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Contents

[edit] Root crops (NRI, 1987, 308 p.)

[edit] Yam (Dioscorea spp.)

YAM.

Botanical name

Dioscorea spp.

Family name

Dioscoreaceae.

Other names

Car� (Braz.); Gname (It.); Ighnam (Ar.); Igname (Fr. and It.); Ignamekolle (Ger.); Inhame (Port.); Iniamas (Nether.); Name (Sp.); Nyambi (Fr.); Nyami (Sen.); Yamswurzel (Ger.).

Botany

Dioscorea is a large genus of over 600 species with subterranean tubers or rhizomes. The tubers are storage organs and often grown to a considerable size; they produce short, fibrous roots and annual shoots, which are twining (except in dwarf species), the direction of twining being specific.

The leaves are petiolate, often cordate, with strongly marked reticulate veining (unusual for a monocotyledon), sometimes lobed, occasionally palmately compound. Many species produce bulbils in the axils of the leaves which have the morphology and appearance of condensed stems and in a few instances are relatively large and tuberous. The flowers are small, and borne in long racemes, with male and female separate and usually borne on different plants. The female flowers are followed by dehiscent capsules, usually trilocular, with 6 seeds, usually winged for wind dispersal, though many of the cultivated forms have become partially or highly sterile.

The genus Dioscorea is divided into a number of taxonomic sections; the important food yams are grouped in the following: Enantiophyllum-D. alata, D. rotundata/D. cayenensis complex, D. opposita and D. japonica; Lasiophyton-D. dumetorum and D. hispida; Combilium-D. esculenta; Macrogynodium-D. trifida; Opsophyton-D. bulbifera. There are some 60 species that have been used for food, but most are of little importance; the above (with the exception of D. japonica) are described in the chapters following this general chapter.

Origin and distribution

The genus Dioscorea is considered to be among the most primitive of the Angiosperms and was present and well diversified in part of the southern world at the end of the Cretaceous period (approximately 75 million years ago), and the early spread appears to have been via an antarctic continent (whose climate was totally different in early geological times). The occurrence of Dioscorea spp. in southern Asia, Africa and South America long pre-dates human history and domestication of the different species in these areas appears to have been by aboriginal man. Wild yams and domesticated cultivars occur throughout the tropical and subtropical world, with one dwarf species (D. pyrenaica Bub. and Borders) as far north as the Pyrenees. West Africa is the most important cultivation zone, where yam is a major staple, producing about 93 per cent of the world's edible yams, but the crop is also of considerable importance in parts of eastern Africa, the Pacific area (including Japan), the Caribbean and tropical America.

Cultivation conditions

Temperature-most edible yams cannot withstand frost and make poor growth below 20°C. Optimum growth occurs at about 30°C; temperatures much above this have an adverse effect, especially if associated with drought. An exception is D. opposita (see Chinese yam).

Rainfall-although generally considered drought resistant, yams require adequate moisture throughout their growing period and there is a positive correlation between high and regular rainfall, vine growth and tuber yield. For optimum yields adequate moisture between the 14th and 20th weeks of growth is of great importance. The major areas of production are centred where there is a sharply demarcated dry season of 2-5 months and a rainfall of 120-150 cm or more during the growing season. In parts of West Africa yams are grown where the rainfall is as low as 60 cm per year, but yields are very poor, while crops are also obtained where the annual rainfall reaches 300 cm.

Soil-good drainage is essential and for optimum yields a deep well-drained sandy loam is required. On heavy, waterlogged soils the tubers are liable to rot, while on poor soils the weak root system is unable to obtain sufficient water or nutrients to produce reasonably-sized tubers. Most yams are grown on land after it has been cleared from bush; fallow mulching is often practiced and FYM at the rate of 17.5-25 t/ha gives greatly increased yields. Fertilisers are not widely used but there is a wide response to treatment, particularly to the application of phosphorus and potassium. The use of potassium sulphate at the rate of 125-370 kg/ha or a 12:12:8 NPK mixture at the rate of about 60 g per mound has been recommended in Nigeria; while in Trinidad potassium muriate at 376 kg/ha and superphosphate at 376 kg/ha are used. In Barbados yams grow well as a rotation crop in sugar cane land that has residual nitrogen and potassium from the previous cane crop, but yields have been substantially increased by the application of 225 kg/ha of a 9:10:23 NPK fertiliser. Application should be 2-3 months after growth commences. The application of potash alone has given yield increases in Nigeria, and it is also reported to increase the storage life of the tubers, while chlorine in the fertiliser adversely affects the starch content.

Altitude-most yams can be grown successfully at low or medium elevations and some, such as D. alata, are reported to be grown at altitudes up to 2 700 m in the Himalayas, but in general yields are considerably reduced above 900 m.

Day-length-the majority of Dioscorea spp. exhibit a photoperiodic response and although day-lengths greater than 12 hours favour the growth of the vine, tuber development is normally most satisfactory under short-day conditions (10-11 hours of daylight).

Planting procedure

Material-edible yams are normally propagated by the use of small tubers (seed yams), cuttings off the tubers, setts (pre-sprouted tubers or pieces of tuber), or bulbils. It is possible to use vine cuttings, but tuber production by this method is generally uneconomic. (All types of vegetative planting material other than vine cuttings are commonly referred to as setts.) The best planting material is the small whole tuber and species such as D. esculenta and D. trifida, which produce a fairly large number of tubers, can be propagated very easily by reserving a few of the tubers and planting these at the beginning of the next season. Other species such as D. bulbifera and some forms of D. alata produce aerial bulbils, which can be used, but the majority of the more important food yams only produce 1-3 tubers a season and in this case setts cut from the tuber are often used and are referred to as tops or 'heads' (proximal), middles, and bottoms or 'tails' (distal). In general, tops are preferred and the larger the sets, the earlier and greater is the rate of germination. The weight of sett used varies from about 0.25 to 2 kg but occasionally whole tubers weighing up to 4.5 kg are used, especially when extra large yams are required for ceremonial purposes. Sometimes the body of the yam is cut off and the head left in the soil to grow and produce seed yams for propagation; this practice is known as 'topping' or 'milking'. In addition, planting material may be produced in specialised yam nurseries, where one to four small setts of 85-150 g are planted in small heaps of soil, normally up to 12 500/ha, and these yield small yams suitable for use as seed. Most yams have a definite period of dormancy, but this may be broken by the use of a chemical such as ethylene chlorhydrin, where production of out-of-season tubers is required.

A recent development has been the production in Barbados of virus-tested planting material, in which yams grown from virus-free meristem tip cultures are being multiplied in the field and, after inspection, distributed for planting. This material has been tested in a number of Caribbean islands and has given approximately double the yield obtained when conventional seed yams are planted, and the operation is now commercial.

Method-yams are usually intercropped with maize and vegetables, such as cucurbits, pumpkins, peppers and okra, but mono-culture, normally on small plots, is increasing in certain areas of West Africa and the Caribbean. Three types of planting systems are practiced: the setts may be planted on the flat, they may be planted in trenches or holes, or they may be planted on mounds, ridges or raised beds. The last method is the most widespread and the mounds can vary from about 50 cm high and perhaps twice as wide at the bottom, to nearly 100 cm high and twice this width at the base. In the smaller mounds one sett is normally planted and in the larger ones three or four, or even eight to ten setts. In general, larger mounds are preferred and the setts are planted in holes dug in the sides near the natural ground level. They are planted deeply to avoid drying out of the young shoots and for this reason the head of the sett is also placed downwards. Sometimes, instead of individual mounds, ridges are used and the setts are planted along the sides of the ridges. Planting on the flat is only practiced in areas such as river flood plains, where the soil is deep and soft. In this system, the setts are planted in holes just below the soil surface. Support for the growing vines is usually provided, most often stakes or trellises, or strings attached to horizontal ropes or trees, sometimes corn stalks left from an intercrop of maize, or even bushes: there is evidence indicating that such support is necessary for satisfactory plant and tuber development. However, a few cultivars, notably of D. alata and D. esculenta, are adapted to trailing on the ground without support, and in Barbados D. alata is cultivated, without staking, on the plantation scale. Recent work suggests that with close planting other species may also give satisfactory yields without support.

Planting is normally by hand, but a mechanical planter is now being used in Barbados. For optimum yields yams must be kept free from weeds, at least for the first three months of growth, and the following herbicides have been used successfully: diuron 2.6 kg/ha, together with TCA at 4.4 kg/ha, and atrazine at 1.5-2.9 kg/ha.

Time of planting-yams are not normally grown under irrigation and in areas where the rains last 8-10 months planting normally takes place just before or at the beginning of the rains. Where the rainy season is less than 8 months it has been found that early planting, up to 3 months before the rains, can give a 30 per cent increase in yield.

Field spacing-a wide range of planting distances is used, depending upon the species, the soil type and the water table and whether intercropping is practiced; mounds are often irregularly spaced and planting distances ranging from 0.7 to 2.3 m2 have been reported. Generally, the wider the spacing the lower the yield and common spacings are 1.2 x 1.2 m, 1.2x0.9 m and 1.8x0.6 m.

Seed rate-the number of setts used obviously varies according to the species and the cultivar, but for most large-tubered yams 10 000-15 000/ha are used, requiring at least 2.5 t/ha of setts.

Pests and diseases

Weeds-can be serious competitors with yams. While hand-weeding is the most common practice, pre-emergence spraying with atrazine or ametryn will control weeds until the plants have sprouted; subsequently paraquat carefully applied with a shielded spray may be used. In due course the foliage should become thick enough to cover the ground and eliminate weeds, especially when the vines are unslaked.

Pests-yam beetles of several species are important, especially in Africa: these include the greater yam beetle (Heteroligus meles), the lesser yam beetle (H. appius), also Heteronychus licas, Prionoryctes rufopiceus, P. caniculus and Lilioceris spp. These attack the tuber setts and may prevent sprouting. Dusting the plant setts with 2 per cent aldrin or 0.5 per cent gamma-HCH will normally prevent attack. In the Caribbean, the yam weevil Palaeopus costicollis causes similar damage and control is as for the yam beetles. The termite, Amitermes evancifer, is occasionally a serious pest of yam tubers in Africa. Yam scale (Aspidiella hartii) attacks stored yams in Africa, Asia and the Pacific; in the Caribbean its principal damage is to young vines which may be destroyed completely. It is important to use scale-free planting material; this, together with the dusting recommended above, should provide adequate control. Mealy bugs, Geococcus coffeae, Phenacoccus gossypii, Planococcus citri, and (in Papua New Guinea) P. dioscoreae, feed on the tubers and roots of yams, and can multiply to considerable numbers, causing shrivelling of the stored tubers. Only clean and healthy material should be planted (again the dusting treatment recommended for yam beetles should be used), and, if aerial parts of the plant are affected, spraying with malathion or malathion plus an oil emulsion (eg Triona or Albolineum) is recommended.

Several species of nematodes attack yams. The yam nematode, Scutellonema bradys, is widely distributed in both Old and New World tropics and causes 'dry rot' of the tubers. Pratylenchus coffeae, causing rather similar lesions, has been reported to attack yams in Puerto Rico, Jamaica and the Solomon Islands, while the root knot nematodes, Meloidogyne spp. of world-wide distribution, sometimes attack this crop. Chemical control has not proved entirely satisfactory, though D-D and dibromochloropropane have given some reduction in the infestation rate. Absence of host plants and a fallow period are recommended, and care must be taken to avoid planting infected material.

Diseases-these include anthracnose (caused by Glomerella cingulata), which produces black necrotic lesions on leaves and stems, and can kill the plant by attacking the terminal bud, and leaf spot, caused by various species of Cercospora, Colletotrichum, and Phyllosticta. Control involves sanitation by removal of crop debris, and fungicide treatment: maneb, benomyl, benomyl+ propineb, zineb and mancozeb have all been reported to give reasonably good results. During storage of the tubers, severe losses are caused by rotting due to Botryodiplodia theobromae, Aspergillus spp., Rosellinia bunodes, Lasiodiplodia sp., Fusarium oxysporum, F. solani and other Fusarium spp. These rots may also affect the growing plant when the setts consist of cut pieces of tuber, but are controlled by simple measures such as the painting of the cut surfaces with limewash or Bordeaux mixture, or coating with wood ash. Rotting during storage may be minimised by treating cut or bruised surfaces of the harvested tubers in the same manner.

Virus diseases have been reported from the Caribbean and West Africa, but are probably world-wide. Most are of the mosaic type causing leaf mottling, and most are serious only when the infection occurs early and is severe, leading to stunting and sometimes causing the production of numerous basal shoots, giving the plant a bushy appearance. In the Lesser Antilles an internal brown spotting (first observed in Barbados) is associated with virus infection; the affected tubers develop hard brown nodules in the flesh, often surrounded by necrotic areas, and the foliage has (not always easily discernible) mosaic symptoms. Yields of affected plants may be reduced by half. No vector has been identified for any of these viruses, but a meristem culture technique and the production of virus-tested yams has been developed and carried through to the commercial scale in Barbados.

