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Early bulking was the second most important farmer-preferred trait after high FSRY.
Farmers suggested a number of attributes to be incorporated into early bulking cassava genotypes, the most important of which were: high dry mass content (DMC), sweetness and high FSRY.
Farmer participation in this research helped to identify the key factors that limit cassava production in Uganda. For example, CMD and CBSD are diseases of great economic importance not only in Uganda, but also in the whole of the east African region (Legg et al., 2011). Lack of early bulking cultivars is another limiting factor to cassava production in Uganda, which has also been identified in other east African countries (Mtunda, 2009;
Kamau et al., 2011). Initially, information on the existing and potential impact of early bulking cassava cultivars in Uganda was rather limited, but the data collected from the PRA will form the basis for further research on the development of adapted early bulking cultivars.
More than 10 cassava cultivars were identified in each of the surveyed districts. Each cultivar had special attributes attached to it by the farmers, indicating that they select cultivars on the basis of a series of preferred traits effectively constituting an informal selection index. Early bulking was the second most important farmer-preferred trait.
Farmers indicated that early bulking cultivars escape late season pests, diseases and droughts, they allow for multiple sequential cropping, and they are sweet. Above all, it was believed that early bulking cultivars generate food and income quicker. Farmers suggested a number of other attributes that they want incorporated into early bulking cultivars. Their key suggestions in order of importance were: high DMC, sweetness, high FSRY and resistance to pests and diseases. It was evident that to design a successful breeding programme to meet smallholder production system requirements, the farmers’
trait preferences and production constraints must be clearly understood and taken into consideration. Accordingly, the farmers’ trait preferences, production constraints and general suggestions were considered in the cultivar evaluations and selections conducted in this research and breeding programme.
7.2.2 Variation between selected cassava genotypes for storage root bulking and other important traits
High variability for FSRY and all the other traits assessed was observed among the genotypes.
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Relatively high broad sense heritability estimates were observed for all the traits evaluated.
The genotype mean squares for FSRY, harvest index (HI), dry storage root yield, storage root number, storage root girth, postharvest physiological deterioration, cassava brown streak disease root necrosis (CBSD-RN), cassava mosaic disease severity and DMC were significant for the selected cassava genotypes evaluated as potential parents for a breeding programme. The high variability between genotypes for the traits evaluated implied that the genotypes could serve as parents for the improvement of all the traits evaluated. The relatively high broad-sense heritability estimates for each of the traits indicated that a large proportion of the phenotypic component of variance for all the traits was accounted for by the genotypic variance component relative to the environmental and genotype x environment variance components. In turn, this indicated considerable genetic variation among the genotypes in the expression of the traits that were unaffected by the environment. This genetic variability is important in a hybridisation and/or selection programme because it means that significant genetic gain through phenotypic selection is practically feasible for all the traits under study.
7.2.3 Genotype x environment interaction effects on early fresh storage root yield and other important traits
Genotype effects were significantly different for early FSRY and other traits.
Location effects were significantly different for all traits except CBSD-RN
The GEI effects were non-significant for early FSRY, but significant for all other traits.
Significant genotype effects for early FSRY and all other traits indicated significant variation in the mean performance of the genotypes across locations for these traits.
Significant location effects for all traits except CBSD-RN meant that the overall means of the genotypes at each location i.e. the location means were significantly different for most traits. The variation between genotypes and locations underlines the need to conduct multi-locational trials in order to identify both the generally and specifically adapted genotypes with good performance for the traits under consideration. Non-significant GEI effects for early FSRY, but significant GEI for all other traits indicated stable performance of the genotypes across the three locations for early FSRY, but unstable performance for other traits. Indeed, the greater percentage (67%) of the genotypes viz., Akena, CT2, CT4, NASE14, CT3, Nyara, CT5 and TME14 was stable for early FSRY.
179 7.2.4 Diallel analysis for early fresh storage root yield and related traits at the F1 seedling and clonal evaluation stages
A number of similarities and differences were observed regarding the diallel analyses conducted at the F1 seedling and clonal evaluation stages, but the most crucial included the following:
Similarities:
A high degree of variation among individual genotypes and families was observed for all traits assessed at both stages.
Both additive and non-additive gene action were involved in the expression of the traits at the seedling and clonal stages.
Parent genotypes that had been developed by hybridising CIAT and Ugandan germplasm had good combining ability for most of the traits at both stages.
Differences:
Parent CT4 was the best general combiner for early FSRY and most other traits at the seedling stage while CT5 was the best general combiner for FSRY and most other traits at the clonal stage.
The first three families with the best SCA effects for early FSRY at the seedling stage were: NASE3 x CT2, CT5 x CT3 and CT1 x B11 while the first three families at the clonal stage with the best SCA effects: CT1 x Nyara, CT1 x B11 and CT5 x TME14.
The high degree of variation between individual genotypes and families observed for all traits at both the seedling and clonal stages indicated high potential for selection of all traits assessed. High variation between individual genotypes and families for all traits at the seedling and clonal stages was also reported by Mtunda (2009) and Chikoti (2011).
At both the seedling and clonal stages, both the additive and non-additive gene actions were involved in the expression of the traits. Traits with predominant additive genetic effects could be further improved through cycles of recurrent selection. For traits where there was a predominance of non-additive gene effects in their expression, specific hybrid combinations could be made of parents drawn from different heterotic groups to maximise the expression of non-additive gene action. Following hybridisation of the selected parents, the non-additive gene effects that predominate in some traits could be “captured”
or fixed in subsequent generations by exploiting the vegetatively propagatable nature of cassava. A reciprocal recurrent selection breeding strategy should be employed when breeding for traits under both additive and non-additive gene action. The challenge
180 remains to combine maximal expression of additively and non-additively determined traits within single new genotypes.
Combining ability analysis at the seedling stage was conducted to investigate the genetic control of storage root related traits given the high number of storage roots, from 1 - 23 plant-1, which was produced by the seedling plants in this study. This high number of storage roots at the seedling evaluation stage was attributed to a combination of the technique that was used for germinating the botanical seeds and the good growing conditions (rainfall and temperatures) in Uganda. Seeds were germinated in plastic bags and the resulting seedlings with undamaged roots transplanted to the field. Ceballos et al.
(2004) indicated that cassava seeds germinated in seedling containers and later transplanted as seedlings to the field often develop adventitious storage roots and that the mature plants that develop from such seedlings are similar in terms of the storage root formation to the plants derived from stem-cuttings. Usually, diallel analysis for storage root traits cannot be undertaken in areas where storage root development of transplanted seedlings is affected by high in-field variability. The diallel analysis conducted at the seedling stage in this study reduces the potential loss of useful background genetic data and breeding material when advancing to the clonal selection stages, and could be replicated wherever conditions permit to improve the effectiveness of cassava breeding programmes. Overall, parents and families with good combining ability for early FSRY and other pertinent traits at both the seedling and clonal evaluation stages were identified and will be exploited in the future cassava breeding programmes for early bulking and relevant traits in Uganda.
7.3 Progress in breeding for high yielding, early bulking cassava in Uganda