ABSTRACT

1. Evaluation of Latin American Cassava Varieties for Adaptation and Productivity in Nigeria Eunice Ekaette1, Emmanuel Okogbenin1*, Olalekan Akinbo1, Chiedozie Egesi1, Favour Ewa1, Ezenwaka Lydia1,Eluwa Kenneth1 and Martin Fregene2 1National Root Crops Research Institute (NRCRI), Umudike, Nigeria; 2Donald Danforth Plant Center, USA; email address: eokogbenin@yahoo.com ABSTRACT Cassava, valued most for its starchy storage root, is an important food and economic crop in Nigeria. A principal constraint to breeding for improved productivity in Nigeria is the narrow genetic base identified in elite cassava varieties released in Nigeria in the 80s and 90s. Early initiatives to introduce Latin American (LA) germplasm to Africa and Nigeria were severely hampered by its susceptibility to the cassava mosaic disease (CMD) in the 90s. The discovery and genetic mapping of the CMD2 dominant gene has significantly contributed to the efficient transfer of CMD resistance into Latin American cassava. The LA germplasm consisted of those genotypes developed for CMD resistance using CMD2 gene. A total of 35 pre-selected genotypes of Latin American origin from the clonal evaluation stage in the breeding scheme in Nigeria were evaluated for adaptation and productivity based on disease resistance, yield and tuber characteristics in Nigeria. The randomised complete block design was used for the study in Nigeria in 2010 and 2011. Results showed that there was high variation among the clones for fresh root yield, harvest index, dry matter content and starch. The fresh root yield ranged from 24.00 – 33.00 ton ha-1 with CW 482 - 3 having the highest yield (33.74 ton ha-1) while the highest dry matter content was 34.06 % from CR 15B - 7. The analysis of variance showed a significant difference (P ≤ 0.05) among the evaluated clones for fresh root yield at 12 months after planting. The results indicates that the best four LA genotypes were significantly better in root yield than TMS30572, a widely grown improved variety used as check in this trial. A good number of the LA genotypes were found to be relatively well adapted with good performance for biotic and agronomic traits which could explored to further improve African germplasm. The LA germplasm would be more beneficial for enhanced productivity if they represent different heterotic group from the African germplasm to maximize heterosis. INTRODUCTION Cassava’s increasing importance in Nigeria for food, animal feed and industry has spurred rapid expansion of cassava production. However, an array of constraints in combination with the crops long reproductive cycle limits efficient introduction of genes for key useful traits. African cassava varieties could be improved by using cassava germplasm from Latin America (LA), the crop's center of origin, to enhance production and root quality attributes in Africa. However Latin American varieties are highly susceptible to cassava mosaic disease (CMD) which makes it highly unadapted in Africa where CMD endemic and inimical to its production. Following the genetic mapping of the single dominant gene CMD2 the gene has been highly deployed in many parts of Africa including Nigeria with the aid of molecular markers. Markers associated with CMD2 have been used to introgress the gene into LA germplasm (Akanoet al., 2002; Fregeneet al., 2006). A good number of the LA genotypes were found to be relatively well adapted with good performance for biotic and agronomic traits which could be explored to further improve African germplasm. In cassava improvement, the breeders’ interest is to increase the root yield of varieties that are resistant to major diseases and pests (Mahungu, 1987; Ceballoset al., 2004). Root yield is usually assessed visually as the number, size and shape of roots. Root quality and yield related characteristics are usually often considered. The aim of this study was to evaluate a uniform yield trial of Latin American cassava specifically developed for root yield characteristics with a view to increasing the genetic diversity of improved cassava varieties on farmers’ field. MATERIALS AND METHODS The experiment was carried out during the 2010/2011 planting seasons at the western farm of the National Root Crops Research Institute Umudike (NRCRI), which represents the forest transition zone of Nigeria. Selected clones from the clonal evaluation trial of pre-selected 35 genotypes were based on yield and tuber characteristics and planted for the uniform yield trial (UYT). The trial was planted using matured stem cuttings in a plot of 36 plants (6x6) with 1m spacing between plants with 3 replications using the randomized complete block design. The variety TMS 30572 was used as local check in the experiment because of its outstanding agronomic performance and moderate resistance to major pests and diseases. Pre-emergence herbicide as recommended and hand weeding were used to control weeds. NPK 20:10:10 fertilizer was applied at 6 weeks after planting at the rate of 450 kg ha-1 according to the recommendation based on the soil analytical result to supplement the soil nutrients. The germplasm was evaluated for resistance to CMD at 6 weeks after planting (WAP) based on incidence and severity. The other required diseases and pest data such as the cassava bacterial blight (CBB), cassava anthracnose disease (CAD) and cassava green mite (CGM) were taken subsequently at the different growth stages (3, 6, 9 and 12 MAP). At twelve months after planting (12MAP), plants were harvested and evaluated for fresh root yield, dry matter content, and harvest index. Harvest index was measured as the ratio between fresh root weight and the total fresh biomass. Root dry matter content was estimated using the specific gravity methodology (Kawano et al., 1987). Approximately 3 kg of roots was weighed in a hanging scale (WA) and then the sample was weighed with the roots submerged in water (WW). Dry matter content was estimated using the formula: All data generated was subjected to Simple Statistical Analysis and Analysis of variance (ANOVA) using SAS 9.0. Mean with significant difference will be separated using the Least Significant Difference (LSD) method. RESULTS AND DISCUSSION Developed elite breeding clones with good adaptation and useful traits were harvested from the uniform yield trial. The selection into the uniform yield trial was based on CMD resistance, however the LA materials showed good tolerance to the cassava bacterial blight (CBB) and the cassava anthracnose disease with an average score of 2. The CMD disease scores indicated that 90% of the clones were highly resistant while the remaining 10% were resistant (Figure 1). All the LA genotypes evaluated therefore showed good resistance to the disease. The gene designated as CMD2 controls the currently deployed resistance and was the source of resistance used in developing CMD resistance in the LA materials which showed that they are generally better than the check variety (TMS 30572) usually grown in Nigeria. In the harvest evaluation, there was high variation among the clones for each of fresh root yield (FRY), harvest index (HI), dry matter content (DMC) and starch. The summary statistics of the variables are shown in Table 1. The analysis of variance showed a significant difference (P ≥ 0.05) among the evaluated clones for fresh root yield at 12 months after planting (Table 2). Fresh root yield ranged from 24-33 ton ha-1 with CW482-3 having the highest yield. The root yields of these genotypes were comparable to the checks used in the trial. Also the result showed that the highest percentage dry matter content (DMC) was 34.06% and was attained by the genotype CR 15B-7 but there was no significantly difference among the clones and the checks used. Results obtained in this study indicated that the LA germplasm introduced were well adapted because of their resistance to CMD. Some of the yields obtained were higher than the average yield of 21 ton ha-1 for TMS 30572, an improved high yielding clone, generally used as standard check for yield in IITA breeding program. The variety TMS 30572 is moderately resistant and tolerant to CMD. In this study, high yields (20–33.74 ton ha-1 at 12 MAP) (Table 3) were observed in LA clones. The top yielding genotype had very good commercial roots (Figure 2). Important traits of interest for which useful genes are sought from introduced germplasm include increased dry matter content, morphological and agronomic traits, pest and disease resistance. Results showed that 25% had good architecture (Figure 3) with late branching that supports mechanization, and intercropping. Because this type of architecture does not support heavy foliage, population density can be increased beyond 10,000 ha thus enhancing the possibility of these types of genotypes. A systematic and well-coordinated approach is required to exploit fully the true potential of cassava germplasm in LA for the benefit of African cassava. Efficient use and rapid deployment of genetic resources from LA to Africa are key elements in current efforts to meet the flexibility sought for in the crop’s food and industrial attributes by farmers, processors, and end users. Thus, this most versatile of crops can be enabled to provide food security, income, and new commercial opportunities for a growing population. Table 1. Simple statistics of agronomic variables evaluated in Latin American Cassava Clones aStandard deviation; bFresh root yield(ton ha-1) ; cDry root yield(ton ha-1) ; dHarvest index(0-1); eDry matter content(%); fCassava mosaic disease(1-5) Table 2. Analysis of variance for fresh root yield of Latin American cassava clones Table 3. Mean yield of selected Latin American cassava clones and one check evaluated at 12 months after planting aHarvest index (0-1); bFresh root yield (ton ha-1); cDry matter content (%); dDry root yield (ton ha-1) Figure 1. Resistant Latin American cassava genotype(CR100-8) Figure 2. Commercially sized cassava root (CW482-3) with high dry matter content from harvest Figure 3: Frequency distribution of architectural pattern for the Latin American cassava clones CONCLUSION AND PERSPECTIVE The CMD2 gene is facilitating the development of LA germplasm with good attributes to Africa for possible release. Previously, this was not possible. It therefore opens the opportunity to increase the genetic diversity of materials released to farmers. The chances of releasing these introduced elite lines as varieties; their true potential for other desirable traits could be evaluated and used in crosses to effectively transfer these traits to farmer-preferred cassava cultivars in a predictable and timely manner. The deployment of LA germplasm to Africa may possibly lead to the identification of materials representing separate heterotic groups from the African germplasm which may be explored to develop superior hybrids with good agronomic characters. REFERENCES Akano, A., E. 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