Growth period

Most edible yams normally reach maturity 8-11 months after planting, though in certain species a first harvest may be obtained after 5 months. The growth period usually comes to an end at, or shortly after, the end of the rainy season: neither late planting nor subsequent irrigation will prolong growth beyond the normal annual periodicity for the particular species.

Harvesting and handling

At the start of the dry season yam plants normally die back and the tubers are ready for harvesting, though in most cases they may be left in the ground for several weeks as deterioration is usually not rapid. In some species, eg D. rotundata and D. alata, an early crop may be taken as well as the main harvest; in this case the tuber is carefully cut below the head and removed, leaving the top to grow again and produce another tuber, or tubers. Large yams are usually dug out by hand with wooden spades or digging sticks, or with forks-a laborious task since great care has to be taken to avoid damaging the tubers. Yam species which produce a number of small tubers can be harvested mechanically with a potato spinner, but recent work in the Caribbean has developed a mechanical harvester suitable for the large-tubered D. alata planted on ridges. Aerial tubers or bulbils are usually plucked by hand from the vine as required.

Yam tubers, if unaffected by pests and diseases, may be stored until their natural period of dormancy is broken. It has already been noted that many species may be stored by leaving them unharvested during part, at least, of the dry season. Normally, however, yams are harvested and stored. The tubers must be clean and undamaged, excessive temperatures must be avoided and good aeration provided. Some varieties will keep in good condition for about 6 months, though the tubers may lose 10-40 per cent or more of their weight, and the storage life differs between different species or even cultivars. Several methods of storage are used: in some parts of Asia and Africa tubers are stacked into heaps which may be small or which may contain several tonnes; these heaps are covered with straw or leafy branches. In West Africa yam barns are common, consisting of a wooden vertical or nearly vertical framework to which the tubers are individually tied: the frames are usually 1-2 m high and are built in the shade of a tree or under a thatched roof. These frames provide excellent ventilation and the tubers can be protected from termite attack and flooding. In the Pacific Islands specially constructed thatched huts with a raised platform, on which the yams are stacked, are used. In many countries, yams are stored simply in ordinary storerooms, sheds, or under houses that are built on piles or stilts; often the yams are arranged in piles or rows about 1 m high and 1 m broad, allowing for ventilation and access.

Most yams, with the exception of D. trifida, are liable to chilling injury if exposed to temperatures below about 12°C, an important consideration when yams are to be exported to temperate climates.

Insect damage to yams during storage is usually not serious, though lesions produced by insects (often during the growth of the tubers) may permit entry of fungi or bacteria, as do cuts and bruises received during harvesting. Rots caused by fungi (see Pests and diseases) may be rapid, destroying a tuber within a week or two.

Prolongation of dormancy by chemical methods is not normally successful but recent experiments have achieved a degree of success using gamma irradiation of sound tubers at a dosage of 12.5 head; this treatment has also somewhat reduced weight loss. The method is claimed to be economically and practically superior to storage at 15°C.

Primary product(s)

Tubers-the subterranean tubers consist mainly of starchy tissues covered by a suberised layer forming a skin. There is great variation in the size, form and colour of tubers, in their texture, flavour, thickness of skin, and in storage behaviour.

The principal economic species are the Enantiophyllum yams (see Botany), and these produce few tubers, normally one to three, which may be globular, flattened, cylindrical or elongated and sometimes branched or lobed, under normal conditions weighing from 3 to 15 kg, though specially grown 'giant tubers' can exceed 50 kg. The Lasiophyton yams form several medium-sized tubers, sometimes fused into an irregular cluster. The Asian Combilium yams and the American Macrogynodium yams produce a large number of small spindle-shaped tubers, similar to sweet potatoes.

Bulbils-many yam species also produce bulbils in the axils of leaves, which may become similar to underground tubers, but smaller. In a few species, eg D. bulbifera (Opsophyton), the bulbils are the main storage organs.

Yield

Under optimal conditions yams are among the most efficient producers of human food: yields of 70 t/ha (D. esculenta) have been recorded from West Irian and 58 t/ha (D. alata) from St. Vincent. Under normal farming conditions, however, yields are considerably lower, the normal range for yams in pure stands being: West Africa 7.5-18 t/ha; South-East Asia 12.5-25 t/ha; the Caribbean 20-30 t/ha. These yields are gross, and because a substantial quantity is used for propagation, net yields are generally about 2.5 t/ha less.

Main use

Yams are a staple carbohydrate food, commonly eaten as a vegetable, either boiled, baked or fried. In West Africa a major proportion of the yam crop is eaten as 'fufu', a stiff, glutinous dough.

Subsidiary uses

Yams are sometimes dried and made into flour; this is often the case with damaged yams or with yams that are surplus to requirements, essentially as a method of storage. In recent years there have been attempts at more sophisticated processing for export, eg fufu from Nigeria, canned yams and yam soup from Puerto Rico, and yam flakes from Barbados. In general these attempts have not been commercial successes, largely owing to the high cost of the raw material.

Secondary and waste products

Peelings and waste from yams are often used for feeding poultry and livestock. The possibility of using yams for the production of starch or of alcohol has been considered, but not developed, as there are many cheaper sources of both these products in the countries where yams are grown. Poisoned bait for hunting and fishing is sometimes prepared from D. hispida and D. dumetorum in parts of Asia and Africa. Diosgenin (used as a basis for the corticosteroid family of drugs) has been commercially prepared from some species notably D. mexicana, D. floribunda and D. composita. Some species, eg D. cirrhosa, are used as a source of tannin.

Special features

Yams are essentially carbohydrate foods with relatively high protein and ascorbic acid contents. Typical figures for the composition of the edible portion of fresh tuber are: energy 439 kJ/100 g; water 72.4 per cent; protein 2.4 per cent; fat 0.2 per cent; carbohydrate 24.1 per cent; fibre 0.6 per cent; calcium 22 mg/100 g; iron 0.8 mg/100 g; thiamine 0.09 mg/100 g; riboflavin 0.03 mg/100 g; niacin 0.5 mg/100 g; ascorbic acid 10 mg/100 g.

Proximate analyses of different species will be found in the specific chapters.

Many yam cultivars contain substantial amounts of mucilage which affects the culinary properties of the tubers. In addition, several Dioscorea spp. contain alkaloids, tannin and sapogenins. For example, the toxic alkaloid dioscorine is present in the edible species D. hispida and the presence of diosgenin in D. mexicana and others has already been noted.

Processing

Fufu-is prepared by cutting peeled yams into small pieces and boiling in water until soft, then pounding in a mortar until a sticky dough is formed, which is usually firm enough to be cut into slices.

Yam flour-traditionally a method of long-term storage, in which the tubers are sliced to a thickness of about I cm, peeled and dried in the sun. When dry, the hard slices are ground to give a coarse flour. In a slightly more sophisticated process, washed tubers are cut into slices about 5 cm thick, cooked until soft, then peeled and mashed into a pulp which is spread out to a depth of about 2 cm and dried for 6-8 hours at a temperature of 50-70°C, until the moisture content is reduced to 10 per cent. The dried material is finely ground and passed through a sieve before being packed into polyethylene bags. When reconstituted the product somewhat resembles fufu.

Yam flakes-have been prepared in the Caribbean from D. alata and in Nigeria from D. rotundata: the process is outlined in the chapter on Greater yam.

Production and trade

Production-despite the increasing consumption of introduced foods such as cassava and rice in the main yam-consuming area of the world (West Africa), it appears that yam production is remaining constant at about 20 million tonnes per year. Table 1 is taken from FAO statistics (however, the last year for which FAO published separate figures for yams was 1975): it may be seen that some 97 per cent of the world's production derives from Africa (over 90 per cent from West African countries).

P286.GIF
Table 1: Yam - Area and production in selected countries

P287.GIF
Table 1: Yam - Area and production in selected countries (continued)

World production per hectare increased slightly during the decade 1965-1975 (Table 2), but this was almost entirely due to improvements in Africa (and, to a lesser extent, Asia). Productivity fell slightly in South America.

Table 2: Yam - Average yields (t/ha)


1961-65

1975

World

7.6

9.37

Africa

7.55

9.54

North and Central America

10.04

11.12

South America

5.7

4.69

Asia

8.29

11.35

Oceania

14.84

13.51

All developed countries

17.91

17.92

All developing countries

7.58

9.54

Trade-accurate figures are not available, but trade sources estimate imports of yams to the UK for 1982 to have been about 10 000 t, made up as follows: Brazil 5 000 t; Jamaica 3 000 t; Barbados 100 t; West Africa 400 t; others 1 500 t. Until 1980 Colombia supplied 5 000 t/a to the UK, but this fell to zero during 1981 and 1982. It is believed that the Colombian export trade is now entirely to the USA and that the USA also imports about 5 000 t from Brazil and a substantial quantity from Puerto Rico. Canadian trade sources indicate average imports of about 2 000 t/a from Jamaica for the past decade, but expect a slow decline unless there is a reduction in price. It appears that the total quantity of yams entering overseas trade is fairly steady at about 22 000 t/a.

Major influences

Yam in most parts of the tropical world is more costly (mainly because of high labour inputs and relatively low yields) than most of the competing carbohydrate foods, but in spite of this continues to be highly favoured, at least partly for traditional and ethnocultural reasons. It appears to be holding its own in terms of production and the adoption of full mechanisation as currently being developed (mainly in the Caribbean), along with breeding programmes for high-yielding, shallow-rooting cultivars and elimination of staking, should ensure that yam production will not decrease, but possibly increase. A small export trade seems likely to continue.

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KANG, B. T. and WILSON, J. E. 1982. Effect of heap size and fertilizer application on yam (Dioscorea rotundata) in southern Nigeria. Proceedings of the 5th International Symposium on Tropical Root and Tuber Crops (Philippines, 1979), pp. 245-258. Los Ba�os, Laguna, Philippines: Philippine Council for Agriculture and Resources Research, 720 pp.

KASASIAN, L. 1971. Root crops. Weed control in the tropics, pp. 157- 158. London: Leonard Hill, 307 pp.

L�ON, J. 1977. Origin, evolution and early dispersal of root and tuber crops. Proceedings of the 4th Symposium of the International Society for Tropical Root Crops (Colombia, 1976), IDRC-080e (Cock, J., MacIntyre, R. and Graham, M., eds), pp. 20-36. Ottawa, Canada: International Development Research Centre, 277 pp.

LINNEMAN, A. R. 1981. Preservation of certain tropical root and tuber crops. Abstracts on Tropical Agriculture, 7 (1), 9-20.

MANTELL, S. H. and HAQUE, S. Q. 1979. Disease-free yams: their production, maintenance and performance. Yam Virus Project Bulletin, No. 2. St. Augustine, Trinidad: Caribbean Agricultural Research and Development Institute, 22 + vii pp.

MANTELL, S. H., HAQUE, S. Q. and WHITEHALL, A. P. 1979. A rapid propagation system for yams. Yam Virus Project Bulletin, No. 1. St. Augustine, Trinidad: Caribbean Agricultural Research and Development Institute, 19 pp.

MANTELL, S. H., HAQUE, S. Q. and WHITEHALL, A. P. 1980. Apical meristem tip culture for eradication of flexous rod viruses in yams (Dioscorea alata). Tropical Pest Management, 26, 170-179.

MARTIN, F. W. 1977. A collection of West African yams. Proceedings of the 3rd Symposium of the International Society for Tropical Root Crops (Nigeria, 1973) (Leakey, C. L. A., ed.), pp. 23-27. Ibadan, Nigeria: International Society for Tropical Root Crops in collaboration with the International Institute of Tropical Agriculture, 492 pp.
MOHAMED, N. A. 1976. Virus-like particles and cytoplasmic inclusions associated with diseased Dioscorea spp. in the Eastern Caribbean. Tropical Agriculture, Trinidad, 53, 341-351.

MOHAMED, N. A. and MANTELL, S. H. 1976. Incidence of virus symptoms in yam (Dioscorea sp.) foliage in the Commonwealth Caribbean. Tropical Agriculture, Trinidad, 53, 255-261.

MONTALDO, A. 1972. Names. Cultivo de ra�ces y tub�rculos tropicales, pp. 24-50. Lima, Peru: Instituto Interamericano de Ciencias Agricolas de la OEA, 284 pp.

NOON, R. A. 1978. Storage and market diseases of yams. Tropical Science, 20, 177-188.

