Root crops (NRI, 1987, 308 p.)
Sweet potato (Ipomaea batatas)
SWEET POTATO, Louisiana yam, Spanish potato.
Botanical name
Ipomaea batatas (L.) Lam.
Family
Convolvulaceae.
Other names
Apichu (Peru); Artichaut des Indes (Fr.); Batata(s)' (Lat. Am.); Batate(s) douche(s) (Ant.); Batate s�sskartoffel, Bataten-winde (Ger.); Camote (Lat. Am.); Chaco (Venez.); Cumar (Polyn.); Dam long (Camb.); Dankoli, Doukali (W. Afr.); Getica (Braz.); Glycopata (Cy.); Gumbili (Mol.); Imo (Japan); Jetica (Braz.); Kamote (Philipp.); Kara-imo (Japan); Ketala rambet (Indon.); Khoai day, Khoai fang (Viet.); Kumala (Fiji); Kumana, Kumara (N.Z.); Lardak-lahori (Ir.); Mabi(y) (Carib.); Mita-alu (Indon.); Moniato (Cuba); Myonk-ni (Burma); Obi djalar, Obi-djawa (Indon.); Pappas (S. Am.); Patata (douce), Patate (jaune) (Fr.); Patata dolce (It.); Satsuma-imo (Japan); Skirrets (Ir.); Skurar-kanda (Ind.); Trouffe douce (Fr.); Uala (Haw.); Uara (Pacif. Is); Ubi (Indon.); Umala (Sam.); Umara (Haw.); Veeazee (C. Afr.); Vomanga (Madag.); Yam; Ycam; Yeti; Zardakalahori (Ir.).
Botany
A herbaceous, perennial vine cultivated as an annual. There are several hundred cultivars of sweet potatoes and great variation is found in the form and growth habit. Forms that have twining and trailing long stems (0.9-4.5 m) of slender to moderate thickness and moderately to widely spaced leaves are most prevalent, but types with short thick stems, short internodes and semi-erect to erect growth habits also occur. Leaves are spirally arranged along the stems with phyllotaxis 2/5; they may be shouldered, toothed, entire, deeply cleft or variously lobed and the petioles vary in length, thickness and degree of erectness. Pigmentation also is variable, from green to deep-purple. The flowers are solitary or cymose and vary in colour from white to purple. The fruit is a glabrous or hirsute dehiscent capsule 5-8 mm in diameter containing 2-4 angular, brownishblack seeds with a very hard testa. The root tubers are formed by a thickening of parts of the adventitious roots close to the subterranean part of the stem or at the nodes which rest on the soil.
Origin and distribution
The plant is believed to have originated in South America, and in pre-Columbian times was cultivated in Central America, the Caribbean and parts of South America, Polynesia and New Zealand. It is now grown throughout the tropics, subtropics and warm temperate zones of both hemispheres.
Cultivation conditions
Sweet potatoes are widely grown throughout the world from 40°N to 32°S, under contrasting systems of agriculture, ranging from intensive horticultural practice to subsistence farming, and cultivars differ very considerably in their adaptability to soil and other conditions.
Temperature - for optimum growth a temperature of 24°C or more, with abundant sunshine and warm nights, with a minimum of cool cloudy weather, is required. Growth is restricted by cool weather and the plant damaged by temperatures below 10°C so that in warm temperate areas there must be a minimum frost-free growing period of 4-5 months.
Rainfall - sweet potatoes require at least 50 cm of rain during their growing season and an annual rainfall of 75-100 cm is considered to be best, with a low humidity as the crop reaches maturity. They can tolerate considerable periods of drought, but yields are very much reduced if a water shortage occurs 50-60 days after planting when storage root initiation has begun. They are successfully grown under irrigation, four to eight irrigations supplying 112-150 cm of water being generally recommended.
Soil - sweet potatoes can be grown on a variety of soils but sandy loams, reasonably high in organic matter, with a permeable sub-soil, are ideal.
They are sensitive to alkaline and saline conditions and good drainage is essential. Heavy clay soils or soils very rich in humus normally result in low yields and a poor quality product. Yields are usually best on soils with a pH range of 5.6-6.6; on acid soils deficient in calcium or magnesium, liming is often carried out the year before the crop is planted, as infections with certain disease organisms (soil rot or pox and scurf) are more prevalent when the soil approaches neutrality.
