Breeding for Cassava brown streak resistance in coastal Kenya.

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 I thank my supervisor, Prof. Rob Melis, for his patience, understanding, guidance, encouragement and suggestions during this study and the writing of the thesis.To Dr. Paul Shanahan, thank you for your patience, empathy, guidance and suggestions while writing the thesis, especially in the analysis of the diallel data.I would like to recognise the contributions of Dr. John Derera for the genetic analyses and suggestions offered while compiling the thesis.The guidance received from Dr. Githiri Mwangi is acknowledged with gratitude.The data analysis would not have been possible without the valuable contributions and guidance of Mr Harvey Dicks, Dr H. Mwambi, and Prof. P. Njuho, and I appreciate with gratitude the sacrifices you made during data organisation and analysis.My in-country supervisors,