Sweetpotato Forum

Fig. 1. Sweetpotato grown by farmers in East Africa (courtesy of Ted Carey)

Sweetpotato (Ipomoea batatas (L.) Lam.) is of American origin. The centre of origin of I. batatas was somewhere between Yucatán Peninsula of Mexico and the mouth of the Orinoco River in Venezuella¹. The crop was spread widely by Portuguese explorers to Africa, India and the East Indies in the 16th century^2,3. It is a dicotyledonous plant which belongs to the family Convolvulaceae⁴. Sweetpotato is a hexaploid (2n = 90)⁵. It has been suggested that it was derived by amphidiploidy from a tetraploid (2n = 60) and a diploid (2n = 30) to produce triploid (2n = 45), followed by subsequent doubling of the chromosomes to produce the hexaploid⁴.  Its tendency to mutate readily, as well as its obligate outcrossing nature and its capacity to flower and set seed^{6,7,8,9,10,11,12} have given rise to large numbers of landraces grown by farmers in several areas of Asia, Africa and Oceania^2,13,14,15. In these secondary centres of genetic diversity the crop is exposed to selection pressures (diseases, pests, climatic conditions and market forces) that may not exist in the centre of origin, and locally adapted genotypes and populations may emerge.

Fig 2: An African farmer (in Tanzania) is transporting the storage roots to her home(courtesy of Ted Carey)

Sweetpotato crop is widely grown in tropical, sub-tropical and temperate areas. A wide range of cultivars are suitable for different soils and climates. The crop is grown on a variety of soils, but well-drained light and medium textured soils with ph range of 4.5-7.0 are more favourable for the plant^16,17. The crop is very sensitive to aluminium toxicity and will die about six weeks after planting if lime is not applied at planting in this type of soil¹⁶. Best growth is obtained with temperatures above 24^o C, abundant sunshine and warm nights. Annual rainfalls of 750-1000 mm are considered most suitable, with a minimum of 500 mm in the growing season. The crop is sensitive to drought at the tuber initiation stage 50-60 days after planting and is not tolerant to water logging, as it may cause tuber rots and reduce growth of storage roots if aeration is poor¹⁷.

The edible storage roots are important for food for resource-poor farmers in many countries^13,14,16,18. In the Far-East including China, Thailand, Indonesia, Japan and Taiwan the sweetpotato has evolved from being a food security crop, to being important for starch production (sugar syrups and alcohol), other value-added products and animal feed^16,19. Breeding has been successful in many places (USA, Japan, China and Peru), but has been somewhat less successful in places where sweetpotato has been retained its character of 'subsistence' crop¹⁶. Recent efforts, particularly in sub-Saharan Africa have been promoted by international agricultural research centres (CIP, IITA and AVRDC/IBPGR) in collaboration with national programmes. The challenges include selecting for marginal environments, as well as ensuring adequate attention to quality, necessary adaptations and resistances to biotic and abiotic constraints. To be most successful, the breeding efforts need to be accompanied by concerted efforts at seed dissemination²⁰.

Keywords: Ipomoea batatas, resource-poor farmers, sweetpotato forum, CIP, IITA, AVRDC,IBPGR, low-input systems, crops in East Africa, sub-Saharan Africa, allopolyploid, crop of American origin.

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