OBIGBESAN, G. O., AGBOOLA, A. A. and FAYEMI, A. A. A. 1977. Effect of potassium on tuber yield and nutrient uptake of yams. Proceedings of the 4th Symposium of the International Society for Tropical Root Crops (Colombia, 1976), IDRC-080e (Cock, J., MacIntyre, R. and Graham, M., eds), pp. 104-107. Ottawa, Canada: International Development Research Centre, 277 pp.

OGUNDANA, S. K., NAQVI, S. H. Z. and EKUNDAYO, J. A. 1970. Fungi associated with soft rot of yams (Dioscorea spp.) in storage in Nigeria. Transactions of the British Mycological Society, 54, 445-451.

ONAYEMI, O. 1983. Observations on the dehydration characteristics of different varieties of yam and cocoyams. Abstracts of the 6th Symposium of the International Society for Tropical Root Crops (Peru, 1983), p. 96. Lima, Peru: International Potato Center, 113 pp.

ONWUEME, I. C. 1982. A strategy package for reducing the high labour requirement in yam production. Yams: Ignames (Mi�ge, J. and Lyonga, S. N., eds), pp. 335-344. Oxford: Oxford University Press, 411 pp.

ONWUEME, I. C. 1982. Tuber physiology in yams (Dioscorea spp.) and its agricultural implications. Proceedings of the 5th International Symposium on Tropical Root and Tuber Crops (Philippines, 1979), pp. 235-243. Los Ba�os, Laguna, Philippines: Philippine Council for Agriculture and Resources Research, 720 pp.

THOMPSON, A. K., BEEN, B. O. and PERKINS, C. 1973. Nematodes in stored yams. Experimental Agriculture, 9, 281-286.

THOMPSON, A. K., BEEN, B. O. and PERKINS, C. 1977. Fungicidal treatments of stored yams. Tropical Agriculture, Trinidad, 54, 179-183.

VANDEVENNE, R. 1977. Mechanization of yam cultivation in the Ivory Coast. Proceedings of the 3rd Symposium of the International Society for Tropical Root Crops (Nigeria, 1973) (Leakey, C. L. A., ed.), pp. 263-271. Ibadan, Nigeria: International Society for Tropical Root Crops in collaboration with the International Institute of Tropical Agriculture, 492 pp.

WICKHAM, L. D. 1983. Dormancy mechanism in yams, Dioscorea spp. Abstracts of the 6th Symposium of the International Society for Tropical Root Crops (Peru, 1983), p. 94. Lima, Peru: International Potato Center, 113 pp.

WILSON, J. E. 1982. Present and future roles of yams (Dioscorea spp.) in West Africa. Proceedings of the 5th International Symposium on Tropical Root and Tuber Crops (Philippines, 1979), pp. 205-211. Los Ba�os, Laguna, Philippines: Philippine Council for Agriculture and Resources Research, 720 pp.

WILSON, J. E. 1982. Recent developments in the propagation of yam (Dioscorea spp.). Yams: Ignames (Mi�ge, J. and Lyonga, S. N., eds), pp. 55-59. Oxford: Oxford University Press, 411 pp.

WOOD, T. G., SMITH, R. W., JOHNSON, R. A. and KOMOLAFE, P. O. 1980. Termite damage and crop loss studies in Nigeria-Pre-harvest losses to yams due to termites and other soil pests. Tropical Pest Management, 26, 355-370.


[edit] Bitter yam (Dioscorea dumetorum)

Common names

BITTER YAM, Cluster yam.

Botanical name

Dioscorea dumetorum (Kunth) Pax. (Lasiophyton).

Family

Dioscoreaceae.

Other names

Esuri yam, Esur�, (W. Afr.); Ikamba (Gab.); Name amargo (S. Am.); Ono (W. Afr.); Three leaved or Trifoliate yam.

Botany

The stems twine clockwise (to the left), unlike most other yams of economic importance, and are robust, hirsute and spiny. The leaves are trifoliate with tomentose leaflets that have the typical reticulate venation of yams, 12-16 cm long, 6-9 cm broad: the petioles are pubescent and often spiny. The flowers are small dioecious and fertile, forming oblong fruit, about 4 cm in length and 2 cm in diameter. The tubers may be single or (usually) produced in clusters: bulbils are rarely formed. The wild forms are often very poisonous, but the cultivated forms usually have little toxicity. This African species is closely related to the Asian species
D. hispida Dennst.

Origin and distribution

The species is found wild throughout tropical Africa between 15°N and 15°S, and is cultivated in West Africa, especially in Nigeria.

Cultivation conditions

See Yam

Planting procedure

Setts are normally used but satisfactory results have been obtained with vine cuttings.

Pests and diseases

Among other pests of stored yams (see Yam) the insects Araecerus fasciculatus and Lepidobregma minuscula have been identified in Nigeria as infesting tubers of D. dumetorum.

Growth period

Usually 8-10 months.

Harvesting and handling

D. dumetorum is easily harvested by hand (and could be mechanically harvested were it grown on the large scale). The tubers do not store well, a high proportion becoming hard and inedible within 4 weeks after lifting. Drying of sliced tubers is used as a method of storage.

Primary product

Tubers-which can show great variation in colour, form and quality. The tubers may be single or form a cluster. The flesh may be white, pale-yellow, or dark-yellow, the last being bitter. The wild forms are usually toxic and the degree of toxicity is generally in inverse proportion to the depth to which the tubers penetrate in the soil.

Yield

No reliable data appears to have been recorded, but yields are generally reported to be higher than for most other edible yam species in Africa.

Main use

Bitter yam is used as a vegetable, but not pounded into 'fufu'. Owing to its soft texture it is favoured by old people with poor teeth. The wild forms are regarded as famine food, and the tubers are detoxified by slicing and soaking and boiling, frequently with the addition of salt; the slices may be subsequently dried. It is becoming a preferred yam in Cameroon.

Subsidiary uses

The dried tubers can be used to prepare flour. In the Sudan wild detoxified tubers have been ground into a flour which has been used as a base for the preparation of beer.

Secondary and waste products

The tubers of wild varieties mixed with bait are sometimes used for poisoning animals in parts of Africa, and cases of their use for criminal purposes have been recorded. This species is sometimes deliberately planted in fields of other edible yams in order to discourage thieves.

Special features

A typical analysis of the edible portion of the tubers is: water 79 per cent; protein 2.78 per cent; fat 0.28 per cent; carbohydrate 17 per cent; fibre 0.3 per cent; ash 0.72 per cent; calcium 92 mg/100 g; ascorbic acid 6.6 mg/100 g.

The carbohydrate consists mainly of starch, the granules of which are small, rounded or polyhedral, average size 1-4 microns, with a gelatinisation temperature of 77-85.5°C. Analysis of the starch of D. dumetorum has been given as: moisture 13.5 per cent; protein 1.49 per cent; ash 0.39 per cent; amylose 15 per cent; pH 4.4; iod. val. 3.9. Many forms of D. dumetorum contain a convulsant alkaloid which is a mixture of stereoisomers of dihydrodioscorine.

Major influences

D. dumetorum continues to be of importance in Nigeria, Cameroon and several central African nations, and a breeding programme has been started on this species. Further, it has been shown to be less demanding of manpower in its cultivation than D. alata and D. cayenensis.

Bibliography

BEVAN, C. W. L., BROADBENT, J. L. and HIRST, J. 1956. Convulsant alkaloid of Dioscorea dumetorum. Nature, London, 177 (4516), 935.

BEVAN, C. W. L. and HIRST, J. 1958. A convulsant alkaloid of Dioscorea dumetorum. Chemistry and Industry, (4), 103.

CORKILL, N. L. 1948. The poisonous wild cluster yam Dioscorea dumetorum Pax., as a famine crop in the Anglo-Egyptian Sudan. Annals of Tropical Medicine and Parasitology, 42 (3/4), 278-287.

COURSEY, D. G. 1967. Yams. London: Longmans, Green and Co. Ltd, 230 pp.

COURSEY, D. G. 1983. Yams. Handbook of Tropical Foods (Chan, H. C. (Jr.), ed.), pp. 555-601. New York: Marcel Dekker, 639 pp.

LYONGA, S. N. 1981. The economics of yam cultivation in Cameroon. Tropical Root Crops: Research Strategies for the 1980s: Proceedings of the 1st Triennial Root Crops Symposium of the International Society for Tropical Root Crops-Africa Branch (Nigeria, 1980), IDRC-163e (Terry, E. R., Oduro, K. A. and Caveness, F., eds), pp. 208-213. Ottawa, Canada: International Development Research Centre, 279 pp.

PASSAM, H. C. 1982. Dormancy of yams in relation to storage. Yams: Ignames (Mi�ge, J. and Lyonga, S. N., eds), pp. 285-293. Oxford: Oxford University Press, 411 pp.

PLUMBLEY, R. A. and REES, D. P. 1983. An infestation by Araecerus fasciculatus (Degeer) (Coleoptera: Anthribidae) and Decadarchis minuscula (Walsingham) (Lepidoptera: Tineidae) on stored fresh yam tubers in southeast Nigeria. Journal of Stored Products Research, 19, 93-95.

PURSEGLOVE, J. W. 1972. Dioscorea dumetorum (Kunth) Pax. African bitter or cluster yam. Tropical crops: Monocotyledons 1, p. 106. London: Longman Group Ltd, 334 pp.

RASPER, V. and COURSEY, D. G. 1967. Properties of starches of some West African yams. Journal of the Science of Food and Agriculture, 18, 240-244.

TRECHE, S. and DELPEUCH, F. 1982. Le durcissement de Dioscorea dumetorum au Cameroun. Yams: Ignames (Mi�ge, J. and Lyonga, S. N., eds), pp. 294-311. Oxford: Oxford University Press, 411 pp.

WAITT, A. W. 1963. Yams, Dioscorea species. Field Crop Abstracts, 16, 145-157.

WILSON, J. E. 1982. Progress in the breeding of yam, Dioscorea spp. Yams: Ignames (Mi�ge, J. and Lyonga, S. N., eds), pp. 17-22. Oxford: Oxford University Press, 411 pp.

WILSON, J. E. 1982. Recent developments in the propagation of yam (Dioscorea spp.). Yams: Ignames (Mi�ge, J. and Lyonga, S. N., eds), pp. 55-59. Oxford: Oxford University Press, 411 pp.


[edit] Chinese yam (Dioscorea opposita)

Common name

CHINESE YAM

Botanical name

Dioscorea opposita Thunb. (Enantiophyllum).

Family

Dioscoreaceae.

Other names

Chinese potato, Cinnamon vine, Igname de Chine (Fr.); Name (de) Chino (Venez.).

Botany

The vine stems are round, spineless and climb to a height of about 3 m twining anticlockwise (to the right). The leaves are opposite, acuminate, 4-8 cm long. Bulbils are formed in the leaf axils. The flowers are cinnamon-scented, sessile in 1-2 simple raceme-like spikes from the axils.
The tubers are variable in form but are often spindle-shaped and long, reaching I m, descending vertically into the ground; some cultivars have been selected for shorter and thicker tubers.

Origin and distribution

D. opposita is a native of China and widely grown there and in Japan, Korea and the Ryukyu Islands. It is subtropical and can tolerate much colder conditions, including frost, than most yam species. It was grown experimentally in Europe during the 19th century at the time of the potato famine, and is still grown in France to supply the immigrant food market.

Cultivation conditions

Chinese yam requires a good supply of moisture and fertile soil and responds to the application of fertiliser. In Japan it is estimated that for optimum yields 394 kg/ha of nitrogen, 296 kg/ha of phosphorus and 350 kg/ha of potassium are required.

This species requires day-lengths of 10-11 hours for tuberisation.

Planting procedure

In Japan pieces of tuber weighing approximately 280 g are cut into four pieces and after disinfection with formalin are planted in ridges 80 cm apart with 36 cm between the yam setts. Higher yields are obtained if the setts are first germinated in warm seed beds to the stage when the adventitious bud appears as a small knob. The usual planting rate is about 2 750 kg/ha. Bulbils are sometimes used, but these may take 3 years to produce an economic yield of tubers.

Growth period

When setts are used the tubers are ready for harvesting in about 6 months.

Harvesting and handling

A wooden digging stick is widely used for harvesting the long cylindrical tubers; the shorter, thicker tubers can be lifted by forking. In Japan most of the crop is consumed almost immediately after harvesting, but the tubers can be stored successfully in clamps, and cold storage units are used for storing part of the crop.

Primary product

Tubers-occur in three recognized forms in Japan: thin cylindrical tubers often measuring I m in length, known as Naga-imo; a palmer form known as Icho-imo; and a globular form known as Tsukune-imo. Naga-imo is grown throughout the islands, but the other forms are grown only in the warmer regions.

