Cassava varieties resistant to cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) are needed for the food and income security of the rural poor in eastern and southern Africa (ESA). The International Institute of Tropical Agriculture led five national cassava breeding programs (Malawi, Mozambique, Kenya, Tanzania and Uganda) in virus-cleaning and exchanging elite cassava germplasm resistant to both diseases. This paper documents the experiences and lessons learned from the process. Thirty-one clones (25 elite, two standard and four national) were submitted by the five breeding programs to the Natural Resources Institute and Kenya Plant Health Inspectorate Services for virus cleaning and indexing. Subsequently, ca 75 invitro virus-indexed plantlets per clone were sent to Genetic Technologies International Limited (GTIL), a private tissue culture (TC) lab in Kenya, and micro-propagated to produce ≥1500 plantlets. After fulfilling all the formal procedures of germplasm exchange between countries ≥300 plantlets per clone were sent to each partner country. National check clones susceptible to CMD/CBSD were sent only to their countries of origin. In each country, the in-vitro plantlets were acclimatized under screen house conditions and transferred to clean isolated sites for field multiplication. All the clones were cleaned of the viruses, except Tomo. The cleaning process was slow for F19-NL, NASE1, and Kibandameno and TC micro-propagation at GTIL was less efficient for Pwani, Tajirika, NASE1, and Okhumelela than for the other clones. Difficulties in cleaning recalcitrant clones affected the timeline for establishing the multi-site evaluation trials in target countries. The initiative is the one of the kind to successfully clean and exchange elite germplasm as a joint action to combat CBSD in ESA. Adequate preparation in terms of infrastructure and personnel are critical to successfully receiving and adapting the indexed in-vitro plants as new germplasm.

Cassava (Manihot esculenta Crantz) production in sub-Saharan Africa (SSA) is constrained by the two biotic constraints namely, cassava mosaic disease (CMD) and cassava brown streak disease (CBSD). The aim of this study was to evaluate elite cassava genotypes for variation in agronomical traits, correlate them to CMD and CBSD parameters and identify stable genotypes in Alupe, Kakamega and Kibos in Western Kenya. Twenty three (23) elite cassava genotypes that had shown resistance to either one or both of CMD and CBSD in Eastern Africa were evaluated. The trial was conducted using an alpha lattice balanced design with twenty three (23) genotypes, replicated three times at Alupe, Kakamega and Kibos in Western Kenya for an extended cropping cycle between 2016 and 2017. Results showed significant differences (P ≤ 0.05) between genotypes and location (or agro-ecology), but not interaction (P ≥ 0.05), for all the agronomic performance parameters evaluated. All the 23 cassava genotypes evaluated across the three locations had mean cyanide potential levels ranging from of 3.00–6.00 and were therefore, sweet and not bitter. The significant but negative relationship between CMD and CBSD incidence and severity with agronomic performance implied that their relationship was inverse. Confirmation of stability for agronomic performance was achieved through AMMI analysis, using AMMI stability value (ASV). Stable genotypes based on AMMI stability values (ASV) for fresh root yield across Alupe, Kakamega and Kibos were KBH/2002/066, Kibandameno (a local standard check), NASE-18, Kizimbani and NASE-3. These genotypes need to be further evaluated in more environments to assess their wider adaptability and stability.

This study aimed to assess elite cassava genotypes for resistance to cassava mosaic and brown streak diseases at Alupe, Kakamega and Kibos in Western Kenya. The trial was conducted using alpha lattice balanced design using 24 genotypes with three replicates, for an extended cropping cycle between 2016 and 2017. Results for combined analysis of variance showed that genotype, location, month after planting (MAP) and their interactions significantly influenced (P ≤ 0.05) incidence and severity of CMD and CBSD. High CMD incidence and severity was recorded across all cassava genotypes at Alupe (mean 0.730; 1.256) as opposed to Kakamega (mean 0.000; 1.000) and Kibos (mean 0.031; 1.006). Similarly, CBSD root incidence and severity were high in Alupe (mean 0.848; 1.310), as opposed to Kakamega (mean 0.020; 1.006) and Kibos (mean 0.188; 1.078). Within location analysis for CMD and CBSD incidence and severity among cassava genotypes 12 MAP gave varied results. Genotypes Kibandameno and Kalawe had the highest CMD and CBSD incidence and severity in all three locations. Whiteflies abundance was significantly influenced (P ≤ 0.05) by genotype, location, MAP time and interaction. Significant interaction (P ≤ 0.05) between all disease resistance traits further confirmed dual resistance amongst the cassava genotypes, however, this was location specific and not generalized. These findings should be of value to cassava breeding and development efforts throughout Kenya, and other parts of sub Saharan Africa affected or threatened by CMD and CBSD and will hopefully contribute to the development of much improved and/or resistant genotypes and, ultimately more effective management of two of Africa's most pernicious threats to food security