Sweetpotatoes are vegetative propagated from vines, root slips (sprouts) or tubers, and farmers often take vines for propagation from their own fields year after year. Thus, if virus diseases are present in the field they will inevitable be transmitted with the propagation material to the newly planted field, resulting often in a marked decrease in yields. Yields differ greatly in different areas or even fields in the same location. Thus, the average yield in African countries is about 7.02 tons/ha, with yields of 9.4, 4.4, 2.5 and 3.2 ton/ha in Kenya, Uganda, Sierra Leone and Nigeria, respectively. The yields in Asia are significantly higher, averaging 12.41 tons/ha. China, Japan, Korea and Israel have the highest yields with about 21.6, 25.8, 16.4 and 44.4 tons/ha, respectively.