One of the major yield-limiting factors in sweetpotato production is the lack of clean planting material owing to infection by sweet potato virus disease (SPVD). Viruses are spread by insect vectors such as whiteflies and aphids and they build up over time in the plants, reducing yields with each season. Some sweetpotato varieties are more resistant to virus infection than others, showing less yield loss, and breeding programs in Africa focus on selecting for this very important trait. Socalled susceptible varieties can disappear completely in one or two seasons in areas with high virus pressure.