The crop responds well to organic manure. Responses reported to artificial fertilisers are conflicting and appear to be influenced by cultivar and climate. Too much nitrogen may encourage vine growth at the expense of tubers. In the absence of local fertiliser experiments, the following application rates are suggested: nitrogen 35-45 kg/ha; phosphorus 50-100 kg/ha; potassium 85-170 kg/ha, or 560-1 120 kg/ha of a 6:9:15 complete (NPK) mixture. With a yield of 15 t/ha the removal of nutrients is nitrogen 70 kg/ha, phosphorus 20 kg/ha and potassium 110 kg/ha. Minor elements such as boron, calcium and magnesium are also required for sound growth, and an inadequate supply of these results in characteristic deficiency symptoms in the vines and tubers.
Altitude - on the equator sweet potatoes can be grown from sea level up to 2 100 m.
Day-length - short days with low light intensity promote root development and sweet potatoes require a day length of 11.5 hours or less to promote flowering; at 13.5 hours flowering ceases, but tuber yields do not appear to be affected.
Planting procedure
Material - except in breeding work, when seeds are used, sweet potatoes are propagated vegetatively, usually by 'vine cuttings' obtained from the previous season's crop or by 'sprouts' or 'transplants' raised from tubers. In addition, occasionally, small pieces of tuber are planted directly into the field, as in the planting of potatoes, but yields are usually low and the quality poor when this method of propagation is used.
(i) Vine cuttings - are favoured, particularly in the tropics as they are cheaper, the plants are free from soil-borne diseases, and the tubers produced are of a more uniform size and shape. Apical cuttings are generally used as they give better growth and yields than basal or middle cuttings.
Generally the length of cutting used varies from 20 to 45 cm; cuttings with seven or more nodes are favoured since they normally give higher yields than cuttings with only a few nodes. It is generally recommended that 10-20 cm of the cutting is placed below the soil surface at an angle and often the cuttings are left to wilt for 24-48 hours before planting, although there is no experimental evidence to support this practice.
(ii) Transplants - in subtropical and warm temperate areas, notably the USA, sweet potatoes may be propagated by transplanting shoots which have grown from adventitious buds on tubers that have been planted in nursery beds. In many of the more temperate regions the plants are often raised under glass, or in hot beds heated by manure, hot water pipes, or electrically, since for successful germination a soil temperature of about 21°C is required. High quality tubers, free from cracks, bruises, decayed or diseased tissue, are selected and often subjected to temperatures of 21-24°C for 2 weeks before planting about 1.25 cm apart and 5-7.5 cm deep. Each tuber produces a number of 'slips' or 'draws', which reach 22.5-30 cm in 4-6 weeks, when they are pulled and transplanted into the field, either by hand or with single or multiple row transplanters which often incorporate a device which applies a dilute nutrient 'starter' solution to each transplant. Since the slips do not all develop at the same rate growers normally pull the nursery bed twice. For planting one hectare, a nursery bed of about 35 m2 is required.
In order to reduce the amount of seed stock, the cost of setting up and operating large nursery beds, and to avoid disease problems, many growers plant part of their land with slips obtained from the early vine growth of field plantings of transplants, but the use of these vine cuttings often results in reduced yields owing to the delay in planting because the mother plants must be grown first.
Method - sweet potatoes are normally planted on ridges or mounds, the former being preferred although experiments in Zaire suggest that mounds are better, as they encourage tuber formation. In the tropics, small farmers sometimes interplant sweet potatoes with beans or cassava. Once planted in the field sweet potatoes normally receive little attention apart from weed control at the early stages of growth and the maintenance of ridge height and shape. In the USA, extensive use is now made of a wide range of effective herbicides for the control of weeds, including naptalam, allidochlor, chloramben, vernolate, diphenamid, prometryn; normally application is pre-planting or pre-emergence.
Field spacing - the spacing used is determined by the following factors: growth habit and root-setting characteristics of the cultivar; type and fertility level of the soil; length of the growing season; and the purpose for which the crop is required. In the last case, if the tubers are required for the fresh market, then high yields of tubers of uniform shape and size are of primary importance, while for canning or freezing small tubers with a diameter of 2.5-5 cm and a length of 7.5-15 cm are required; for industrial uses, such as the manufacture of starch and dehydrated flakes, large roots are preferred because they are easier to handle and losses during preparation are less.
In the USA, sweet potatoes are commonly planted 30-37.5 cm apart in rows which are 90-105 cm apart in well-drained light soils and 120 cm apart in heavier soils. In the tropics, the vine cuttings are usually spaced 22.5-30 cm apart in ridges 60-75 cm apart.
