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Sweet potato (Ipomoea batatas (L.) is an important food, feed and vegetable crop in most developing countries, which account for about 97 percent of the world production. They produce 131 million tonnes per year on approximately 9 million ha and obtain mean estimated yields of 13.7 tonnes ha-1 (FAOSTAT, 2009). The crop is ranked fifth economically important crop after rice, wheat, maize, and cassava, sixth in dry matter production, seventh in digestible energy production, and ninth in protein production in the developing countries (Stathers et al., 2005; Thottappilly and Loebenstein, 2009). Sweet potato is widely grown in Africa, Asia, and Latin America, with China accounting for 52 percent of the crop, grown on approximately 4.7 million ha (FAOSTAT, 2009). It is among the world’s most important root crops; it originated from Latin America and was introduced into Africa by Portuguese navigators in the 16th century. Africa is the second largest producer of sweet potatoes, accounting for 10.6 percent of total production. It follows Asia, which accounts for 86.5 percent of the world’s production. In sub-Saharan Africa (SSA), it is the most widely grown root crop, where about 9.9 million tonnes of storage roots are produced on an estimated 2.1 million ha (FAOSTAT, 2009; Ndamange 1987).

In Kenya, sweet potato is an important staple crop (Hagenimana et al., 2001; Irungu and Kidanemarium, 1992; Ewell, 1990). Acreage under the crop has been increasing for various reasons, notably: decreasing soil fertility in most of the agro-ecological zones of the country; removal of subsidies for fertiliser and seeds for major crops such as wheat and maize as part of trade liberalisation policies; devastating pests and diseases of major crops such as wheat and maize; a growing understanding by consumers that sweet potato is a healthy crop to consume rather than a poor man’s food; and the growing shift of sweet potato from a subsistence to a commercial crop (MoALF 2015; Muli and Agili, 2013; Kwach et al., 2009; Kwach et al., 2010; Stathers et al., 2005).