Yield

Typical yields from Japan have been reported as 2.25 t/ha.

Main use

The tubers are eaten as a vegetable, usually by slicing or grating and boiling, but sometimes they are ground to make 'tororo', a traditional Japanese dish.

Subsidiary uses

In Japan about 50 per cent of the Chinese yams are used as a raw material in the preparation of various food products, such as pastry, beanjam bun, fish paste and yam flour.

Special features

A typical analysis of the edible portion of the tubers is: water 70-80 per cent; protein 1.11-3.1 per cent; fat 0.06-1.1 per cent; carbohydrate (mainly starch) 16-29 per cent; fibre 0.33-1 per cent; ash 0.69-1.1 per cent.

The starch consists mainly of fairly large granules (5-60 microns) and the gelatinisation temperature is from 65.5° to 75.5°C. Seventeen amino acids have been isolated from the tubers, which have been found to have a high tryptophan and serine content. Stored tubers when cut or grated, rapidly discolour and it has been suggested that field spraying with maleic hydrazide will suppress browning of the tubers which is due to the presence of polyphenolic compounds. Unlike most other yam species grown in Japan, the tubers of D. opposita do not contain any sapogenins.

Major influences

There is a growing demand for yam flour produced by modern freeze-drying methods in Japan and production of D. opposita is reported to be expanding.

Bibliography

AKAHORI, A. 1965. Studies on the steroidal components of domestic plants: steroidal sapogenins in Japanese Dioscorea spp. Phytochemistry, 4, 97-106.

COURSEY, D. G. 1967. Yams. London: Longmans, Green and Co. Ltd, 230 pp.

COURSEY, D. G. 1983. Yams. Handbook of Tropical Foods (Chan, H. C. (Jr.), ed.), pp. 555-601. New York: Marcel Dekker, 639 pp.

IMAKAWA, S. 1967. [Browning of Chinese yam (Dioscorea batatas).] Hokkaido Daigaku Nogakubu, Hobun Kiyo, 6, 181 - 192. (Chemical Abstracts, 68, 21025).

KAWAKAMl, K. 1970. Yam culture in Japan. Tropical Root and Tuber Crops Tomorrow: Proceedings of the 2nd International Symposium on Tropical Root and Tuber Crops (Hawaii, 1970) (Plucknett, D. L., ed.), Vol. I, p. 102. Honolulu, Hawaii: College of Tropical Agriculture, University of Hawaii, 171 pp. (2 vole).

L�ON, J. 1977. Origin, evolution and early dispersal of root and tuber crops. Proceedings of the 4th Symposium of the International Society for Tropical Root Crops (Colombia, 1976), IDRC-080e (Cock, J., MacIntyre, R. and Graham, M., eds), pp. 20-36. Ottawa, Canada: International Development Research Centre, 277 pp.

SATOH, I. and TANABE, K. 1971. [Studies on the use of MH (Maleic hydrazide) in the cultivation of Chinese yams (Dioscorea batatas).] Bulletin of the Faculty of Agriculture, Tottori University, 23, 47-52. Field Crop Abstracts, 25, 2371.

SAWADA, E. et al. 1959. [Studies on the cultivation of Chinese yam. 4. Experiments on the planting time and pre-sprouting treatment of seed pieces.] Journal of the Horticultural Association of Japan, 28 (2), 123-129. (Field Crop Abstracts, 13, 1330).

TONO, TETSUZO. 1968. [Chinese yams. II. Analysis of glycoproteins and free amino acids, using an amino acid analyzer.] Tottori Nogakkaiho, 20, 129-135. (Chemical Abstracts, 70, 19021 z).

TONO, TETSUZO. 1970. [Chinese yams. IV. Isolation of the browning compounds from the tubers of Chinese yams.] Tottori Daigaku Nogakabu Kenkyu Hokoku, 22, 13- 18. (Chemical Abstracts, 73, 127768 U).


[edit] Cush-cush yam (Dioscorea trifida)

Common names

CUSH-CUSH YAM, India yam, Mapuey yam, Yampee.

Botanical name

Dioscorea trifida L. syn. D. brasiliensis Willd. (Macrogynodium).

Family

Dioscoreaceae.

Other names

Ajale (Cuba); Bell yam (Guy.); Car� doce, Car� mimosa (Braz.); Couche couche (W.l.); Cousse couche blanche (Mart.); Igname Indienne (Ant.); Name de la India (C. Rica); �ame morado, �ame vino (P. Rico); Name ycampi (C. Am.); Yampi(e) (W.l.).

Botany

Normally, 5-8 stems arise from the base of the plant, each with 2-8 membranous wings (sometimes absent on young stems). Twining is clock wise. The leaves are alternate (rarely opposite), deeply divided into 3-7 lobes, but not into separate leaflets. They are large, measuring about 15-25 cm long and broad. The petiole is long with wings continuous with the leaf blade. The male flowers are small, borne in racemes or panicles reaching 80 cm in length. The female flowers are in racemes, up to 20 cm in length, and are 12-14 mm long, with a long inferior ovary. Seed sets freely. The tubers are clustered at the base of the plant as the terminal enlargement of (usually) short stolons. These vary in number from 5 to 50 per plant and are commonly spherical to club-shaped but differ consider ably in form and are usually 15-20 cm long.

Origin and distribution

This species is believed to have originated in the Guyana region of South America, and is now cultivated throughout the northern parts of South America and the Caribbean Islands (to which it was taken by the Arawaks), as far north as the Greater Antilles. It is by far the most important of the indigenous American yams. It has not been successfully introduced into other parts of the tropics except Sri Lanka and New Caledonia, where it is grown on a small scale. In recent years it has received intensive study in Guadeloupe.

Cultivation conditions

D. trifida developed under equatorial conditions where rainy seasons are long and day-length changes minimal, and the growth season of this plant (10-11 months) is not as closely related to annual cycles as is the growth season of many other species. It may therefore be grown not only in equatorial regions but also where there are very distinct wet and dry seasons, if irrigation is provided. Heavy rainfall can be tolerated, but not flooding. A range of soils may be used but, unless they are rich in organic material, fertilising is required: 120-150 kg/ha each of nitrogen, phosphorus and potassium have given high yields in trials in the Caribbean.

Planting procedure

Material-usually small whole tubers are used, but these are more likely to be affected by disease than small cut pieces of whole tubers, which have been treated with benomyl. Seed may also be used. Stem cuttings are not normally successful.

Method-the tubers (or pieces) are planted in hills or ridges, which should be kept moist after planting if rainfall is inadequate. Seed is planted in nurseries with good drainage and aeration, and transplanted when twining stems arise. When the plants have begun to grow, long (2.5-3 m) stakes are normally used for the vines, but recent work in Guadeloupe suggests that staking may not be essential for good yields.

Time of planting-normally at the start of or just before the rainy season. With year-round rains or supplemental irrigation, year-round planting is possible.

Field spacing-50 x 50 cm-80 x 80 cm (about 40 000-15 000 plants/ha) is recommended.

Pests and diseases

In most cases there are no problems from pests and diseases, but occasionally serious problems may occur. Heavy nematode infestation can cause root damage or cracking, pitting or gall formation on tubers. Meloidogyne incognita and Pratylenchus coffeae have been implicated. Mealy bugs (Planococcus citri and Phenacoccus gossypii) can kill individual stems or the whole plant: clean or treated planting material is important for control. A virus condition, appearing as a leaf mosaic, causes stunting and distortion of the plant and ultimately reduction or loss of tubers. Destruction of diseased plantings is recommended and planting material should never be taken from affected areas.

Weeds are usually a serious problem: a pre-emergence spray with atrazine at 2.5-3 kg/ha gives good control until emergence; subsequently hand-weeding or shielded sprays of paraquat have been used, though when staking is not used the rapid growth cover formed by the plant minimises weed growth.

Growth period

10-11 months, after which the foliage of the plant dies back.

Harvesting and handling

The tubers are normally dug by hand and care must be exercised to avoid damage. Mechanical harvesting is being developed with cultivars specially bred for this purpose. Storage life is normally short under tropical conditions, 1-8 weeks before sprouting: loss in weight during storage is rapid (over I per cent per day), and fungal rots and insects may cause severe damage even during such short periods. However, tubers treated against insects and fungi with malathion and benomyl (permitted for grain storage), and stored in a cool dark room, have been maintained in good condition for long periods (up to one year has been claimed). This species appears not to be liable to chilling injury, so refrigerated storage might be a possibility, but this aspect needs further investigation.

Primary product

Tubers-which have relatively thin skins and flesh that varies in colour from white, yellow, pink to purplish. They have a richer flavour than most yams.

Yield

Commercial yields of about 15 t/ha are normal, although in virus-affected areas they may be as low as 1-2 t/ha. However, recent experimental selection and breeding work in Guadeloupe has produced plants that yielded up to 55 t/ha.

Special features

Few analyses of the tubers have been done but the following composition has been reported: protein 2.54 per cent; (6.4-7.6 per cent on dry weight basis); fat 0.44 per cent; carbohydrate 38 per cent. The carbohydrate con sists mainly of starch of granule size ranging from 10 to 65 microns. The ascorbic acid content is approximately 5.5 mg/100 g edible portion, but it is reported to be rapidly lost during storage.

Major influences

The high acceptability of this species suggests a potential for increasing use, especially where year-round production is possible. The plant's sexual fertility holds potential for improvement that is already being realised.

Bibliography

CAMPBELL, J. S. and GOODING, H. J. 1962. Recent developments in the production of food crops in Trinidad. Tropical Agriculture, Trinidad, 39, 261-270.

COURSEY, D. G. 1967. Yams. London: Longmans, Green and Co. Ltd, 230 pp.

CZYHRINCTW, N. and JAFFE, W. 1951. Modificaciones qu�micas durante la conservaci�n de ra�ces y tub�rculos. Archivos Venezolanos de Nutrici�n, 2 (1), 49-67.

DEGRAS, L. M. 1970. Morphology, physiology and selection in three tropical tuber crops. Tropical Root and Tuber Crops Tomorrow: Proceedings of the 2nd International Symposium on Tropical Root and Tuber Crops (Hawaii, 1970) (Plucknett, D. L., ed.), Vol. 1, pp. 163-165. Honolulu, Hawaii: College of Tropical Agriculture, University of Hawaii, 171 pp. (2 vole).

DEGRAS, L. 1977. Vegetative and sexual management in food yam improvement. Proceedings of the 4th Symposium of the International Society for Tropical Root Crops (Colombia, 1976), IDRC-080e (Cock, J., MacIntyre, R. and Graham, M., eds), pp. 58-62. Ottawa, Canada: International Development Research Centre, 277 pp.

DEGRAS, L. 1982. Les probl�mes d'am�lioration g�n�tique de l'igname vus � travers celle de Dioscorea trifida L. Yams: Ignames (Mi�ge, J. and Lyonga, S. N., eds), pp. 3-16: Oxford: Oxford University Press, 411 pp.

L�ON, J. 1977. Origin, evolution and early dispersal of root and tuber crops. Proceedings of the 4th Symposium of the International Society for Tropical Root Crops (Colombia, 1976), IDRC-080e (Cock, J., MacIntyre, R. and Graham, M., eds), pp. 20-36. Ottawa, Canada: International Development Research Centre, 277 pp.

MARTIN, F. W. and DEGRAS, L. 1978. Tropical yams and their potential: Part 5, Dioscorea trifida. United States Department of Agriculture, Agriculture Handbook, No. 522, 26 PP.

ROUANET, G. 1967. Experiments on yams in Guadeloupe. Proceedings of the International Symposium on Tropical Root Crops (Trinidad, 1967) (Tai, E. A., Charles, W. B., Haynes, P. H., Iton, E. F. and Leslie, K. A., eds), Vol. 1, Section 111, pp. 152-158. St. Augustine, Trinidad: University of the West Indies (2 vols)


[edit] Greater yam (Dioscorea alata)

Common names

GREATER YAM, Greater Asiatic yam, Ten months yam, Water yam, Winged yam.

Botanical name

Dioscorea alata L. (Enantiophyllum).

Family
Dioscoreaceae.

Other names

Ambi (N. Guin.); Avase (Togo); Batatilla (Ang.); Bobayassi (W. Afr.); Cabeza de negra (Col.); Car� de Angola, Car� branco, Car� cultivado, Car� inhama (Braz.); Couche couche (Lat. Am.); Cucam, Cucui-mo (Viet.); Dandaba (Sen.); Gbara-gu� (Guin.); Goradu (Assam); Huwi (Sud.); Igname aile� (Gab.); Igname de Chine (Fr.); Kachil, Katula, Khanulu (Ind.); Khoai-mo (Viet.); Kiseba (Ug.); Kuvi (Pacif. Is.); Lisbon yam (W.l.); Name asiatico (Venez.); �ame blanco (C. Rica); Name chino (Cuba); Name de ague (Venez.); Name de mine (P. Rico); Name grande (Venez.); Nangate (Mex.); Obbi, Oewi, Oowi kelapa (Indon.); Ovy (Mal.); Pacala (Fr.); Ratula, Sakourou (Ind.); Tus (C. Rica); Ubi (Philipp.); Ubi kemali (Mal.); Uwi (Indon.); White Manila yam (Philipp.).