Seed rate - there is great variation in the number of vine cuttings planted to the hectare, depending upon whether they are planted singly, in pairs, one each side of the ridge, or two or more cuttings per hole. It has been found that there is in fact relatively little difference in the overall yields in plant populations ranging from 25 000 to 125 000 plants/ha but when the population dropped to 12 500 plants/ha there was a significant reduction in yield.
When transplants are used, approximately 25 000-30 000 slips /ha are required and, as a general guide, with a good sprouting cultivar approximately 380-470 kg of tubers will produce enough slips in the first one or two pullings to plant one hectare.
Pests and diseases
Pests - among the insects attacking the leaves of sweet potato are leaf folders (Brachmia spp.), Bihar hairy caterpillar (Diacrisia obliqua), both in South-East Asia and Indonesia, and sweet potato hornworm (Agrius convolvuli) throughout the eastern hemisphere; in the western hemisphere, army worm (Spodoptera sp.), Alabama argillacea and Manduca singulata are often serious. In Brazil and Trinidad the stem borer (Megastes grandalis) may lay its eggs in the leaf axils, from where the larvae bore through the stems into the tubers. In South-East Asia Omphisa anastomosalis is a stem and vine borer that may also penetrate to the tubers. While crop hygiene is an important factor in minimising attack, chemical control is sometimes necessary: carbaryl, endosulphan or diazinon have frequently been recommended. Tubers are also attacked by insects, notably (in Africa) weevils (Cylas spp.), and in the western hemisphere and Pacific islands by the scarabee beetle (Euscepes postfasciatus); infected tubers develop a strong and unpleasant flavour. Cultural methods are particularly important, including crop rotation and the use of insect-free cuttings. Spraying of the plants or soil treatment with such pesticides as carbaryl, carbofuran, chlorfenvinphos, diazinon, etc has been recommended but resistance to chemicals appears to be developing and the breeding of resistant cultivars is being attempted.
In the USA, several species of nematodes are also of importance, in the southern states five species of root-knot nematodes (Meloidogyne spp.) cause considerable losses, while the reniform nematodes (Rotylenchus reniformis) also cause severe damage in some areas. Many other nematodes are also reported as affecting the crop.
Diseases - sweet potatoes are subject to a number of diseases both in the field and in storage. In the USA, losses due to disease are estimated to average 20-40 per cent, but under small-scale methods of cultivation and harvesting, such as occur in many tropical areas, losses due to disease are usually only of minor importance, apart from virus diseases, which include those producing internal cork and russet crack in roots, feathery mottle, mosaic, chlorotic spotting and banding in foliage, and little leaf and witches broom. Losses of up to 50 per cent of the crop as a result of viruses have been reported from the Republic of South Africa. Transmission is usually by aphids, including Myzas persicae, Aphis gossypii, whitefly (Bemisia tabaci) and others, or by the use of infected planting material. Control methods include rogueing and chemical control of the vectors, and attempts are being made by several countries to produce virus-resistant cultivars and virus-free planting material.
Of the fungal diseases affecting the crop in the field several are of importance. Stem rot (due to Fusarium oxysporum f. baratas) is widespread and can destroy 10-50 per cent of the crop of susceptible cultivars and has been reported to kill 99 per cent of infected plants in certain circumstances.
Black rot (caused by Ceratocystis fimbriata), reported in the USA, the West Indies, New Zealand, Hawaii and Australia, can develop in stored tubers as well as affecting the plants in the field. Scurf rot or soil stain (caused by Monilochaetes infuscans) is widespread, and produces a brown or black discoloration on the surface of the tubers, which considerably reduces their market value. Foot rot (due to Plenodomus destruens) frequently affects plants raised from transplants and infected plants often produce no tubers although they make reasonable vine growth. Other field diseases of sweet potatoes (and their causal organisms) are root rot (Phymatotrichopsis omnivora), mottle necrosis (Pythium spp.), phyllosticta leaf blight (Phyllosticta batatas), septoria leaf spot (Septoria bataticola) and white rust (Albugo ipomoeae-panduratae). Control methods rely mainly on the use of disease-free and sterilised planting material, eg dipped in thiabendazole or benomyl.
Storage losses due to disease, particularly soft rots, can be very substantial. Soft rot, ring rot or collar rot, caused by Rhizopus stolonifer, is of considerable economic importance, since under favourable conditions it can destroy the entire tuber in a few days. Other storage rots affecting sweet potatoes (and their causal organisms) are Erwinia chrysanthemi (in the USA), black rot (Ceratocystis fimbriata), surface rot (Fusarium oxysporum), dry rot (Diaporthe phaseolorum var. batatatis), charcoal rot (Macrophomina phaseolina) and Java black rot (Botryodiplodia theobromae), which is often a serious problem in the tropics (optimum growth temperature is about 28°C).
