41% crude protein on dry weight basis), vitamins and minerals (Hahn, 1984). Cassava has in recent years transformed from famine reserve commodity and rural staple to a cash crop in Africa.
Africa contributes to more than half of global supply, with Nigeria recording more than a third of African production. Nigeria is now the world’s largest cassava producer and its cassava transformation is the most advanced in Africa (Egesi et al.,
2006; 2010). The scope for increasing the use of cassava in industries and export market is determined by the development of improved varieties with high traits of protein, beta- carotene; delayed post-harvest physiological deterioration (PPD) with resistance to pests
and diseases. Thus cassava production has been on the increase as a result of availability of new improved varieties (Egesi et al., 2006).
Cassava is also used as a source of ethanol for fuel, energy in livestock feeds and its conversion to starch in industry is increasing. The crop is amenable to agronomic and genetic improvement and has high yielding potential under good conditions and performs better than other crops under sub-optimal conditions (Anon, 1990).
For a long time, cassava languished as a low research priority crop. However development in the past thirty years has enhanced interest in cassava and now, priority has been given to research on its improvement, increased production and utilization (Dixon et al., 1993; Coyne 1994; Talwana et al.. 1996, 1997; Eke-Okoro et al., 1999; Hillocks, 2002; Okparah and Baiyeri; 2006; Akinpelu et al., 2006; Anyaegbunam et al.,
2008; Mbah et al., 2009).
Among the various constraints facing cassava production today are the plant- parasitic nematodes. As many as 45 nematode species have been reported to be associated with cassava in many different geographical areas but only few have caused economic damage to the crop (Caveness, 1980, 1981; Bridge et al., 1991; Coyne, 1994). The plant-parasitic nematodes most frequently associated with cassava are Meloidogyne incognita, M. javanica, M. arenaria, M.hapla, Pratylenchus brachyrus, Rotylenchus reniformis, Helicotylenchus erythrinae, H.dihystera, and Sculellonema bradys (Tanaka et al. 1979; Luc et al., 1990). Of the above, the most widely reported parasitic nematodes on cassava are the root-knot nematodes Meloidogyne spp. (Caveness, 1979, 1982, Sikora et al., 1988) and the most important species are M. incognita (Kofoid and White) Chitwood and M. javanica (Threub) Chitwood; (Ponte et al., 1980; Luc et. al., 1990; Bridge et al.,
1991). They produce galls on the cassava roots, devitalize the root tips of infected plants and halt their growth. There is a wide range of susceptibility to galling between cultivars ranging from resistance to high susceptibility (Ponte et al., 1980; Nwauzor and Nwankwo, 1989). The most widespread and severe galling due to root-knot nematode was reported from Uganda (Bridge et al, 1991) where 94% of 88 fields examined were affected and 17% of the damaged roots were in the severe category (Coyne and Namaganda, 1994). Direct effects of Meloidogyne spp. on cassava root-yield have been difficult to demonstrate (Talwana et al., 1996). In Uganda, yields were found to be consistently lower in fields with greater root-knot nematode damage and, for two of the more susceptible cultivars Bukalassa 11 and TMS 30337, yields were decreased by 24-
38% (Coyne, 1994). In Nigeria, root-knot nematodes cause galls which exceed 1cm. diameter on susceptible cultivars. In severe attacks the feeder roots are greatly reduced causing stunting and decrease in stem diameter to which losses of root yields of 17-50% have been attributed (Therbege, 1985).
Severe galling by the root-knot nematodes on cassava also causes reductions in plant height and weight which also decrease the quality and quantity of planting material (Caveness, 1980, 1981). One of the greatest effects of root-knot nematodes on the storability of the cassava roots was reported by Caveness (1982) in which post-harvest losses of up to 87% were obtained due to rapid deterioration under severe attack.
Control measures in Latin America, using soil fumigation, have resulted in yield increases but such measures are expensive and inappropriate and would rarely provide an economic return (Ponte and Franco, 1981). Also Hillocks and Wydra (2002) insisted that root-knot nematode infections can be best controlled by using resistant cultivars and crop rotation to avoid excessive nematode population increases. Following the successful biological control of the cassava mealy bug Phenacoccus manihoti with its natural enemy the predatory Epidinocarsis lopezi ( Herren, 1994; Fabres et al.,1994), measures advocated for cassava pests are being focused on Integrated Pest Management (IPM) strategy.
Nematodes as constraints in cassava production have been highlighted but have not received adequate research attention (Hillocks, 2002). The reasons for this may be partly due to the fact that (a) nematode damage and effects go regularly unnoticed on the crop as they affect mostly the feeder roots. This is especially so because of the naturally knobby and rough texture of the roots which can disguise nematode damage and the long duration over which cassava is left in the ground resulting in nematode-infected root systems decomposing in the ground or are not exposed at all at harvest (Caveness, 1982; Talwana et al., 1997; Coyne, 1994). (b) Researches in cassava nematology, are limited and work done so far have tried to address issues of screening a few varieties (and not a wide germplasm) and classifying the reactions of cassava to root-knot infections (Nwauzor and Ihediwa, 1992; Talwana 1996, 1997). (c) Pest control work entails knowing the plant pathogens to be able to manage them (Rossel, 1991). Nematode problems develop with time following the introduction of a new crop or cultivar into a region (Noe and Sikora, 1990). Severe nematode problems can be avoided by studying the effect of ecological factors on plant and pest distribution.
In addition research is now evolving from a strict commodity approach towards an ecosystem approach in which the center of attention is not a particular crop but a system with a variety of crops and the analysis of constraints and manipulations
necessary to bring about changes that are in harmony with the environment and bring optimum and sustainable yields (Onyenobi, 2000).
The objectives of this study therefore, were to:
(i) screen and evaluate the cassava germplasm for resistance to root-knot nematode, Meloidogyne javanica.
(ii) determine the economic threshold level of the root-knot nematode.
(iii) assess the contribution of resistant varieties and intercropped legume types as an integrated nematode pest control strategy.
(iv) investigate the outcome and contribution (if any) of the legumes to soil fertility; and
(v) estimate the productivity of the cassava varieties and their intercropped components.
This material content is developed to serve as a GUIDE for students to conduct academic research
EVALUATION OF CASSAVA CULTIVARS AND INTERCROPPING WITH LEGUMES AS AN INTEGRATED NEMATODE PEST CONTROL STRATEGY IN CASSAVA(MANIHOT ESCULENTA CRANTZ) PRODUCTION
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