Germplasm of the Tropical Manihot Series (TMS), from the International Institute of Tropical Agricultural (IITA) Nigeria, was introduced into Uganda in 1989 (Otim-Nape, 1993). The main objective of the research was to develop high yielding, early bulk cassava genotypes that combine resistance to CBSD and CMD with traits preferred by farmers for cultivation in Uganda.
Introduction
Agricultural origin of cassava
Cytology and taxonomy of cassava
Based on cyanogenic glycoside, cassava cultivars are classified as bitter and sweet (Nassar and Ortiz, 2006). The glycoside content of a variety may be high under some conditions and low under others.
Genetic diversity in cassava
Cassava is also classified according to its morphological characteristics, such as leaf shape and size, plant height, stem color, petiole length and color, inflorescence and flower color, storage root shape and color, suitability and cyanogenic glycoside content (Nassar and Ortiz, 2006 ). Sweet cultivars are reported to have a short growing season and their storage roots mature early (Nassar and Ortiz, 2006; Amenorpe et al., 2007).
Environmental conditions for cassava growth
Onwueme (1982), however, stated that caution should be exercised in using glycoside levels as a distinguishing characteristic for cassava cultivars, as the exact level of glycosides in a particular variety varies according to the environmental conditions under which the plant grows. .
Growth and development of cassava
Storage root development and bulking
- Storage root bulking process
- Dynamics of storage root bulking
- Storage root number
- Storage root yield
Starch deposition in the first parenchyma cells produced occurs 25 days after planting (Indira and Sinha, 1970; Hunt et al., 1977). The rate of bulk formation of cassava stock root fluctuates over a long period due to changes in the agro-climatic conditions (Ekanayake et al., 1998).
Flowering in cassava
- Cassava pollen grains
- Fruits and seed formation
- Factors that affect hybridisation and seed production
An average of one seed per fruit is commonly obtained through controlled pollination from potential three seeds from a tri-locular ovary (Osiru et al., 1996; Jennings and Iglesias, 2002). According to Osiru et al. 1996), a ripe fruit is a spherical capsule, 1.0 to 1.5 cm in diameter with six narrow longitudinal wings, along which it naturally splits explosively to release the seed.
Mating designs in breeding
- General and specific combining ability
- Gene action
Gene function and effects have been extensively studied in many crop species (Sleper and Poehlman, 2006; Acquaah, 2009; Brown and Caligari, 2009). Four types of gene action are recognized; additive, dominant, epistatic and overdominance (Sleper and Poehlman, 2006; Acquaah, 2009).
Variation of traits
- Genetic variation
- Environmental variation
- Genotype by environment interaction
Genotypic variation is generally divided into two components, which are additive and non-additive components (Falconer and Mackay, 1996; Sleper and Poehlman, 2006; Brown and Caligari, 2009). The non-additive variation usually receives little attention as only the additive component of genetic variation is heritable (Falconer and Mackay, 1996; Sleper and Poehlman, 2006; Brown and Caligari, 2009).
Cassava breeding
- Cassava selection cycle
- Breeding methods
Cassava breeding methods are essentially defined by the method of propagation, available genetic variability and breeding objectives (Fukuda et al., 2002). Crossing between parental genotypes of the same cassava species followed by selection among the progeny is the most common method used in cassava breeding (Fukuda et al., 2002; Jennings and Iglesias, 2002; Ceballos et al., 2012).
Breeding and selection for early bulking
Hershey (2012) suggested that root storage quality may be a major indicator of maturity for farmers because some genotypes appear to reach a certain starch content or quality earlier than others. Studies conducted at CIAT by Kawano (1990) and Ojulong et al. 2010) showed that HI determined for seedlings and first clonal generations remained constant in subsequent clonal generations under a wide range of environmental conditions, indicating that HI is a better trait to select for than storage root yield in breeding for early bulking .
Breeding and selection for high storage root yield
He also indicated that another definition of maturity is related to storage root shape, whereby genotypes with short storage roots will generally produce storage roots of commercially useful diameter earlier than those with long storage roots. This results in a delicate balance between shoot and storage root growth for maximum FSRY (Ramanujam, 1985).
Cassava pests and diseases in Uganda
Cassava insect pests include whiteflies, whiteflies (Phenacoccus manihoti) and the green spider mite (Mononychellus tanajoa) (Ojo et al., 1989). Cassava green mite was accidentally introduced into Uganda, where it was first reported in 1971 (Ojo et al., 1989).