Once infection has occurred, little can be done; control lies in prevention. Sound, disease-free material should be used for propagation: resistant cultivars are available for some diseases. Every effort should be taken to avoid bruising on lifting, and curing (see Harvesting and handling) should be carried out to assist in wound healing. Washing or dipping in a chemical sterilant such as calcium hypochlorite, or a fungicide, eg benomyl, dicloran or thiabendazole has been recommended.
Growth period
Sweet potatoes, although perennial, are normally cultivated as an annual and the crop is normally harvested 3-8 months after planting, depending upon the cultivar and climatic conditions; in the tropics, if grown in the wet season, the crop normally takes longer to mature than when grown as a dry season crop.
Harvesting and handling
The crop is ready for harvesting when the leaves turn yellow and begin to drop or when a tuber can be cut without the sap rapidly turning black. In many areas the tubers are dug by hand as required, but where there is large-scale production, such as in the USA or Japan, the vines are usually cut away and the tubers harvested by ploughing out or by being dug out with combine-type harvester units. The methods used to harvest sweet potatoes have a very considerable effect upon their market quality and storage life since they are very easily damaged and very susceptible to fungal rots. Unless great care is taken to avoid mechanical injury heavy losses are likely to be incurred and it is for this reason that efficient mechanical harvesters have proved difficult to design and operate effectively. However, substantial advances have recently been reported, including machines which dig the sweet potatoes, detach them from the vines, and deposit them in a container.
Sweet potatoes are highly perishable and are not normally stored for any length of time in the tropics. In the USA, the tubers are often stored for use during the winter and spring by subjecting them to curing immediately after harvesting, a process which toughens the skin and reduces the incidence of infection by fungal disease-producing organisms. This is accomplished by subjecting the tubers to a temperature of 27-29.5°C at 85-90 per cent RH for a period of 4-7 days, care being taken to ventilate the curing room sufficiently to prevent the accumulation of carbon dioxide. After curing, sweet potatoes may be successfully stored at 13-16°C and 85-90 per cent RH. Interleaving with layers of paper soaked in MENA (methyl ester of naphthalene acetic acid) in the ratio of 40 ml/100 kg tubers has been recommended. Different cultivars have different storage lives: in the USA, cv. Jewel has proved to have the longest, keeping satisfactorily for 9 months when treated in the above manner. At temperatures between 0 and 10°C sweet potatoes are susceptible to chilling injury, which manifests itself in several ways, such as internal breakdown of tissue, increased susceptibility to decay, impaired culinary quality (including the condition known as hardcore), etc, though recent work has shown that under certain circumstances hardcore may be reversed.
Many attempts to achieve long term storage in the tropics have been reported, eg using heaps (which in some cases are arranged on staging over kerosene hurricane lanterns to achieve some measure of curing), pits dug in the soil and lined with leaves or other material, and shelves of various design, etc. There is some indication that pre-storage curing may have been beneficial, though it should be noted that in many tropical areas ambient conditions are very close to those required for curing (30°C and about 9() per cent RH). No method was satisfactory; 4 weeks or less appeared to be the best that could be expected by any of these methods, though with increase in elevation and the associated reduction in ambient temperature, storage life was considerably prolonged. In Trinidad, where storage of up to 4 weeks is usually regarded as attainable, pre-harvest spraying with maleic hydrazide or treating the harvested tubers with MENA in acetone inhibited sprouting when the tubers were stored for 4-8 weeks.
Primary product
Tubers - the starchy tubers of the sweet potato are the fleshy enlargements of the adventitious roots and a single plant may produce 40-50 tubers, which vary greatly in size, shape, colour, storage, nutritional and processing characteristics. The tubers may range from a few centimetres to over 30 cm in length, and they may be spindle-shaped or almost spherical and weigh from approximately 100 g to I kg; tubers weighing as much as 5 kg have been reported on some soils. The tubers may have a smooth or irregular or ribbed surface, and the skin and flesh may range from almost pure white through cream, yellow-orange or pink, to a very deep purple, depending upon the amount of various carotenoid pigments present and the presence or absence of anthocyanins. The tubers are often classified into three groups: those with a dry mealy flesh when cooked; those with a soft watery flesh when cooked (because of their tendency to convert much of their starch to sugars); and those with very coarse flesh which are suitable only for animal feed or industrial uses.
