Evaluation of Cassava ( Manihot Esculentum Crantz) Genotypes in Nsukka Agro-Ecology of Southeastern Nigeria

Field experiments were conducted during the 2008 and 2009 cropping seasons to evaluate 15 new cassava genotypes and one local check (Okwoko) for adaptation to Nsukka agro-ecology in the face of global climate change. The experiments were sited at the research farm of the Faculty of Agriculture, University of Nigeria, Nsukka. Pre-planting soil samples were collected at 0 – 15 cm depth in each experimental unit after land preparation and subjected to routine chemical analysis. The new cassava genotypes were obtained from the National Root Crops Research Institute, Umudike, Abia State, Nigeria while the local (Okwoko) was obtained from the farmer’s field in Obukpa, near the University of Nigeria. The experiment was a randomized complete block design (RCBD) with three replications per trial. The cassava sticks were cut 30 cm long and planted at a spacing of 1 m apart. Data were collected on survival count, plant height, number of branches, tuber weight and disease incidence. Highest stem yield (780 bundles/ha) was obtained from TMS01/1368 while the lowest stem yield (320 bundles/ha) came from NR03/1555 in 2008 and 2009. Tuber yield (t/ha) was significantly (P ≤ 0 .05) lowest with the local genotype in both years. Among the improved genotypes, TMS98/2132 significantly (P ≤ 0 .05) recorded the highest root yield (42.04 t/ha) in 2008 and (43.50 t/ha) in 2009. Cassava mosaic disease (CMD) incidence was significantly higher than root rot among the genotypes. Keywords : Germplasm, cassava, evaluation, environment


INTRODUCTION
Cassava (Manihot esculantus) belonging to the family of Euphorbiaceae is one of the important root crops in West and Central Africa. Although the crop is grown in every country of the African continent, cultivation is concentrated in the humid tropical regions (Nweke, 1996). In Africa generally, food production is fundamentally based on rainfed farming systems and is therefore faced with inherent risks resulting to marked variations in seasonal and annual food supply. Cassava crops can relatively adapt to marginal soils and erratic rainfall conditions compared with other crops and have the capability of maintaining continuity of supply throughout the year. The need to increase food production is always a priority in Africa.
To feed the ever increasing urban population in Nigeria, food supply from every farm household has to increase by at least 63% in 10 years (Sanni et al., 2009). Cassava is a food security crop worldwide (Nweke, 2003) because of its ability to grow under a wide range of conditions, some of which are quite unsuitable for other crops. Farmers prefer the improved varieties of cassava because of their higher yields, earlier maturity and high suppression of weed and greater resistance to diverse diseases and pests (Akoroda et al., 1985(Akoroda et al., , 1987Ikpi et al., 1986). Research in cassava has witnessed tremendous progress with the development of improved varieties of the crop and accompanying improved management recommendations by IITA, Ibadan and NRCRI, Umudike.
Events, particularly flooding, excessive rise in temperature, frequent dry spells, desert encroachment, resurgences of new pests and diseases among others are evidences of climate change which invariably affect establishment and development of crop in a region (Adesina et al, 2007;UmuBig and Camer, 2007). Vulnerability to climate change refers to the degree in which a system or region is susceptible to, or unable to cope with adverse effects of climate change, including climate variability and extremes (Leary et al, 2008). Against this background, this research was designed to determine the adaptability potentials of new cassava genotypes to Nsukka agro-ecology in response to the global climate change. Thus, the specific objective was to evaluate the agronomic parameters of 15 cassava genotypes and one local check for adaptation to Nsukka agro-ecology