Postharvest physiological deterioration
Visually observed as blue fluorescent tissue under ultraviolet light and blue-black line of vascular tissue (Beeching et al., 1999). Breeding has great potential to improve PPD resistance of cassava, however, little progress has been made on this trait (Morante et al., 2010).
Literature summary
Accumulation of hydroxycoumarins during postharvest deterioration of tuberous roots of cassava (Manihot esculenta Crantz). Development of cassava (Manihot esculenta Crantz) cultivars for resistance to cassava mosaic disease in Zambia.
- Introduction
- Materials and methods
- Study sites
- Data collection and sampling method
- Data analysis
- Results
- Cassava production constraints
- Crops grown other than cassava
- Cassava based cropping systems practiced by farmers
- Cassava cultivars grown by farmers
- Farmers’ awareness of early storage root bulking in cassava
- Indicative signs of early storage root bulking cassava cultivars
- Perceived importance of early storage root bulking cassava
- Cassava cultivar selection criteria by farmers
- Traits suggested to be incorporated into early bulking cassava
- Discussion and conclusions
54 Figure 2.4: Percentages of farmers with/without knowledge of early cassava varieties in the three study districts in Uganda. 56 Figure 2.6: Importance of early emerging cassava cultivars as mentioned by farmers in the three study districts in Uganda.
Participatory rural appraisal questionnaire used during the survey
Yes/No If yes, what are they and how long do they take to collect.
Introduction
Its improvement can be achieved by exploiting the genetic variability within the cassava germplasm (Aina et al., 2007). Storage root collection, which determines storage root yield, is a consequence of the formation and growth of storage roots that undergo secondary thickening (Izumi et al., 1999).
Materials and methods
- Experimental sites
- Experimental germplasm
- Experimental design
- Data collection
- Data analysis
The SRG of the four largest storage plant-1 roots of each genotype was determined as the circumference (cm) at the widest point of the mid-section of a storage root. TLjk = interaction effect between j-th harvest time and k-th location GLik = interaction effect between i-th genotype and k-th location;.
Results
- Combined analysis of variance across harvest time and location
- Estimates of phenotypic, genotypic and environmental variance components .88
- Phenotypic correlations of agronomic and disease resistance traits
Namulonge recorded the lowest mean SRG at all harvest times except at 5 MAP where it was recorded at Jinja. Namulonge recorded the highest mean PPD at all harvest times except at 7 MAP where it was recorded at Nakasongola.
Discussion and conclusions
Significant differences between harvest times for all the evaluated traits were a clear indication of significant differences in the growth and development of the test genotypes over time. PC2 accounted for 19.6% of the total variation in the genotypes and was largely contributed by DMC, SRN and PPD.
Introduction
Despite this, cassava exhibits significant differential genotypic responses under varying environmental conditions, a phenomenon termed genotype x environment interaction (GEI) (Egesi et al., 2007; Ssemakula and Dixon, 2007). Stability analysis methods are divided into two main groups; univariate and multivariate (Lin et al., 1986).
Materials and methods
- Experimental sites
- Experimental germplasm
- Data collection
- Data analysis
The GSI for each genotype is calculated as the sum of the corresponding average performance rankings and the AMMI stability value (ASV). The ASV is a measure of the stability of a genotype (the lower the value, the greater the stability) based on weighted IPCA1 and IPCA2 scores (Purchase et al., 2000).
Results
- Variation in traits in response to genotypes and locations
- Performance and genotype x location interaction effects for early fresh storage
107 Table 4.2: Ranking of 12 cassava genotypes according to average performance, IPCA1 scores and genotype selection index for fresh storage root yield evaluated nine months after planting across three locations in Uganda. 111 Figure 4.4: Biplot of mean harvest index and IPCA1 scores for 12 cassava genotypes evaluated nine months after planting at three locations in Uganda.
Phenotypic correlations among agronomic and disease traits
116 Table 4.7: Ranking of 12 cassava genotypes by mean performance, IPCA1 score and genotype selection index for severity of cassava mosaic disease assessed six months after planting at three locations in Uganda.