Yields
Yields vary greatly according to cultivar, local climatic conditions and cultural techniques. FAO figures show that in 1981, the average in Africa and the Caribbean was approximately 6.5 t/ha, in Asia 13 t. The yields (t/ha) of some major producers were: Burundi 9.9; Madagascar 5.1; Cuba 4.1; Mexico 14.1; Argentina 10.1; Brazil 8.9; Bangladesh 10.9; China 13.9; India 7.2; Indonesia 7.5; Korea 22.2; the Philippines 4.7; Vietnam 6. Israel, a small producer using intensive farming methods, reported an average production of 40 t/ha. During the period 1969-1981 there was little change on a world wide basis, as shown by Table 1.
Table 1: Sweet potato - Average yields (t/ha)
|
1969-71 |
1979-81 | |
|
World |
11.6 |
12.2 |
|
All developed countries |
15.8 |
15.4 |
|
All developing countries |
11.5 |
12.1 |
These yields fall very far behind those obtained experimentally, especially in tropical countries. Table 2 shows some published comparisons.
Table 2: Sweet potato - Yields obtained in experimental stations compared with the national average (t/ha)
Main use
Sweet potatoes are utilised primarily as a human foodstuff. In the tropics, the major proportion of the crop is eaten straight from the ground as a vegetable, after boiling, baking or frying. In Malawi, they are sometimes boiled or roasted and pounded with groundnuts to produce 'futali'. In some areas, notably India and parts of East Africa, the peeled tubers are sometimes sliced and dried in the sun to produce chips, which are often ground into flour. In the USA, about 60-70 per cent of the sweet potato crop is utilised for human food and they are eaten fresh, canned, frozen or dehydrated, and used in a variety of products such as pie fillings, purees, candied pieces, souffles, baby foods, etc.
Subsidiary uses
Animal feed - large quantities of sweet potatoes, mainly culls, are used in the USA and certain other countries, as a high carbohydrate feedingstuff for cattle, pigs and poultry.
Starch - sweet potatoes can be used as a source of starch; in Japan, the tubers have been used to produce low-grade starch for over 100 years. In 1968, about 45 per cent of the total Japanese crop was utilised in preparing 350 000 t of starch for use in the textile, paper, cosmetic and food manufacturing industries, and for the preparation of adhesives and glucose.
Flour - sweet potato flour, made by drum-drying or cabinet-drying the peeled sliced tubers, can be used as a partial substitute for wheat flour in bread and pastry making.
Secondary and waste products
A variety of products such as alcohol, acetone, lactic acid, vinegar and yeast may be prepared from the tubers. In the USA, pre-baked or boiled tubers are sometimes pulped and pureed and mixed with certain additives before being baked to give a product 'alayam', which is used as a cookie or ground into a powder for use in ice-cream and certain other food preparations.
Pectin is sometimes obtained as a by-product from the skins and other residues left after processing the tubers for food products or in the preparation of starch.
The tips of the vines and the leaves are sometimes eaten as a vegetable. A typical analysis of the edible portion shows: water 87.1 per cent; nitrogen 0.57 per cent; ether extract 0.67 per cent; fibre 1.4 per cent; ash 1.59 per cent; calcium 81.2 mg/100 g; iron 10.37 mg/100 g; phosphorus 67.3 mg/100 g; carotene 3.61 mg/100 g; thiamine 0.06 mg/100 g; riboflavin 0.17 mg/100 g; niacin 0.94 mg/100 g; ascorbic acid 25 mg/100 g. In many parts of the tropics sweet potato vines are used as a green feed for livestock, often as silage. Their feeding value is comparable with that of lucerne hay and yields average 3 600-17 500 kg/ha. In addition, they are also occasionally used as food for Tilapia in fish ponds.
Bacterial and fungicidal substances have been isolated from both the tubers and the vines, and sweet potatoes are used in a number of countries for various traditional medicinal purposes.
Special features
The chemical composition of sweet potato tubers varies widely according to cultivar, climatic conditions, degree of maturity and the duration of storage after harvesting. The usual range of values for the edible portion is: energy 490 kJ/100 g; water 65-81 per cent; protein 0.95-2.4 per cent; fat 0.4-6.4 per cent; carbohydrate 25-32 per cent; fibre 0.9 per cent; ash 0.9-1.4 per cent; calcium 30-34 mg/100 g; iron 0.8-1 mg/100 g; magnesium 24 mg/100 g; phosphorus 49 mg/100 g; potassium 373 mg/100 g; sodium 13 mg/100 g; carotene trace-12 mg/100 g; thiamine 0.1 mg/100 g; riboflavin 0.05-0.06 mg/100 g; niacin 0.6-0.9 mg/100 g; ascorbic acid 23-25 mg/100 g.