Botany

A large climber, which can reach 15 m in height, with quadrangular winged stems, twining is anticlockwise (to the right). Leaves opposite, variable in size and shape, but essentially ovate to cordate with a deep basal sinus, acuminate. The male flowers are borne on panicles, up to 30 cm long; the female flowers are on small axillary spikes. Few cultivars produce fertile seed and most are completely sterile. Bulbils are sometimes formed in leaf axils, but not so freely as with certain other species. The tubers are usually single and show a great deal of variation in size, shape and colour: they are generally cylindrical but may be long and serpentine to almost globular, and are often branched or lobed, or even flattened and fan-shaped. Their weight is usually 5-10 kg though special cultivation can produce giant tubers of 60 kg or more. The flesh of some cultivars can be pink or even deep reddish-purple and these forms have been classified as D. purpurea Roxb. and D. afropurpurea Roxb. but this is not generally accepted.

Origin and distribution

D. alata is not known in the wild state but appears to have been developed from native species originating in the Assam-Burma region, by selection from deeper-rooting forms. Subsequently, it was spread through Thailand and Vietnam into the Pacific region, westwards and southwards to India and Malaysia and thence apparently to Madagascar and East Africa, later to be taken by the Portuguese and Spaniards to West Africa, northern South America and the Caribbean; in the eastern Caribbean and in the Pacific it is the most popular species of yam. It is cultivated throughout the tropical world.

Cultivation conditions

Rainfall-for optimum yields rainfall of 150 cm evenly distributed over 6-7 months is required, though it will perform moderately well on 100 cm.

Soil-D. alata will tolerate poorer soils than most other species of yam, but it responds well to fertilising. In India FYM at the rate of 25 t/ha has been recommended. In Barbados, where the crop is frequently grown as a rotation crop with sugar cane which has been fertilised with a 22:0:22 NPK mixture, yields of about 10 t/ha are normal, but additional fertilising with NPK at the rate of nitrogen 22 kg, phosphorus 25 kg and potassium 57 kg per hectare gave significant and economic increases in yield. Smaller increases were given when phosphorus was omitted Application should be about 10 weeks after planting, when the plant is completing its dependence upon the parent sets.

Altitude-it is usually cultivated at low or medium elevations, but is grown as high as 2 700 m in India.

Day-length - a day-length of less than 12 hours is required for tuberisation.

Planting procedure

Material-normally setts with two or three sprouts, occasionally small whole tubers are used. In Barbados, small pieces of approximately 100 g are usually cut from stored yams and are often dried for several hours before planting. Owing to the incidence of virus infections, a virus-free planting material has been developed by meristem culture in the West Indies; virus-tested planting material is currently being multiplied and commercially grown in Barbados and is on trial throughout the eastern Caribbean.

Method-the setts are usually planted by hand on mounds or ridges, being placed in holes 5-10 cm deep. Recently, mechanical planting has been developed in Barbados for planting on ridges. It is important to keep the crop weed-free for the first 3 months. The use of pre-emergence herbicides has been suggested: eg atrazine at 1.5-3 kg/ha to which TCA 5 kg/ha may be added on heavy soils to improve grass control, and chloramben at 3-6.5 kg/ha. After emergence, dalapon at 5 kg/ha may be applied for grass control, provided a shield is used. If the yams are not staked, complete ground cover is attained 3-4 months from sprouting and weeds are virtually eliminated.

Field spacing-when grown under monoculture, plantings on ridges 1.7 m apart, with 0.75-1 m between the plants is recommended, since at these spaces the vines need not be staked. Closer spacing can be used in areas of low rainfall.

Seed rate-in India, approximately 1 400 kg/ha of setts are used, in Barbados 650 kg/ha.

Pests and diseases

In addition to yam beetles and scale insects (see Yam) the larvae of three species of Lepidoptera attack the greater yam; they are Loxura atymnus, Theretra nessus and Tagiades gana. The first named is the most destructive as, after initially feeding on the leaves, the larvae attack the stems, often causing them to break off. D. alata is also susceptible to attack by the yam nematode, Scutellonema bradys.

One of the most troublesome diseases affecting this species is anthracnose caused by Colletotrichum gloeosporioides, sometimes in association with other fungi, notably Botryodiplodia and Fusarium spp.; crop losses can sometimes amount to 70-80 per cent, but spraying at 10 day intervals with zineb or ferbam is stated to be effective. Leaf spot, due to Cercospora spp., is reported to be serious in Sri Lanka. In Guadeloupe crown-gall, a bacterial condition caused by Agrobacterium tumefaciens, has been observed. An internal brown spot has caused serious losses in yams exported from Barbados; this has been traced to a virus infection which also leads to considerable reduction in yield (see Yam).

Growth period

Maturity is normally reached in 9-10 months, though some 'early' varieties can be harvested at about 6 months.

Harvesting and handling

Harvesting is normally done manually by forking, though owing to the size and irregular shape of the tubers of many cultivars damage is often high, in the order of 20-25 per cent of the tubers. Recent developments in the Caribbean have led to the production of a mechanical harvester and a reduction in damaged tubers to about 8 per cent.

Storage under ambient tropical conditions is normally for 4-6 months. If the tubers are sound, storage is terminated by the breaking of dormancy: if sprouts are removed as they develop storage may be extended to about 8 months.

Primary product

Tubers-normally large, weighing 5-10 kg, usually basically cylindrical but extremely variable: eg the common 'White Lisbon' of the Caribbean tends to be broad and lobed at the distal end; the 'Coconut Lisbon' is ovoid, the 'Hunt' cultivar is elongated and relatively narrow, much prized for roasting. A great variety of cultivars exists near the South-East Asian centre of origin. The skins are thick and dark and the flesh may be white, pink or purplish. The tubers of D. alata have a definite period of dormancy of 2-4 months, which may be broken by treatment with ethylene chlorhydrin.

Yield

Yields vary widely, but the following average farm yields have been reported: Malaysia 42.5 t/ha; Trinidad 46.8 t/ha; St. Vincent 57.5 t/ha
Fiji 25.2 t/ha; Barbados 5-6 t/ha.

Main use

Used mainly as a vegetable, similarly to the potato, and some cultivars can be used to make French fries and chips, claimed to be superior to similar potato products. Although it is the preferred yam in many parts of the tropics, especially by those accustomed to European dietary habits, it is less highly regarded in West Africa, because it is not suitable for the preparation of 'fufu'.

Subsidiary uses

In several countries, eg the Philippines, Barbados and Puerto Rico, attempts are being made to develop processed products such as yam flakes or powder from surplus supplies of D. alata. Coloured cultivars have been utilised as a colouring and flavouring agent for ice cream.

Secondary and waste products

Badly-damaged tubers are often fed to pigs.

Special features

A typical analysis of the edible portion of the tubers is: water 65-73 per cent; protein 1.12-2.78 per cent; fat 0.03-0.27 per cent; carbohydrate 22-29 per cent; fibre 0.65-1.4 per cent; ash 0.67-2.06 per cent.

The starch contains a high proportion of fairly large granules: sizes ranging from 5 to 50 microns have been reported. The gelatinization temperature ranges from 69° to 88°C and the viscosity from 100 to 200 Brabender units. Unlike most other yam species, starch from D. alata has a high gel strength. Starch from white-fleshed and purple-fleshed cultivars have similar typical composition averaging: moisture 13.6 per cent; protein 0.14 per cent; ash 0.22 per cent; amylose 21.1 per cent; reducing sugars 0.18 per cent; pH 7.1; iod. val. 5.5. Ascorbic acid contents ranging from 4.9 to 8.2 mg/100 g of edible portion have been reported, while certain cultivars in the South Pacific have been found to contain 6 mg/100 g of carotene. Three anthocyanins have been isolated from D. alata var. atropurpurea and rubella and found to be cyanidin glycosides.

Processing

Yam flakes-dehydrated yam flakes may be prepared from the tubers by Iye or hand peeling (average losses, depending upon condition of yams, 15-35 per cent), slicing into I cm thick pieces, cooking in water or steam until soft, ricing (to avoid breaking the cell walls and so releasing starch which would give a glutinous texture to the reconstituted product), gently mixing to a slurry and dehydrating on a single drum dryer with suitably spaced applicator rolls. The resulting product of about 4 per cent moisture content is packed in plastic bags and has a storage life of 2 years or more under ambient tropical conditions; it reconstitutes to a mashed yam.

Powder-an acceptable yam powder, suitable for blending into food products, can be prepared by cooking unpeeled tubers, then peeling, grating and drying at 50°C to 10 per cent moisture.

Production and trade

No figures are available for the production of D. alata separately from other yams. There has been a small export trade in D. alata from some of the Caribbean islands to the UK since the early 1960s. In 1968 approximately I 000 t of tubers of D. alata were exported from Barbados, but the occurrence of chilling injury at the receiving point, and the incidence of internal black spot (virus), reduced the trade almost to zero. However, the recent production of virus-free yams has allowed the trade to re-start, and in 1982 Barbados exported 116 t (to the UK and other West Indian territories), and in the first half of 1983, 324 t of which 272 t were to the UK market.

Major influences

D. alata is the world's most popular yam after the D. rotundata/cayenensis complex, and appears to have held its place. Although traditional methods of production (especially in Africa) are more costly in manpower than for other yams, the introduction of complete field mechanisation, which is now a reality, should reduce production costs and make this crop more competitive as a tropical carbohydrate food and also enable it to maintain or improve its position on the export market.

Bibliography

AFABLE, L. A. 1970. The preparation of ubi powder. Philippine Journal of Plant Industry, 35 (1 -2), 19-25.

BERWICK, J., CHAND, D. and QALIBOKOLA, J. 1972. Yam (Dioscorea alata) planting rate, staking, variety and palatability trials. Fiji Agricultural Journal, 34 (2), 44-50.

CAMPBELL, J. S., CHUKWUEKE, V. O., TERIBA, F. A. and HO-A-SHU, H. V. S. 1962. Some physiological investigations into the white Lisbon yam (Dioscorea alata L.) I The breakage of the rest period in tubers by chemical means. Empire Journal of Experimental Agriculture, 30, 108-114; Some physiological investigations into the white Lisbon yam (Dioscorea alata L.). II. Growth period and out-of-season production. Empire Journal of Experimental Agriculture, 30, 232-238; Some physiological experiments with the white Lisbon yam (Dioscorea alata L.) in Trinidad. III The effect of chemicals on storage. Empire Journal of Experimental Agriculture, 30, 335-344.

CHAPMAN, T. 1965. Some investigations into factors limiting yields of the white Lisbon yam (Dioscorea alata L.) under Trinidad conditions. Tropical Agriculture, Trinidad, 42, 145- 151.

CIBES, H. R. and ADSUAR, J. 1966. Effects of chlorethanol and thiourea on the germination and relative yield of the yam (Dioscorea alata L.). Journal of Agriculture of the University of Puerto Rico, 50, 201-208.

COURSEY, D. G. 1967. Internal brown spot-a condition of yams in Barbados. Journal of the Agricultural Society of Trinidad and Tobago, 67, 473-482.

COURSEY, D. G. 1967. Yams. London: Longmans, Green and Co. Ltd, 230 pp.

COURSEY, D. G. 1968. Low temperature injury in yams. Journal of Food Technology, 3, 143-150.

COURSEY, D. G. 1983. Yams. Handbook of tropical foods (Chan, H. C. (Jr.), ed.), pp. 555-601. New York: Marcel Dekker, 639 pp.

COURSEY, D. G. and MARTIN, F. W. 1970. The past and future of the yams as crop plants. Tropical Root and Tuber Crops Tomorrow: Proceedings of the 2nd International Symposium on Tropical Root and Tuber Crops (Hawaii, 1970) (Plucknett, D. L., ed.), Vol. 1, pp. 87-90. Honolulu, Hawaii: College of Tropical Agriculture, University of Hawaii, 171 pp. (2 vole).

FERGUSON, T. U. and HAYNES, P. H. 1970. The response of yams (Dioscorea spp.) to nitrogen, phosphorus, potassium and organic fertilizers. Tropical Root and Tuber Crops Tomorrow: Proceedings of the 2nd International Symposium on Tropical Root and Tuber Crops (Hawaii, 1970) (Plucknett, D. L., ed.), Vol. 1, pp. 93-96. Honolulu, Hawaii: College of Tropical Agriculture, University of Hawaii, 171 pp. (2 vole).