MATERIALS AND DMETHODS
Experimental sites: Field experiments were conducted in 2008 and 2009 in the rainforest zone of South Eastern Nigeria. The area is located by latitude 6 0 52 1 N and longitude 7 0 23 1 E, altitude 400 m above sea level and has a humid tropical climate. The mean annual rainfall ranges from 1600 to 2000 mm. The temperature is uniformly high throughout the year but the annual mean maximum temperature does not exceed 35 0 C (Asadu, 1990). The soil was derived from falsebedded sandstone parent material. It is sandy loam and has been classified as Typic Kandpaleustult or Dystric Nitosol, belonging to Nkpologu series (Nwadialo, 1989) and the vegetation has been described as derived savannah. The experiments were sited in the same location but in different fields for the two years. The first experiment was on 12 th April 2008 while the second was 3 rd March 2009. Land was prepared each year by disc ploughing, harrowing and ridging to obtain a smooth seed bed.
Soil sampling and analysis: Pre-planting soil samples were collected at 0-15 cm depth in each treatment plot one week after land preparation. Soil samples were air-dried and sieved using 2 mm sieve. The samples were subjected to routine chemical analysis as described by Tel (1984).
Diseases assessment: Cassava mosaic disease (CMD) was characterized by a pathologist using known observable symptoms. The visual assessment was done using a 5 point hedonic scale where: 0 = No sign of infection or symptom; 1 = mild distortion at the base of the leaflet only 2= moderate narrowing and distortion of the 1-3 of the leaflet 3 = severe mosaic distortion of the 2-3 of the leaflets and general reduction of leaf size 4 = severe mosaic distortion of the entire leaf The assessments were done at intervals of 1, 3 and 6 months after planting and the average was calculated and used as the disease parameter.
Similarly, root-rot was determined based on the evidence of rot on the harvested tubers using visual assessment where: 0 = No root-rot 1 = < 25% root-rot 2 = 26 -50% root-rot 3 = 51-75% 4 = 76% and above Data collection: All data were collected from the middle rows containing six plants. Data were collected on the survival counts, plant height, number of branches, tuber weights, tuber length, presence or absence of disease and severity. Statistical data analysis: The data were analyzed by one way analysis of variance test using Statistical Analysis System (SAS) package. Treatment means were compared by the Least Significant Difference technique (Steel & Terrie, 1980) at P ≤ 0.05 levels. Cluster analysis was performed to group the genotypes into characteristic similarities. Disease incidences and severities were graphically presented using excel chart wizard.

RESULTS
Pre-planting soil properties showed that the soils of the study area were slightly acidic, moderate in organic matter content and low in Nitrogen content (Table 1). The meteorological data for the period of studies in the area are summarized in Table 2. Rainfall stability was earlier and also higher in intensity in the year 2009 than 2008. Soil temperature was higher in 2008 than in 2009 from January to April and very low in May, 2008. Wind speed was very high in 2009 and maintained higher speed rate from January to December than what was obtained in 2008.

Disease parameters:
Cassava mosaic disease (CMD) incidences were observed in all the 16 genotypes though with varying levels of infection while root rot disease was totally absent in some genotypes (Fig. 1).The Local genotype appeared to be more susceptible to both CMD and root rot diseases among all the genotypes evaluated. Incidence of CMD was mild among the improved genotypes except TMS30572 and TMS01/1368 where it was severe whereas root rot was totally absent.

Diseases parameters:
Cassava is known to be attacked by many diseases but only cassava mosaic disease (CMD) and cassava root rot (CRR) were assessed in this study because of their prevalence in the study area. All the genotypes evaluated in this study were attacked by the CMD but the attack did not inflict any noticeable yield reduction especially on the majority of the improved genotypes. This result is similar to the observation by Nweke et al (1999) that CMD and CRR did not seem to have adverse effects on yield because the improved varieties which were widely grown had become more tolerant of the diseases than the local land races. The less tolerant improved genotypes in this study are likely to have similar characteristics with the local land race and this is confirmed by the cluster analysis. The inability of the improved genoytpes to tolerate these diseases could probably be part of the reasons for the consistent low yield maintained by few improved genotypes and the local land race in the two cropping seasons.

CONCLUSION
All the genotypes evaluated in this study expressed distinct genetic capability in terms of growth, disease tolerance, components of yield and actual root yield. Root yield of TMS98/2132 was consistently the highest in the two years of evaluation. All other genotypes manifested change-order interaction with respect to root yield. However, the root yield of CR12-45 and TMS01/1371 were high with slight rank-order change across the two years. Averaged over two years, the highest root yielder in order of importance were TMS98/2132, CR12-45, and TMS01/1371; suggesting that these genotypes were the most adaptable to Nsukka agroecology. It was also apparent in this study that seasonal differences in environmental factors such as rainfall (timing, amount and intensity), temperature, wind speed and total radiation probably caused variability in yield parameters of cassava genotypes during the two-year evaluation period as reported in this study.