Discussion and conclusions
AMMI analysis of yield trials - Additive main effects and multiplicative interaction analysis of two international maize cultivar trials. Analysis of genotype-by-environment interaction using the additive main effects and multiplicative interaction model and stability estimates.
Introduction
The genetic information generated helps breeders to use effective breeding methodologies to improve yield (Jaramillo et al., 2005; Zacarias and Labuschagne, 2010). In crop improvement, GCA scores can also be used to predict genetic gains (Bhullar et al., 1979; Falconer and Mackay, 1996).
Materials and methods
- Experimental site
- Parental selection and hybridisation
- Trial design
- Data collection
- Data analysis
The analysis of variance (ANOVA) for the traits was performed using Genstat 14th edition (Payne et al., 2011). The relative contribution of the traits to the total variability of the 36 families was analyzed according to Jollife (2002) using principal component analysis (PCA) in Genstat (Payne et al., 2011).
Results
- Performance of the individual genotypes within families
- Mean performances of the 36 F 1 families
- Diallel analysis of variance for traits
- General combining ability
- Specific combining ability
- Traits contribution to the families variability
- Phenotypic correlations
TME14 had a highly significant (P<0.01), positive GCA effect for DMC and negative GCA effect for PHT. Parent CT3 had a highly significant (P<0.001), negative GCA effect for PHT and significant (P<0.05), negative GCA effects for FSRY and DSRY.
Discussion and conclusions
In this study, GCA effects accounted for over 50.0% of the variability expressed by families for FSRY, HI, DSRY, PHT, SRG, SRL, CBSD-RN, and CMD-S, indicating the predominance of additive gene action in the expression of these . eight traits. Specific combining ability effects accounted for over 50.0% of SS families for DMC and SRN alone, suggesting the predominance of non-additive gene action in the expression of these two traits.
Introduction
Among the main traits that farmers look for when selecting cassava cultivars are high storage root yield, earliness, pest and disease resistance, and dry matter content (Tumuhimbise et al., 2012). Diallel analysis provides information on heterosis and effects due to reciprocal, maternal, general combing ability (GCA) and specific combining ability (SCA) of parents in crosses (Glover et al., 2005).
Materials and methods
- Experimental locations
- Plant germplasm
- Experimental design
- Data collection
- Data analysis
Storage root length (SRL) was measured as the length (cm) from end to end of the storage root, and storage root girth (SRG) was measured as the girth (cm) at the widest point of the middle part of the storage root. As the differences were not significant, an unweighted combined analysis of variance was performed on the data for both locations.
Results
- Performance of individual F 1 genotypes
- Diallel analysis of variance for traits
- Mean performance and general combining ability effects
- Mean performance and specific combining ability
- Estimates of mid- and better-parent heterosis of the families
- Phenotypic correlations between traits
- Traits contribution to the families variability
- Selection of F 1 genotypes for advancement
Parents' mean performance and GCA effects for the different traits, averaged across the two sites, were discussed. The average performance and SCA effects of the 36 F1 families for the different characteristics, averaged over the two locations, were discussed.
Discussion and conclusions
The proportion of SS for families due to GCA and SCA effects indicates the relative importance of additive and non-additive genetic effects in trait expression (Calle et al., 2005, Kulembeka, 2012). Both additive and non-additive gene effects have been implicated in trait expression.
Summary of the research findings
- Farmers’ attitudes to and/or perceptions of cassava early bulking, production
- Variation between selected cassava genotypes for storage root bulking and
- Genotype x environment interaction effects on early fresh storage root yield and
- Diallel analysis for early fresh storage root yield and related traits at the F 1
Both additive and non-additive gene action were involved in the expression of the traits at the seedling and clonal stages. At both the seedling and clonal stages, both the additive and non-additive gene actions were involved in the expression of the traits.
Progress in breeding for high yielding, early bulking cassava in Uganda
Combining ability analysis at the seedling stage was performed to investigate the genetic control of storage root-related traits given the high number of storage roots, from 1 - 23 plant-1, produced by the seedling plants in this study. This high number of storage roots during the seedling evaluation stage was attributed to a combination of the technique used for germinating the botanical seeds and the good growing conditions (rainfall and temperatures) in Uganda.
Implications of the findings of this study for future cassava breeding
181 selected genotypes had a score of 1.0 for both CMD-S and CBSD-RN and were therefore resistant to both CMD and CBSD.