GONZ�LEZ, M. A. and COLLAZO DE RIVERA, A. 1972. Storage of fresh yams (Dioscorea alata) under controlled conditions. Journal of Agriculture of the University of Puerto Rico, 56, 45-56.

GOODING, E. G. B. 1970. The production of yams in Barbados. Tropical Root and Tuber Crops Tomorrow: Proceedings of the 2nd International Symposium on Tropical Root and Tuber Crops (Hawaii, 1970) (Plucknett, D. L., ed.), Vol. 1, pp. 97-99. Honolulu, Hawaii: College of Tropical Agriculture, University of Hawaii, 171 pp. (2 vole).

GOODING, E. G. B. 1971. Effects of fertilising and other factors on yams in Barbados. Experimental Agriculture, 7, 315-319.

GOODING, E. G. B. 1972. The production of instant yam in Barbados. Part 1: Process development. Tropical Science, 14, 323-333.

GOODING, E. G. B. and HOAD, R. M. 1967. Problems of yam cultivation in Barbados. Proceedings of the International Symposium on Tropical Root Crops (Trinidad, 1967) (Tai, E. A., Charles, W. B., Haynes, P. H., Iton, E. F. and Leslie, K. A., eds), Vol. 1, Section 111, pp. 137-148. St. Augustine, Trinidad: University of the West Indies (2 vole).

HAQUE, S. Q. and CHANDLER, F. 1981. Virus-tested yam tuber multiplication project: Annual Report 1981. St. Augustine, Trinidad: Caribbean Agricultural Research and Development Institute, 14+v pp.

HAQUE, S. Q. and CHANDLER, F. 1982. Virus-tested yam tuber multiplication project: Annual Report 1982. St. Augustine, Trinidad: Caribbean Agricultural Research and Development Institute, 14+x)x pp.

IMBERT, M. P. and SEAFORTH, C. 1968. Anthocyanins in Dioscorea alata. Experientia, 24, 445-447.

KASASIAN, L. 1971. Root crops: Dioscorea alata. Weed control in the tropics, pp. 157-158. London: Leonard Hill Books, 307 pp.

L�ON, J. 1977. Origin, evolution and early dispersal of root and tuber crops. Proceedings of the 4th Symposium of the International Society for Tropical Root Crops (Colombia, 1976), IDRC-080e (Cock, J., MacIntyre, R. and Graham, M., eds), pp. 20-36. Ottawa, Canada: International Development Research Centre, 277 pp.

LYONGA, S. N. 1981. The economics of yam cultivation in Cameroon. Tropical Root Crops: Research Strategies for the 1980s: Proceedings of the 1st Triennial Root Crops Symposium of the International Society for Tropical Root Crops-Africa Branch (Nigeria, 1980), IDRC-163e (Terry, E. R., Oduro, K. A. and Caveness, F., eds), pp. 208-213. Ottawa, Canada: International Development Research Centre, 279 pp.

MANTELL, S. H. and HAQUE, S. Q. 1979. Disease-free yams: their production, maintenance and performance. Yam Virus Project Bulletin, No. 2. St. Augustine, Trinidad: Caribbean Agricultural Research and Development Institute, 22 + vii pp.

MANTELL, S. H., HAQUE, S. Q. and WHITEHALL, A. P. 1979. A rapid propagation system for yams. Yam Virus Project Bulletin, No. 1. St. Augustine, Trinidad: Caribbean Agricultural Research and Development Institute, 19 pp.

MANTELL, S. H., HAQUE, S. Q. and WHITEHALL, A. F. 1980. Apical meristem tip culture for eradication of flexous rod viruses in yams (Dioscorea alata). Tropical Pest Management, 26, 170-179.

MARTIN, F. W. 1976. Tropical yams and their potential. Pt. 3 Dioscorea alata. United States Department of Agriculture, Agriculture Handbook, No. 495. Washington, DC: USDA Agricultural Research Service, 40 pp.

MARTIN, F. W. and RHODES, A. M. 1973. Correlations among greater yam (Dioscorea alata) cultivars. Tropical Agriculture, Trinidad, 50, 183-192.

MARTIN, F. W. and RUBERTE, R. 1972. Yam (Dioscorea spp.) for production of chips and French fries. Journal of Agriculture of the University of Puerto Rico, 56, 228-234.

NADAKAL, A. M. and THOMAS, N. 1967. Observations of nematodes associated with dry rot of Dioscorea alata L. Science and Culture, 33 (3), 142-143.

NWANKITI, O. A. 1982. Symptomatology, aetiology and incidence of a leaf disease of yam (Dioscorea spp.) originally called 'Apollo' Disease. Yams: Ignames (Mi�ge, J. and Lyonga, S. N., eds), pp. 274-279. Oxford: Oxford University Press, 411 pp.

NWANKITI, O. A. and OKPALA, E. U. 1981. Anthracnose of water yams in Nigeria. Tropical Root Crops: Research Strategies for the 1980s: Proceedings of the 1st Triennial Root Crops Symposium of the International Society for Tropical Root Crops-Africa Branch (Nigeria, 1980),

IDRC-163e (Terry, E. R., Oduro, K. A. and Caveness, F., eds), pp. 166-172. Ottawa, Canada: International Development Research Centre, 279 pp.

RASPER, V. and COURSEY, D. G. 1967. Anthocyanins of Dioscorea alata L. Experientia, 23, 611-612.

RASPER, V. and COURSEY, D. G. 1967. Properties of starches of some West African yams. Journal of the Science of Food and Agriculture, 18, 240-244.

RHODES, A. M. and MARTIN, F. W. 1972. Multivariate studies of variations in yams (Dioscorea alata L.). Journal of the American Society for Horticultural Science, 97, 685-688.

RIVERA-ORTIZ, J. M. and GONZ�LEZ, M. A. 1972. Lye peeling of fresh yam Dioscorea alata. Journal of Agriculture of the University of Puerto Rico, 56, 57-63.

ROYES, W. V. 1967. Yield trials with Dioscorea alata. Proceedings of the International Symposium on Tropical Root Crops (Trinidad, 1967) (Tai, E. A., Charles, W. B., Haynes, P. H., Iton, E. F. and Leslie, K. A., eds), Vol. 1, Section 1, pp. 144-151. St. Augustine, Trinidad: University of the West Indies (2 vole).

SASTRAPRADJA, S. 1982. Dioscorea alata: its variation and importance in Java, Indonesia. Yams: Ignames (Mi�ge, J. and Lyonga, S. N., eds), pp. 44-49. Oxford: Oxford University Press, 411 pp.

SINGH, R. D. and PRASAD, N. 1966. Efficacy of different fungicides for control of anthracnose of Dioscorea alata. Plant Disease Reporter, 50, 385-387.

WAITT, A. W. 1963. Yams, Dioscorea species. Field Crop Abstracts, 16, 145-157.


[edit] Intoxicating yam (Dioscorea hispida)

Common names

INTOXICATING YAM, Karukandu, Nam�.

Botanical name

Dioscorea hispida Dennst. (Lasiophyton).

Family

Dioscoreaceae.

Other names

Gado(e)ng, Gadong mabok (Mal.); Kal�t (Philipp.); Killoi, Koi (Thai.); Maranpash poll (Ind.); �amo (Philipp.); Palidumpa, Pashpoli (Ind.).

Botany

D. hispida is a climber usually with a prickly stem, 6-10 mm in diameter, varying from glabrous to pubescent with fine white to brown hairs. Twining is clockwise (to the left). The leaves are trifoliate with oval to obovate leaflets, about 10 cm long by 8 cm broad, hairy, with small prickles on the underside of the main vein. Male flowers are in large, branched inflorescences; the female inflorescences are unbranched. The tubers are large, weighing 5-15 kg, roughly globose but deeply lobed, pale skinned, but covered with masses of fibrous roots: they are produced near the soil surface and are extremely poisonous.

Origin and distribution

This species grows wild in South-East Asia and Indonesia, and extends to Papua New Guinea and the Philippines and India. It is not cultivated to any great extent, though some cultivation is practiced in Java.

Cultivation conditions

D. hispida thrives in tropical rain forest conditions. It usually grows at relatively low elevations, less than 500 m, though it has been reported growing at altitudes up to 1 200 m in the Himalayas.

Planting procedure

As noted above, the plant is infrequently cultivated. When cultivation is practiced propagation is often by planting pieces of tuber in prepared mounds (see Yam).

Growth period

Maturity is normally reached in about 12 months.

Harvesting and handling

The tubers are usually lifted by hand with a digging stick or fork. It has been reported that if the tubers are exposed to temperatures below about 10°C, subsequent growth is adversely affected.

Primary product

Tubers-which have white or pale-yellow, starchy, highly toxic flesh.

Yields

In cultivation, yields of about 20 t/ha have been reported.

Main use

As a famine food-the tubers, growing near the surface, are easily accessible. Detoxification is essential and one method is to cut the tubers in pieces, cover the surface with wood ashes for 24 hours, then steep in sea water for several days, wash with fresh water, and dry. The process is repeated several times. Another method is to dry the slices mixed with ashes. A third is to salt the pieces of tuber and then press under water until no whitish sap remains. After detoxification the yams are usually tested by feeding to dogs or other domestic animals.

Subsidiary uses

The possibility of using the tubers as a source of starch has been considered, but so far appears not to have been commercially developed.

Secondary and waste products

The tubers are sometimes used to prepare poisons. The pounded tubers are also used in parts of Asia in local medicine for the treatment of open wounds. It has been suggested that the residue left after starch extraction could be used as an insecticide.

Special features

Tubers-an approximate analysis of the tubers has been given as: water 78 per cent; protein 1.81 per cent; fat 0.16 per cent; carbohydrate 18 per cent; fibre 0.93 per cent; ash 0.69 per cent. On a dry weight basis the tubers contained 0.2-0.7 per cent diosgenin and 0.044 per cent of the toxic alkaloid dioscorine.

Flour-the average composition of the flour extracted from the tubers was given as: protein 5.28 per cent; fat 0.23 per cent; starch 88.34 per cent; fibre 5.33 per cent; ash 0.66 per cent. It is suitable for both edible and industrial purposes and can be used for the manufacture of glucose. Starch granules from Indian tubers are non-stratified and oval-shaped, with an average longitudinal diameter of 35-40 microns and a gelatinisation temperature of 85°C. Starch from D. hispida differs from cassava and potato starches in that its viscosity does not fall appreciably after prolonged heating.

Processing

The following method is suggested for the preparation of flour or starch from D. hispida tubers.

(i) The tubers are thoroughly washed in clean water, either by hand or mechanically, to remove adhering soil, etc.

(ii) The tubers are mashed with water; a potato rasping machine is suitable for the preparation of flour, but for the production of starch, the tubers must be ground very finely in order to rupture the cell walls and liberate the starch granules.

(iii) In order to detoxify the material, the pulp is treated with lime water containing potassium permanganate; usually lime water equivalent to five times the weight of tubers and containing 0.005 per cent of potassium permanganate, is used. Any excess potassium permanganate is removed by treating the starch milk with sulphur dioxide.

(iv) The starch is allowed to settle out and is then washed and centrifuged as in the manufacture of sweet potato starch.

In the Philippines it has recently been suggested that starch or flour could be produced on a commercial scale by extracting the tubers with 95 per cent alcohol followed by treatment with 5 per cent sodium chloride or acidified water.

Major influences

Production of D. hispida is normally scattered over a large area so that the large-scale commercial production of starch from this yam is not likely to be economically viable.

BIBLIOGRAPHY

BURKILL, I. H. 1935. Dioscorea hispida. A dictionary of the economic pro ducts of the Malay peninsula, Vol. I (A-H), PP. 818-821. London: The Crown Agents for the Colonies, 1220 pp.

CABATO, F. H. (Jr.). 1965. Taming the wild nam�. Farmers Digest Philippines, I (2), 24-25. (Philippine Abstracts, 6 (3), 89).

COURSEY, D. G. 1967. Yams. London: Longmans, Green and Co. Ltd, 230 PP.

COURSEY, D. C. 1976. The origins and domestication of yams in Africa. World Anthropology (Harlan, J. R., de Wet, J. M. J. and Stemler, A. B. L., eds), pp. 385-408. The Hague, Netherlands: Mouton, 498 PP.

COURSEY, D. G. 1983. Yams. Handbook of tropical foods (Chan, H. C. (Jr.), ed.), pp. 555-601. New York: Marcel Dekker, 639 PP.

COURSEY, D. G. and FERBER, C. E. M. 1979. The processing of yams. Small-scale processing and storage of tropical root crops (Plucknett, D. L., ed.), pp. 189-211. Boulder, Colorado: Westview Press Inc, 461 PP.

