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Understanding direct and indirect defense mechanisms that enhance host plant resistance (HPR) and biological control is critical for successful development of an integrated pest management (IPM) approach. Cassava green mite (CGM) (Mononychellus tanajoa Bondar (Acari: Tetranychidae)) is a major arthropod pest of cassava (Manihot esculenta Crantz) in Africa. Strategies to control CGM include HPR and biological control by use of exotic natural enemies particularly the predatory mite Typhlodromalus aripo DeLeon (Acari: Phytoseiidae). The success of the latter depends on continuous survival of the natural enemy which requires suitable host plants and weather conditions. Various plant morphological traits have been recognized as indirect defense mechanisms that enhance HPR to CGM, and/or attract T. aripo in cassava. It was envisaged that integration of HPR and classical biological control approaches through manipulation of such indirect defense traits would lead to a more sustainable management of CGM in view of anticipated climate change. Lack of information on farmers’ perception of CGM and preferred varietal attributes, and gene action controlling the inheritance of CGM resistance also limits success of resistance breeding and adoption of varieties. This research was undertaken to gather information on farmers’ perceptions of cassava varietal attributes and cultural practices in relation to CGM resistance, identify suitable sources of resistance and environments for future breeding; and to determine the nature of gene action controlling CGM resistance and the inheritance of plant morphological traits that enhance the ability of cassava to host and support continuous survival of natural enemies. High fresh storage root yield (FSRY), high storage root dry mass percentage (SRDM%), earliness combined with extended underground storability, and resistance to foliar pests and diseases are the major factors that influence adoption and retention of genotypes by farmers. Moles, termites and CGM are the most widespread and most damaging pests. However, due to the non-conspicuous nature of CGM, its effects are under-estimated and are given limited attention by farmers. The majority of the farmers are familiar with CGM leaf damage symptoms but they cannot associate them with the actual pest. Participation of farmers in field training and field research activities helps them to know CGM. Crop rotation, intercropping, removal of shoot tips, selective pruning of infested shoots, and burning of cassava fields are some of the ways used by farmers to manage CGM. Farmers associate hairy broad-leaved, tall cassava genotypes and pink leaf pigmentation (anthocyanin) with low CGM damage. There is substantial genetic variability in the Zambian cassava germplasm for CGM resistance and associated plant morphological traits such as leaf pubescence (Pbs), leaf retention (LR), stay green (SG), tip size (TS), tip compactness, and plant height (PH), stem diameter (StD), SRDM% and FSRY. Genotypes with wide or specific adaptability for these traits have been identified, and should be recommended for general or localized production and for use as sources of desired genes in crop improvement. Genotypes L9.304/147, 92/000, TME2, 4(2)1425, I60/42 and L9.304/175 combine wide adaptability with high levels of resistance to CGM. Genotypes Kapeza, L9.304/147 and 4(2)1425 are able to produce 13-15 t ha ¯¹ at 9 months after planting suggesting their potential for early bulking. This study has shown that both additive and non-additive gene effects play a role in the expression of CGM resistance and associated plant morphological traits. The best combinations of parents for resistance against CGM were 4(2)1425 x L9.304/147 and Mweru x L9.304/147, while L9.304/147 x I92/000 displayed combined resistance to CGM and cassava mosaic disease (CMD). The resistance of cassava to CGM is positively correlated with Pbs, LR, and TS, SG, PH, StD. Overall, the study has shown that there is wide diversity in the expression of valuable indirect defense traits among genotypes, indicating that there is scope for integration of biological control and host plant resistance for CGM in Zambia. The release of genotypes that exhibit high level of intra-season and inter-season stability for enhanced expression of LR, SG, and Pbs will minimize the impact of CGM on FSRY and SRDM% that results from seasonal effects. Such genotypes should also provide habitat for and thus help to ensure the survival of T. aripo in cassava fields. The study has contributed to the promotion of food security through identification of early-bulking genotypes which also have good potential for extended underground storability of roots. Early-bulking, high FSRY and SRDM% and SRR resistance are farmer-preferred traits. Therefore, enhancement of such traits through plant breeding is likely to increase the adoption of new genotypes by farmers. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.