From time to time outbreaks of poisoning of cattle have been reported due to the incorporation of mouldy sweet potatoes in their feed. The metabolites ipomeamarone and ipomeamaranol have been isolated from mouldy tubers and found to be toxic to the liver and other organs, and the latter to cause lung oedema. A disturbing feature is that these metabolites may occur in tubers which show only slight blemishes, insufficient to arouse suspicion that they are unsound.
Processing
Canning - considerable quantities of sweet potatoes, particularly the yellow types, are canned, notably in the USA, where several different styles of pack, such as canned whole, slices or puree, in syrup or in water are produced. The essential processing steps are: grading, washing, peeling, either by the use of lye or steam, trimming, cutting, pulping when required, filling into cans, syruping when necessary, heating the filled cans in an exhaust box until the product attains a centre temperature of 82-93°C (normally 6-10 minutes for No. 2 and No. 2 1/2 cans and 10-12 minutes for A10 cans), sealing immediately and then heat processing at 115°C for periods ranging from 55 to 95 minutes, depending upon the initial temperature of the contents, the style of pack and the size of can.
Dehydrated flakes - the washed, peeled, sliced tubers are cooked for about 20 minutes before being reduced to a fine pulp or puree. Approximately 100 ppm of a fungal diastase, Rhozyone S, are then added to the puree, to partially convert the starch into sugar. After holding for 20 minutes the puree, which has a total solids (TS) content of 22-24 per cent, is dried on drum-driers to give a thin sheet of 97 per cent TS content, which is broken into flakes and packed in cans or polyethylene flexible pouches, flushed out with nitrogen or other inert gas in order to avoid oxidation during subsequent storage.
Starch - may be produced from sweet potatoes using conventional wet starch extraction methods, such as those used in the preparation of cassava starch, provided that the process is kept alkaline throughout (approximately pH 8.6), and processing is carried out as quickly as possible to avoid losses due to fermentation. Quality variation, poor colour of the final product and relatively high production costs have made it difficult for sweet potato starch to compete with maize starch when this is readily available at a relatively low cost.
Production and trade
Production - FAO figures for 112 countries showed total world production in 1981 to be in excess of 145 million tonnes, which is second only to the potato among major root crops. Figures for different regions of the world and the highest producing countries are given in the following table; it should be noted that the estimated production from China accounts for about one-half of the world's total.
Table 3: Sweet potato - Area and production in selected countries
Table 3: Sweet potato - Area and production in selected countries (contiued)
Trade - most sweet potatoes are consumed domestically and only a small proportion enters international trade whether in the fresh state or in a processed form and very few statistics are available. In the Caribbean, St. Vincent exports a substantial proportion of its annual production (I 000-4 000 t), mainly to Trinidad. Exports - St. Vincent: 1961-65 average, 1990 t/ha; 1972, 1420 t; 1974, 1 750t; 1976, 1 840 t; 1978, 900 t (estimate). United Arab Emirates: 1961-65 average, 25 t/a; later figures are not available. Japan: 1961-65 average, 18 t/a; later figures are not available. Imports - Trinidad and Tobago: 1961-65 average, 2 000 t/a; 1966-70, 1 990 t/a; 1978-80, 1 500 t/a (estimate).
There is a small import trade of sweet potatoes into the UK and some northern and central European countries, but separate statistics are not published. Most comes from the Canaries and the Mediterranean area, a very small quantity from the Caribbean.
Major influences
The highly perishable nature of sweet potato tubers together with the comparatively low yields per hectare usually attained and high production costs have been a severe limitation to the commercial exploitation of the crop. The complete mechanisation of both planting and harvesting techniques would considerably reduce production costs when the crop is cultivated on a large scale but as yet there is no satisfactory fully-mechanised system for harvesting the tubers for the fresh market (though great progress is being made) and, in the USA, prices have increased steadily and consumption has declined by about 60 per cent over the last 30 years.
High production costs have also handicapped the use of sweet potatoes as a source of industrial starch and even in Japan, where starch has been extracted as a cottage-based industry for many years, production is declining, owing to the increased availability of lower-priced maize starch.
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