L�ON, J. 1977. Origin, evolution and early dispersal of root and tuber crops. Proceedings of the 4th Symposium of the International Society for Tropical Root Crops (Colombia, 1976), IDRC-080e (Cock, J., MacIntyre, R. and Graham, M., eds), pp. 20-36. Ottawa, Canada: International Development Research Centre, 277 PP.

MARTIN, F. W. and DEGRAS, L. 1978. Minor cultivated Dioscorea species. Tropical yams and their potential, Part 6. United States Department of Agriculture, Agriculture Handbook, No. 538, 23 PP.

NOON, R. A. 1978. Storage and market diseases of Yams. Tropical Science, 20, 177-188.

RAO, P. S. and BERI, R. M. 1952. Tubers of Dioscorea hispida Dennst. Indian Forester, 78, 146 - 152.

SACWANSUPYAKORN, C. and CHANTRAPRASONC, C. 1982. Yam of Thailand species: importance and utilization. Proceedings of the 5th International Symposium on Tropical Root and Tuber Crops (Philippines, 1979), PP. 213-215. Los Ba�os, Laguna, Philippines: Philippine Council for Agriculture and Resources Research, 720 PP.

SASTRI, B. N. (ed.). 1952. Dioscorea hispida. The wealth of India: Raw materials, Vol. 3 (D-E), pp. 73-74. New Delhi, India: Council for Scientific and Industrial Research, 236 pp.

STEELE, W. J. E. and SAMMY, G. M. 1976. The processing potential of yams (Dioscorea spp.). Journal of Agriculture of the University of Puerto Rico, 60, 215-223.

SULIT, J. I. 1967. Processing and utilization of nam� (Dioscorea hispida Dennst.) tubers. Araneta Journal of Agriculture, 14, 203-221.

WESTER, P. J. 1924. The food plants of the Philippines. Philippines Department of Agriculture and Natural Resources, Bureau of Agriculture Bulletin, No. 39, p. 134.


[edit] Lesser yam (Dioscorea esculenta)

Common names

LESSER YAM, Asiatic yam, Lesser Asiatic yam.

Botanical name

Dioscorea esculenta (Lour.) Burk. (Combilium).

Family

Dioscoreaceae.

Other names

Apali (Philipp.); Chinese yam (W. Afr. and W.l.); Couche-couche douce (Mart.); Diba (N. Guin.); Hisu (Fiji); Igname des blancs (Fr.); Kangar, Karen potato (Ind.); Kaw(a)i (Fiji); Kizahangu, Kodi (Sri La.); �ame asiatico, Name azucar, �ame chino, �ame papa, �ame pequeno (Lat. Am.); Pana (Sol. Is.); Potato yam, Sasniali, Sathni, Silakandom (Ind.); Taitu(kava) (S. Pacif.); Tongo, Trident yam, Tugi, Tungo (Philipp.); Ufi lei (Pacif. Is.); Wale, War� (N. Cal.).

Botany

The plant is a vine, seldom climbing to more than 3 m. The stems are thin, usually 1-3 mm in diameter, and vary from smooth to prickly. They twine clockwise (to the left) in climbing. The leaves are alternate, almost round, but pointed at the tips and deeply lobed at the base, finely hairy and about 10 cm in diameter. The petioles are thickened at the base with 4 sharp prickles. Flowers are rare in most cultivars, but when they occur are larger than in most other Dioscorea spp. The roots are fibrous, often more or less prickly, and a former classification based on the presence or absence of prickles (var. spinosa and var. fasciculata) is no longer recognised. The tubers are the swollen ends of stolons arising from the crown of the plant; each stolon bears only one tuber. The stolons vary in length from about 5 to 50 cm; the length is a varietal characteristic. About 5-20 tubers are borne per plant; the number and size of the tubers is related to the cultivar. They resemble rather long and narrow sweet potatoes, but occasionally may be spindle shaped or branched. Papua New Guinea cultivars produce very large tubers weighing up to 3 kg: the Caribbean cultivars weigh 100-200 g and are usually 8-10 cm long and 2.5-5 cm in diameter.

Origin and distribution

D. esculenta is among the most ancient species of the genus, and its centre of origin is stated by various authorities as India, Vietnam, or Papua New Guinea and the Philippines. It has long been domesticated and is documented as a staple food in southern China from the 2nd and 3rd centuries. Today it is widely distributed throughout the tropics, but is little used except in South-East Asia, where it is grown to such an extent that it ranks third in production and utilisation of yams after D. rotundata/D. cayenensis and D. alata.

Cultivation conditions

Temperature-D. esculenta is a plant of tropical forests and grows best at high temperatures, though this species may be grown up to about 25°N in southern China.

Rainfall-optimum yields have been obtained with moderately high rainfall (175 cm), though satisfactory yields are reported from areas with 87-100 cm, which is well distributed throughout the year. Dry periods of more than about 2 months can lead to death of the plant.

Soil-sandy soils are not suitable, and very heavy clays can lead to misshapen tubers. Good drainage is essential and a high level of organic matter greatly improves growth. There is little information on the use of fertilisers in the Far East but experiments in Trinidad have shown that nitrogen produces a positive response in the earlier part of the growth cycle, but depresses yield if applied late; potassium is needed especially during tuberisation; phosphate is seldom a limiting factor. A general recommendation is 400 kg/ha of an 11:11:33 NPK mixture applied 6-8 weeks after planting.

Altitude-low or medium altitudes are best, though satisfactory growth at levels of up to 900 m has been reported from northern India.

Planting procedure

Material-small whole tubers of 55-85 g weight are recommended.

Method-the tubers are planted in mounds or in ridges, 8-12 cm below the surface of the ground. Atrazine at 3 kg/ha has been successfully used as a pre-emergence herbicide, and a shielded spray of paraquat at 3 litres/ha is recommended for the later control of weeds. Staking is commonly used and has been shown to double the yield obtained when the vines are unslaked, though the plant is stated to give satisfactory results without staking.

Field spacing-recommended spacings vary, though 90 x 90 cm appears to be the most common for mounds, and from 90 to 130 cm in ridges I m apart: at the latter spacing about 2 000 kg/ha of seed tubers are required.

Pests and diseases

The yam nematode, Scutellonema bradys, and the root knot nematode, Meloidogyne sp., are both reported as serious pests in some areas. Selection of nematode-free tubers for planting and avoidance of nematode-infested soils are important precautionary measures.

Fungal diseases of the aerial parts are rare, but the tubers may be affected by certain fungi, eg Botryodiplodia theobromae, Lasiodiplodia sp. and Fusarium spp. The foliage often shows virus symptoms; it is thought that virus is always present and is tolerated, though virus-free material might well yield better.

Growth period

In Fiji the crop is reported to mature in 6-7 months, in Malaysia 8-9 months and in the West Indies 10 months.

Harvesting and handling

The tubers are thin-skinned and succulent, and easily damaged during harvesting: lifting is normally done by hand. However, as the tubers are small and near the surface, commercial potato diggers, carefully used, may be used to harvest them when they are planted on ridges. The tubers should be cut from the crown, washed and dried, and packed in well-ventilated boxes, not sacks. Damaged tubers should be used as quickly as possible; even superficial damage permits the entry of fungi which can cause rotting (see Pests and diseases). It is claimed that uninjured tubers can be stored for 4 months or longer in well-ventilated conditions under ambient temperatures in the tropics; larger tubers store better than small ones. Respiration and loss of water continue during storage; there is therefore loss of dry matter and shrivelling of the tubers. Sweetness increases and changes in flavour occur, with a reduction in palatability. Sprouting usually occurs, leading to further loss of weight. There is no information as to whether low temperature injury occurs below about 13°C as is the case with some other yams.

Primary product

Tubers-which are very thin-skinned and have a yellow flesh, and thus appear pale-yellow even before the skin is removed. The surface is smooth except for some fine adventitious roots and a few depressions like the eyes of a potato; these are not buds, but are local wounds resulting from minor injuries to the tuber during its growth. The flesh is floury to succulent, crisp, with little fibre and a characteristic bland but rather sweet flavour.

Yield

High yields are common when the yams are planted in pure stands: the following average yields (t/ha) have been reported: Malaysia 25; West Indies 34-38; West Irian 70; Philippines 20 - 30.

Main use

The tubers are cooked and eaten as a carbohydrate foodstuff. They may be boiled in their skins or after peeling (peeling involves only about 5 per cent loss of the tubers); in the latter case they disintegrate badly, though this is minimised by boiling for no more than 10 minutes. They may be baked in their skins, or fried as slices or as chips (french fries).

Special features

The nutritional composition of the edible portion of D. esculenta has been quoted as: water 67-81 per cent; protein 1.29-1.87 per cent; fat 0.04-0.29 per cent; carbohydrate 17-25 per cent; fibre 0.18-1.51 per cent; ash 0.5-1.24 per cent.

The carbohydrate is mainly starch but with a relatively high content of sugars (7-11 per cent). The starch granules are rounded or polyhedral, very small (1-15 microns in diameter), with a rather low amylose content (14-15 per cent).

Production and trade

Although considerable quantities are grown in the Far East, the delicate and perishable nature of the tubers makes any external trade difficult and the tubers are normally traded only within a community or village.

Major influences

D. esculenta is both high yielding and easily adapted to mechanical cultivation, as well as being palatable and easily prepared in the kitchen; it therefore could become more popular than at present.

Bibliography

ANON. 1970. Introducing the 'Chinese' yam. Root crop production bulletin, 1. St. Augustine, Trinidad: University of the West Indies, 5 pp.

BARRAU, J. 1956. Les ignames alimentaires des �les du Pacific sud. Journale d'Agriculture Tropicale et de Botanique Appliqu�e, 3, 386-387.

COURSEY, D. G. 1967. Yams. London: Longmans, Green and Co. Ltd, 230 pp.

COURSEY, D. G. 1977. The comparative ethnobotany of African and Asian yam cultures. Proceedings of the 3rd Symposium of the International Society for Tropical Root Crops (Nigeria, 1973) (Leakey, C. L. A., ed.), pp. 164-169. Ibadan, Nigeria: International Society for Tropical Root Crops in collaboration with the International Institute of Tropical Agriculture, 492 pp.

COURSEY, D. G. 1983. Yams. Handbook of Tropical Foods (Chan, H. C. (Jr.), ed.), pp. 555-601. New York: Marcel Dekker, 639 pp.

COURSEY, D. G. and MARTIN, F. W. 1970. The past and future of the yams as crop plants. Tropical Root and Tuber Crops Tomorrow: Proceedings of the 2nd International Symposium on Tropical Root and Tuber Crops (Hawaii, 1970) (Plucknett, D. L., ed.), Vol. I, pp. 87-90. Honolulu, Hawaii: College of Tropical Agriculture, University of Hawaii, 171 pp. (2 vole).

ENYI, B. A.C. 1970. Growth studies in Chinese yam (Dioscorea esculenta). Tropical Root and Tuber Crops Tomorrow: Proceedings of the 2nd International Symposium on Tropical Root and Tuber Crops (Hawaii, 1970) (Plucknett, D. L., ed.), Vol. 1, p. 103. Honolulu, Hawaii: College of Tropical Agriculture, University of Hawaii, 171 pp. (2 vole).

ENYI, B. A. C. 1972. The effects of seed size and spacing on growth and yield of lesser yam (Dioscorea esculenta). Journal of Agricultural Science, 78, 215-225.

ENYI, B. A. C. 1972. Effect of staking, nitrogen and potassium on growth and development in lesser yams (Dioscorea esculenta). Annals of Applied Biology, 72, 211-219.

FERGUSON, T. U. and HAYNES, P. H. 1970. The Chinese yam as a commercial proposition. Farmer, Kingston, Jamaica, 75, 372-375.

IRVINE, F. R. 1969. Yam (Dioscorea spp.). West African Agriculture, 3rd edn, Vol. 2, West African Crops, pp. 160-173. London: Oxford University Press, 272 pp.

MARTIN, F. W. 1974. Tropical yams and their potential. Pt. I Dioscorea esculenta. United States Department of Agriculture, Agriculture Handbook, No. 457. Washington, DC: USDA Agricultural Research Service, 18 pp.

MI�GE, J. 1948. Le Dioscorea esculenta Burkill, en C�te d'Ivoire. Revue Internationale de Botanique Appliqu�e et d'Agriculture Tropicale, 28 (313-314), 509-514.

MI�GE, J. 1957. Influence de quelques caract�res des tubercules semences sur la lev�e et le rendement des ignames cultiv�es. Journale d'Agriculture Tropicale et de Botanique Appliqu�e, 4, 315-342.

