Doctor of Philosophy in Plant Breeding. University of KwaZulu-Natal, Pietermaritzburg 2016.Cassava (Manihot esculenta Crantz) production in Malawi, like in most of the East African countries, has suffered from cassava brown streak disease (CBSD), a disease that affects both the quality and yield of storage roots. The incidence of CBSD in Malawi has increased in recent years causing yield loss of up to 25%. Currently there is no information that indicates the availability of resistant cassava varieties in Malawi. In addition, most cassava varieties grown in Malawi are late bulking (12-18 months), and this contributes to high CBSD incidences, which increase with plant age. Therefore, there is need to develop CBSD resistant and early storage root bulking varieties, which can be harvested before the disease (storage root necrosis) becomes severe. Early storage root bulking, CBSD resistant varieties will not only provide good storage root quality and productivity per unit area of land, but will also facilitate the release of land for other farming activities. The main objective of the study was to develop cassava varieties that are resistant to CBSD and early storage root bulking, in order to improve the yield and quality of cassava in Malawi. The specific objectives of the study were (1) to assess farmers’ knowledge of CBSD and its management, (2) to identify early storage root bulking cassava genotypes as well as traits associated with early storage root bulking, (3) to assess the effect of harvest time on cassava genotype performance, stability and adaptability, (4) to evaluate cassava genotypes for resistance to CBSD and its associated yield losses and (5) to determine the mode of gene action and the importance of combining ability effects in the inheritance of CBSD resistance and early storage root bulking traits. Assessment of farmers’ knowledge of CBSD indicated that the majority of the farmers did not know the disease through foliar symptoms and only 10.1% of the farmers were able to identify CBSD. The study established that CBSD is a continuing threat to the cassava industry, where high incidence levels were observed. On average, 75.0% and 71.7% of the farms had plants with leaf and storage root symptoms, respectively. The average CBSD leaf incidence per farm was 31.2% with levels up to 86.7% on some farms. At harvest, 88.3% of the farmers’ cassava fields exhibited storage root necrosis. Most farmers were found to lack a source of clean planting material and the lack of new improved varieties was reported as the most important constraint of cassava production, apart from CBSD. Therefore, the results suggest that education of farmers on the efficient management of this viral disease through selection of clean planting material should be provided.
Early storage root bulking and agronomic traits associated with early bulking in cassava, was studied at two sites over two seasons with 16 genotypes. High yields of up to 9.5 t haˉ¹ at 6 months after planting (MAP) and 17.8 t haˉ¹ at 9 MAP were obtained and four varieties were identified as early-bulking (Mulola, Phoso, Mbundumali and Maunjili). The study further identified harvest index and shoot mass as the major selection criteria in improving fresh storage yield and dry storage root yield. The results indicated that both source and sink capacities were important for determining early yield. Therefore, these two traits are the key determinants of early storage root bulking and should be used when selecting early bulking varieties. On the effect of harvest time, the study revealed that genotype, environment and genotype x environment interaction have a significant influence on the performance of varieties, regardless of the harvest time. Most of the cassava varieties exhibited specific adaptation to certain environments. The study identified five varieties (Mulola, Phoso, Maunjili, Beatrice and Unknown) that exhibited consistent performance, stability and adaptability across the three harvest periods (6, 9 and 12, MAP). The results, therefore, showed that multi-location studies in cassava, regardless of the time of harvest, could help discriminate genotypes with superior performance, stability and general adaptation. In terms of resistance to CBSD, high significant differences in CBSD incidence and severity values were observed (some varieties reached as high as 94.9% and severity of up to 3.8). The CBSD storage root severity increased with the prolonged stay of the crop in the field. The study established that yield loss due to CBSD was significantly associated with storage root severity at different harvest times and a maximum yield loss of 43.1% was recorded at 12 MAP on Kalawe, while at 9 and 6 MAP, maximum yield loss was 24.8% and 10.9%, respectively. The study identified five varieties to be resistant/or tolerant to CBSD (Phoso, Maunjili, Mpale, Sauti and TMS4(2)1425). The results, in general, suggest that an integrated approach should be used by farmers in order to effectively manage CBSD, which among others include using varieties that are early bulking and resistant/tolerant to CBSD, selecting planting material free from CBSD, sanitation and roguing infected plants from the field, especially shortly after sprouting. Four parental genotypes (Silira, Mulola, Phoso and Mkondezi) were identified as the best general combiners for the CBSD resistance and early storage root bulking. Thirteen progenies exhibiting CBSD resistance and early storage root bulking traits were identified and selected for advancement. The study established that resistance in cassava for CBSD, as well as for early storage root bulking is controlled by both additive and non-additive gene action. However, additive gene action is more important than non-additive type of gene action in the inheritance of CBSD resistance and early storage root bulking. The implication is that mass phenotypic recurrent selection after hybridisation of elite clones could, therefore, be effective for the development of varieties resistant to CBSD as well as addressing challenges related to late storage root bulking