Despite the increasing number of farmers growing cassava in Zambia, yield per hectare has remained low at 5.8 t ha-1. The major constraints contributing to low yields are pests and diseases of which cassava mosaic disease (CMD) caused by East Africa cassava mosaic virus (EACMV), Africa cassava mosaic virus (ACMV) and South Africa mosaic virus (SACMV) is the most important. Breeding of cassava is restricted by limited information on viruses and associated satellites, and farmer preferences. Most of the farmers cannot manage to institute control strategies that require buying of chemicals. The most feasible option remains improving existing cultivars through resistance breeding. The study therefore was conducted to: i) establish farmers’ perception and knowledge of CMD; ii) to identify viruses of cassava occurring in Luapula province; iii) evaluate the performance of local and improved cultivars for agronomic traits; iv) evaluate the performance of F1 progenies for CMD resistance; and v) determine general combining ability and specific combining ability for CMD resistance. The studies were carried out between 2008 and 2011 at different locations in Zambia. The information generated was important in formulating a local breeding strategy for CMD resistance. A participatory rural appraisal and a structured survey was conducted in Mansa, Samfya and Mwense districts in Luapula province involving farmers to ascertain farmers’ perceptions of CMD. The results of the study showed that the majority of the respondents (97.6%) were not aware of CMD. Most of the farmers grew landraces on small pieces of land. Although, the cultivars (local and improved) were widely grown, they were susceptible to CMD. The farmers preferred cultivars with high yielding and early bulking characteristics among others. A CMD survey conducted between April and May 2009 in Samfya, Mansa, Mwense, Kawambwa and Nchelenge districts in Luapula province established East Africa cassava mosaic virus (EACMV), and Africa cassava mosaic virus (ACMV) as the most prominent viruses in the area. Symptoms of satellites were also observed in the farmers’ fields in most of the areas visited. Satellite II and III were detected in leaf samples. The CMD incidence (59.1%) and severity (2.4) was moderate across the districts surveyed. The CMD symptoms on the cassava plants were variable with plants showing mild and severe symptoms characterised with narrowing and reduced leaf blades. The transmission of CMD infections was mainly through cuttings rather than via whitefly infection which means that most of the planting materials used by the farmers were infected. Evaluation of cassava cultivars for CMD resistance was conducted in 2009/2010 and 2010/11 seasons at Mansa Research Station in Luapula province using a 4 x 4 α lattice design. Both introduced and locally grown cultivars had significant (P<0.001) differences in their reaction to CMD. Bangweulu, Namuyongo, Kalaba, Chikula, Mwakamoya, Chila7 and Chila11 were the most susceptible genotypes. Mweru, Tanganyika, and Nalumino were moderately tolerant to CMD. Eight hundred F1 genotypes developed using a North Carolina II mating design were evaluated in a 4 x 5 α lattice design in 2011 at Mansa Research Station, Luapula province to determine combining ability for reaction to CMD, yield and yield components. The plants were harvested 7 months after planting (MAP). Significant (P<0.001) general combining ability and specific general combining ability were recorded for CMD. The SCA effects were more important for CMD than GCA effects suggesting that non-additive gene action was more prominent than the additive gene action in determining CMD reaction. Parent lines with desired significant, negative GCA effects for reaction to CMD were Bangweulu, Kampolombo, Nalumino and TME2. In general, the survey and participatory rural appraisal established CMD as one of the constraints to cassava production and created a basis for the research study. The findings indicate opportunities that exist in creating genotypes with tolerance to CMD. The study identified cassava lines with resistance to CMD. The lines that expressed the above trait should be selected and tested further for release to the farmers in Zambia. Since the clonal evaluation trial was harvested at 7 MAP, there is need to investigate further for earliness trait in best performing lines in different locations. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.

Cassava (Manihot esculenta Crantz ssp. esculenta) is the second most important food crop and a main source of income for the rural communities with potential for industrial use in the coastal region of Kenya. However, its productivity of 5 to 9 t ha-1 is low due to the low yield potential of the local cassava landraces caused by cassava brown streak disease (CBSD) among other biotic and abiotic constraints. Breeding for CBSD resistant varieties with farmer desired characteristics is hampered by limited information on the current status of the disease and farmers’ preferred characteristics of new CBSD resistant genotypes. In addition, there is a lack of an effective inoculation technique for cassava brown streak virus (CBSV) for screening genotypes for CBSD resistance. Information about the general combining ability (GCA) and specific combining ability (SCA) for CBSD above and below ground symptoms, fresh biomass yield (FBY) and fresh storage root yield (FSRY) (kg plant-1), harvest index (HI), dry matter % (DM %) and picrate score (PS) is limited and conflicting especially for the cassava germplasm in Kenya. These studies were carried out to update information on the status of CBSD, farmer’s preferences for cassava genotypes, and identify the most effective CBSV inoculation technique. In addition, the studies aimed to: determine the GCA and SCA for, and gene action controlling, the incidence and severity of above ground CBSD, root necrosis, FBY, FSRY, HI, DM %, and PS; and identify CBSD resistant progeny with farmers’ desired characteristics. A survey carried out in three major cassava-growing divisions in Kilifi, Kwale and Malindi Districts indicated that there was potential to increase production and productivity by increasing the area under cassava production and developing CBSD resistant genotypes that are early maturing, high yielding and sweet. In addition, CBSD was widely distributed, being present in 98.0% of the farms surveyed at a mean incidence of 61.2%. However, 99.0% of farmers interviewed lacked awareness and correct information about the disease. The genetic variability of cassava within the farms was low as the majority of farmers grew one or two landraces. Highly significant differences (P < 0.01) were observed among inoculation techniques for CBSV for which the highest infection rate of up to 92.0% was observed in plants inoculated by wedge grafting infected scion. Highly significant differences (P < 0.01) were observed among genotypes, between sites and their interaction for incidence of CBSD and root necrosis, while the differences among genotypes and the interaction between genotypes and the period of ratings were highly significant (P < 0.01) for the severity of CBSD and root necrosis. Above ground CBSD symptoms were not always associated with below ground CBSD symptoms and below ground CBSD symptoms were more severe at 12 months after planting (MAP) than at 6 MAP. Therefore, selecting cassava genotypes with resistance to below ground CBSD is more important than selection based on resistance to above ground CBSD and should be done after 12 months. Genotypes 5318/3 (exotic) followed by Msa140 and Plot4 (both local) had high resistance and can be used as new sources of resistance to root necrosis. Both GCA and SCA effects were highly significant with GCA sums of squares (SS) predominant over the SCA SS for most traits evaluated except for DM % at the clonal stage. These results indicate that although additive and non-additive genetic effects are involved in the inheritance of these traits, the additive genetic effects are more important except for DM %. Therefore breeding for CBSD-resistant genotypes that have characteristics desired by farmers in the coastal region of Kenya can be achieved through recurrent selection and gene pyramiding followed by participatory selection or use of a selection index that incorporates characteristics considered important by farmers. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2008.