The importance of cassava as a food security crop in Africa and the world Cassava, originally from South America, is the fourth most important source of calories in the developing world after the cereal crops wheat, maize, and rice. Worldwide, it feeds an estimated 700 million people directly or indirectly. Cassava production has increased steadily for the last 50 years, with 242 MT harvested in 2012. The increase is likely to continue as farmers in more than 105 countries come to recognize the crop's advantages. A semi-perennial root crop, cassava can stay in the ground for up to 3 years. This makes it an excellent food security crop: when all other crops have been exhausted, cassava roots can still be harvested. It is naturally drought resistant and resilient to climatic changes, high temperatures, and poor soils, and in addition, cassava responds extremely well to high CO 2 concentrations, making it a very important crop for the 21st century. Africa alone accounts for more than 55 % of the world's production, and cassava is the first food crop in fresh tonnage before maize and plantain in sub-Saharan Africa. Cassava is also an important source of income, especially for women in sub-Saharan Africa (SSA). Furthermore, cassava is the second most important source of starch in the world. Cassava is thus a highly valuable crop for the world today and in the future. It is critical that it should not be compromised by viral diseases.

Seed systems for roots, tuber, and banana (RTB) crops receive relatively little attention from development-oriented research and commercial seed sector actors, despite their importance for food security, nutrition and rural livelihoods. We review RTB seed systems-with particular reference to potato, sweetpotato, cassava, yam and banana -to reflect on current seed system development approaches and the unique nature of these systems. We refer to our own experiences, literature and 13 case studies of RTB seed system interventions to identify gaps in our knowledge on farmer practices in sourcing and multiplying seed, and processes affecting seed quality. Currently, most approaches to developing RTB seed systems favour decentralised multiplication models to make quality seed available to smallholder farmers. Nevertheless, arguments and experiences show that in many situations, the economic sustainability of these models cannot be guaranteed, among others because the effective demand of farmers for seed from vegetatively propagated crops is unclear. Despite the understudied nature of farmers' agronomic and social practices in relation to seed production and sourcing in RTB crops, there is sufficient evidence to show that local RTB seed systems are adaptive and dynamic. Our analysis suggests the paramount importance of understanding farmers' effective demand for seed and how this affects the sustainable supply of quality seed from specialized producer-entrepreneurs, regardless of the seed system paradigm. From the case studies we learnt that few interventions are designed with a rigorous understanding of these issues; in particular, what types of interventions work for which actors, where, and why, although this is a necessary condition for prioritizing investments to increase the use of improved seed by smallholder farmers.

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