The plant
The sweet potato, Ipomoea batatas, is part of the order Solanales, family Convolvulaceae and genus Ipomoea. It is a perennial herbaceous plant cultivated as an annual plant. The creeping stems can reach 6 metres in length and 3 to 10 mm in diameter. The leaves are alternating, with a long stalk variable in shape, toothed or lobed (5 to 7 lobes), and green to reddish in colour. The flowers with solid white and/or violet-coloured corollas appear on their own or grouped on the leaf axils. The sweet potato is cultivated for its tubers (actually tuberous roots), which are more or less elongated or spherical in shape and variable in colour. The flesh and skin coloration is a varietal characteristic. Hence we can find varieties with white skin and flesh, violet skin and white flesh, red skin and yellow flesh, orange skin and flesh, or also violet skin and flesh.
The sweet potato apparently originates from the American tropics, spreading from two centres, one between Yucatán and Venezuela, and the other between Peru and Ecuador where it was known as far back as 8 000 years BC. A world research centre on the potato and sweet potato has been set up in Lima, Peru (Centro Internacional de la Papa, CIP). Later on it was found as a cultivated plant in the American tropics, as well as in Polynesia and New Zealand before the arrival of the Europeans. Very widely distributed by the Spanish and Portuguese, it is now present in all the tropical, subtropical and Mediterranean zones.
The plant struggles in temperatures below 10°C. Vegetation starts from 15°C, with maximum growth at between 21 and 28°C. Tuberization is faster in 11-hour days; it is inhibited by days of more than 14 hours, which explains its preference for the tropical zones. Flowering is rare in days of more than 13 hours. Water requirements are 600 mm per cycle. The optimal annual rainfall is 750 to 1 000 mm. The plant requires a regular water input. Water shortage during tuberization affects the yield. Sweet potatoes develop better in light and well-drained soils, with a pH of between 5.2 and 6.6. They cannot withstand either salinity or alkalinity. In tropical zones, cultivation can take place year-round. However, it is preferable for the harvest to take place outside of the rainy season, as tubers are sensitive to excess moisture which can cause rotting.
The plant stock used for sweet potato production comes from cuttings or vitroplants. 25-30 cm cuttings bearing three or four buds, and with the lower two-thirds defoliated, are buried at an angle of 45°. Beforehand they are disinfected by soaking to facilitate weevil management. Planting is carried out on nursery beds or ridges, depending on the nature of the soil. Densities vary from 30 000 to 60 000 cuttings per hectare. The cuttings are placed in a single or double row depending on the width of the ridge, with a spacing of 30 cm. In the United States, planting is often carried out with a 30 cm spacing on the row and a gap between rows of 90 cm to 1 m in drained soil, or around 1.20 m in heavier soil. Fertiliser inputs are applied in three stages: before planting, and then 40 and 70 days after. Weeding is carried out in the first two months after planting. The abundant foliage naturally limits growth of invasive weeds. The tubers are harvested 100 to 180 days after planting. The crop is harvested with the plant left in place, enabling a second production 3 to 4 months later.
Feathery Mottle Virus is observed mainly in the United States and South Africa. The only management method appears to be using resistant varieties and selecting sound cuttings.
The fungi Ceratocystis fimbriata and Fusarium oxysporum, which attack the stems and tubers, are fairly widespread. Again, using sound plant stock for planting appears to limit attacks.
Weevils ( Euscepes postfasciatus and Cylas formicarius) probably represent the most common enemies of the sweet potato. Stem borers ( Omphisa anastomosalis) and nematodes ( Meloidogyne, Rotylenchulus) also cause considerable damage. Pheromone traps are a management method.
In sound soil, the sweet potato can be cultivated three or four years in a row without major sanitary problems. In crop rotation systems, alternation should be avoided with plants sensitive to nematodes, which would lead to their proliferation.
The sweet potato needs a maturation phase to achieve its optimum flavour. After harvesting, it is stored in a warehouse at a temperature of 30°C and 95 % relative humidity, to set the skin and enable any injuries caused by harvesting to scar. The starch in the tubers then begins its transformation into sugar. The tubers are then stored at a temperature of approximately 15°C and 85 % relative humidity. The tubers are kept in this way for several months.
The sweet potato is consumed in the same way as the potato: boiled, fried or mashed. Its sweet flavour also enables it to be used for cakes and even jams. It can also be converted into flour. The leaves are also consumable, like spinach. Leaves and tubers are used as livestock fodder. The tubers contain starch and sucrose which can be used for alcohol production by fermentation and distillation, as well as starch production. Hence, American producers were able to unveil at the last edition of Fruit Logistica (Berlin 2016) a sweet potato-based beer.
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