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Cassava brown streak disease (CBSD) presents a serious threat to cassava production in East and Central Africa. Currently, no cultivars with high levels of resistance to CBSD are available to farmers. Transgenic RNAi technology was employed to combat CBSD by fusing coat protein (CP) sequences from Ugandan cassava brown streak virus (UCBSV) and Cassava brown streak virus (CBSV) to create an inverted repeat construct (p5001) driven by the constitutive Cassava vein mosaic virus promoter. Twenty-five plant lines of cultivar TME 204 expressing varying levels of small interfering RNAs (siRNAs) were established in confined field trials (CFTs) in Uganda and Kenya. Within an initial CFT at Namulonge, Uganda, non-transgenic TME 204 plants developed foliar and storage root CBSD incidences at 96–100% by 12 months after planting. In contrast, 16 of the 25 p5001 transgenic lines showed no foliar symptoms and had less than 8% of their storage roots symptomatic for CBSD. A direct positive correlation was seen between levels of resistance to CBSD and expression of transgenic CP-derived siRNAs. A subsequent CFT was established at Namulonge using stem cuttings from the initial trial. All transgenic lines established remained asymptomatic for CBSD, while 98% of the non-transgenic TME 204 stake-derived plants developed storage roots symptomatic for CBSD. Similarly, very high levels of resistance to CBSD were demonstrated by TME 204 p5001 RNAi lines grown within a CFT over a full cropping cycle at Mtwapa, coastal Kenya. Sequence analysis of CBSD causal viruses present at the trial sites showed that the transgenic lines were exposed to both CBSV and UCBSV, and that the sequenced isolates shared >90% CP identity with transgenic CP sequences expressed by the p5001 inverted repeat expression cassette. These results demonstrate very high levels of field resistance to CBSD conferred by the p5001 RNAi construct at diverse agro-ecological locations, and across the vegetative cropping cycle.

General belief has it that cassava is (i) a subsistence crop, grown to avoid hunger (ii) by poor farmers, (iii) predominantly as an intercrop, (iv) requiring less labour than other crops and (v) no inputs. These beliefs influence policy, project development and implementation, and if wrong, may have far-reaching consequences for the success and sustainability of interventions. This study examines five beliefs about cassava and discusses consequences for interventions targeting cassava. From 2004 to 2006, 120 detailed farm surveys were carried out with smallholder farmers in 6 sites in central/eastern Uganda and western Kenya, whereby households were categorised in three wealth categories by local key informants. Through structured interviews and field visits, details on the importance of cassava, socio-economic indicators, food security, crop management and labour aspects were obtained. Our results show that cassava does ensure food security, but that the other beliefs are either myths or half truths. Besides supplying 27-41% of starchy staple food consumption, cassava also provided significant income (84 US$ yr −1 ), similar to that of maize (90 US$ yr −1 ). It is too simplistic to classify cassava as a 'poor man's crop' as in Uganda wealthier households marketed more (+16%), but in Kenya consumed less (-11%) cassava than poorer farmers. Cassava is not predominantly intercropped (30% of acreage in Uganda and 51% in Kenya), farmers do use inputs on cassava (36% of the households hire labour) and total labour requirements (287 mandays ha −1 ) were higher than for most crops. Contrary to expectations, we conclude that increasing cassava production will not improve food security -unless a disease epidemic is present -but instead will improve the scope for commercialisation of cassava. To ensure that projects designed to enhance cassava production benefit poor and/or labour deficit households, specific provisions are needed, including development of labour saving technologies. food security / income / input use / labour / wealth classes / weed management

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