RASPER, V. and COURSEY, D. G. 1967. Properties of starches of some West African yams. Journal of the Science of Food and Agriculture, 18, 240-244.

SAGWANSUPYAKORN, C. and CHANTRAPRASONG, C. 1982. Yam of Thailand species: importance and utilization. Proceedings of the 5th International Symposium on Tropical Root and Tuber Crops (Philippines, 1979), pp. 213-215. Los Ba�os, Laguna, Philippines: Philippine Council for Agriculture and Resources Research, 720 pp.

SASTRI, B. N. (ed.). 1952. Dioscorea esculenta. The wealth of India: Raw materials, Vol. 3 (D-E), pp. 72-73. New Delhi, India: Council for Scientific and Industrial Research, 236 pp.

SEARL, S. 1970. Introducing the versatile 'Chinese' Yam. Farmer, Kingston, Jamaica, 75, 376-380.

TURAGA, P. and YAKU, P. 1944. Cultivation of the 'Kawai (Dioscorea esculenta). Fiji Agricultural Journal, 15, 107- 108.

WAITT, A. W. 1963. Yams, Dioscorea species. Field Crop Abstracts, 16, 145-157.

WESTER, P. J. 1924. The food plants of the Philippines. Philippines Department of Agriculture and Natural Resources, Bureau of Agriculture Bulletin, No. 39, p. 191.

WILSON, J. E. 1982. Present and future roles of yams (Dioscorea spp.) in West Africa. Proceedings of the 5th International Symposium on Tropical Root and Tuber Crops (Philippines, 1979), pp. 205-211. Los Banos, Laguna, Philippines: Philippine Council for Agriculture and Resources Research, 720 pp.


[edit] Potato yam (Dioscorea bulbifera)

Common names

POTATO YAM, Aerial yam, Air potato, Bulbil bearing yam, Turkey liver yam.

Botanical name

Dioscorea bulbifera L. (Osophyton).

Family

Dioscoreaceae.

Other names

Acom (W.l.); Agbanio (Togo); Akam or Akom (W. Afr.); Banalu (Philipp.); Batata de rama (Braz.); Bayag-toro (Philipp.); Cambar� marron (Maur.); Car� de aire, Car� de espinho, Car� de Sao Thom�, Car� de sapateiro (Braz.); Cu mei (Viet.); Danda yam (W. Afr.); Dimoa (N. Cal.); Gaithi (Ind.); Hoi (Tah.); Huwi blichik (Sud.); Igname bois (Ant.); Igname pousse debout (Zar.); Ir�ga (Gab.); Kaile (Fiji); Kasiena (S. Pacif.); Kattala (Sri La.), Khoing� (China), Man nok (Thai.); Name del aire (Col.); Name congo, Name criollo, Name de mate (Venez.); Numwe (N. Cal.); Oobi Singapore (Mal.); Otaheite potato (W.l.); Papa caribe, Papa del aire, Papa voladora (C. Rica); Pousse en l'air (Fr.); Ratulu (Ind.); Soi (Polyn.); Ubi atas (Mal.); Ubi-ubihan (Philipp.); Ycam.

Botany

A strongly climbing vine, reaching 6 metres or more, with smooth stems ranging from I to 8 mm in diameter; twining is clockwise (to the left). The leaves are cordate-orbicular to ovate-orbicular, strongly acuminate, 15-30 cm long and broad; the petioles are thickened at the base with ear like projections that often encircle the stem. The flowers are small (though larger than those of many cultivated yams), about 3 mm long, closely appressed to the pedicel in long axillary or terminal racemes. Winged seeds are produced freely from the trilocular capsules which are 2-5 cm long: the seeds germinate readily. Tubers are produced underground and tuberous bulbils in leaf axils and on terminal racemes. In African races of the plant these bulbils are sharply angled, while those of Asian races are spherical to ellipsoid. The bulbils are grey or brown in colour with white or yellow mucilaginous flesh; they range from about 3 to 10 cm in diameter and usually weigh about 0.5 kg, but can be as heavy as 2 kg. Some varieties may need detoxification by soaking or boiling before they are eaten. The underground tubers arise from a swelling of the young stem and enlarge rapidly as storage organs. Both bulbils and tubers are edible, although tubers are usually hard, bitter and unpalatable, and selection for bulbils appears to have taken place in early times, as some varieties, both Asian and African, lack significant underground tubers. Tuber size ranges from small up to about 25 cm in length.

The species occurs in a wide variety of forms and many synonyms have appeared in the literature, including D. crispata Roxb., D. heterophylla Roxb., D. oppositifolia Campbell, D. papilaris Blanco, D. pulchella Roxb., D. sativa Thunb., D. tamnifolia Salisbury, D. tunga Hamilton, D. Iatifolia Benth., and D. anthropophagum Chev.

Origin and distribution

The African and Asian varieties are so distinct that evolution must have taken place in prehistoric times, and there is disagreement as to whether the original source was in South-East Asia or whether there was also a centre of origin in Africa. The species is now pan-tropical.

Cultivation conditions

Potato yam grows in a wide range of soils and most varieties require long rainy seasons; this plant can be grown at elevations up to 1 800 m.

Planting procedure

Material-bulbils or tubers, either whole or small pieces. The tubers produced by plants grown from bulbils are usually very small in the first year and are often themselves used as setts for planting the following year to produce edible bulbils (and tubers) of a reasonable size.

Method-see Yam. Staking is necessary.

Field spacing-information is scant. In Puerto Rico satisfactory results have been obtained with rows at 160 cm spacing and the plants 70 cm apart in the row (about 9 000 plants/ha).

Pests and diseases

Leaf spot (due to Cercospora spp.) sometimes occurs and the nematode
Scutellonema bradys has been reported to attack the subterranean tubers.

Growth period

Immature bulbils may be harvested 3-4 months after planting, and picking may continue for the life of the plant, up to 24 months. Underground tubers are normally harvested when the vine dies back, after about 15-24 months.

Harvesting and handling

Immature bulbils are hand picked: mature bulbils fall to the ground. The tubers are lifted by fork or other digging tool or may be left for several months in the soil until needed. Both bulbils and tubers are resistant to fungal infections and harvest wounds heal quickly; storage under dry, cool conditions, away from sunlight, appears to give moderate storage life.

Primary products

Bulbils-usually weighing 0.5-2 kg. The flesh is yellow or white and is harder in the African varieties than in the Asian varieties.

Tubers-which also have white or yellow flesh. The African varieties are harder than the Asian.

Yield

This species is seldom grown commercially, but field trials in Puerto Rico of sixteen varieties from all over the tropics gave yields ranging from 0.05 to 19.5 t/ha for bulbils, and from zero to 25.34 t/ha for tubers. Some varieties gave high yields of both bulbils and tubers, eg 10.6 t bulbils and 22.09 t tubers, and 16.9 t bulbils and 13.35 t tubers. There is clearly potential for high productivity but unfortunately the time span of the experiment was not stated.

Main use

The bulbils are normally cooked and eaten in a manner similar to other starchy root crops, though many African forms require detoxification by soaking in water or prolonged boiling before they are safe to consume. A few are very succulent and may be eaten raw. The flavour is reported to be inferior to that of most common yams and some are bitter. Some yellow fleshed varieties darken during cooking. However, these yams have some popularity because of the convenient size of the bulbils for kitchen use.

Subsidiary uses

The bulbils and tubers are occasionally used for the production of flour. In Indonesia a fish poison is made from the bulbils of toxic varieties, and in Africa poisonous varieties may be planted among safe varieties to discourage thieves. In folk medicine in India a paste from the tuber is used as a cure for snakebite and in Jamaica for treatment of scorpion and centipede stings.

Secondary and waste products

Tubers-are used as food in times of scarcity, detoxification is usually necessary.

Special features

Bulbils-the proximate composition of the bulbils, in terms of the fresh weight, has been given as: water 63-67 per cent; protein 1.12-1.5 per cent; fat 0.04 per cent; carbohydrate 27-33 per cent; fibre 0.7-0.73 per cent; ash 1.08-1.51 per cent.

Tubers-the fresh weight composition of the tubers has been given as: water 69.1 per cent; protein 0.89 per cent; fat 0.1 per cent; carbohydrate 26.5 per cent; fibre 6.74 per cent.

Considerably higher values of protein have been found in other analyses, up to 10 per cent of the dry matter, but the sulphur-bearing amino acids are low, limiting the nutritive value. The yellow pigments of D. bulbifera are xanthophylls of no nutritional importance: beta-carotene is absent.

The toxic element found in many varieties is apparently dioscorine, and in some varieties the saponin dioscin has been reported.

Major influences

D. bulbifera has not been thoroughly investigated and its full potential may still be undiscovered. Currently, it seems that it should have a future in the home plot or gardens rather than commercial production. In its favour is the fact that a crop can be harvested for a period of perhaps 20 months starting at only 4 months from planting; in addition, the non-toxic varieties are easy to prepare for consumption.

Bibliography

CHEVALIER, Aug. 1952. De quelques Dioscorea d'Afrique �quatoriale toxiques dont plusiers vari�ti�s vent alimentaires. Revue Internationale de Botanique Appliqu�e et d'Agriculture Tropicale, 32 (351/2), 14-19.

COURSEY, D. G. 1967. Yams. London: Longmans, Green and Co. Ltd, 230 pp.

JACQUES-F�LIX, H. 1947. Ignames sauvages et cultiv�es du Cameroun. Revue lnternationale de Botanique Appliqu�e et d'Agriculture Tropicale, 27 (293/4), 122-123.

L�ON, J. 1977. Origin, evolution and early dispersal of root and tuber crops. Proceedings of the 4th Symposium of the International Society for Tropical Root Crops (Colombia, 1976), IDRC-080e (Cock, J., MacIntyre, R. and Graham, M., eds), pp. 20-36. Ottawa, Canada: International Development Research Centre, 277 pp.

MARTIN, F. W. 1974. Tropical yams and their potential. Pt. 2 Dioscorea bulbifera. United States Department of Agriculture, Agriculture Handbook, No. 466. Washington, DC: USDA Agricultural Research Service, 20 pp.

MARTIN, F. W., TELEK, L. and RUBERTE, R. 1974. The yellow pigments of Dioscorea bulbifera. Journal of Agricultural and Food Chemistry, 22, 335-337.

PURSEGLOVE, J. W. 1972. Dioscorea bulbifera L. Potato or aerial yam. Tropical Crops: Monocotyledons 1, pp. 102-104. London: Longman Group Ltd, 334 pp.

RAO, P. S. and BERI, R. M. 1953. Non-cereal foods, tubers of Dioscorea species. Indian Forester, 79, 568-571.

RASPER, V. and COURSEY, D. C. 1967. Properties of starches of some West African yams. Journal of the Science of Food and Agriculture, 18, 240-244.

SASTRI, B. N. (ed.) 1952. Dioscorea bulbifera. The wealth of India: Raw materials, Vol. 3 (D-E), pp. 71-72. New Delhi, India: Council for Scientific and Industrial Research, 236 pp.

WAITT, A. W. 1963. Yams, Dioscorea species. Field Crop Abstracts, 16, 145-157.


[edit] White yam (Dioscorea rotundata)

Common names

WHITE YAM, Guinea yam, White Guinea yam.

Botanical name

Dioscorea rotundata Poir. (Enantiophyllum).

Family

Dioscoreaceae.

Other names

Common yam, Eboe yam (W. Afr.); Eight months yam, Name blanco, �ame Guineo blanco (S. and C. Am.); Negro yam (Jam.); Portuguese yam (Mart.); Proper yam.


[edit] Yellow yam (Dioscorea cayenensis)

Common names

YELLOW YAM, Guinea yam, Yellow Guinea yam.

Botanical name

Dioscorea cayenensis Lam. (Enantiophyllum).

Family

Dioscoreaceae.

Other names

Afoo (Jam.); Affou (Trin.); Affun yam (W. Afr.); Atous temps (Ant.); Attoto yam (Afr.); Balugu (Ug.); Car� de Par� (Braz.); Congo amarillo (P. Rico); Cut and come again yam, Dye yam (Guy.); Fusaka (Mali); Hard yam (Guy.); Igname Guine�, Igname jaune, Igname pays negre (Fr.); Mapuey morado (P. Rico); �ame amarillo (Sp.); Name chomo (Pan.); Name negro (C. Rica); Negro yam (W.l.); Niame (Cuba); Ovihazo (Madag.); Twelve months yam, Yam a tout tan (W.l.).

Botany

The classification of these yams is confused. In the older literature they are usually separated but most taxonomists now regard them as the same species. Both show considerable variation: at the extremes the differences seem clear, but many intermediate forms occur, possibly as a result of hybridisation, and many current workers are grouping the two under the term Dioscorea rotundata/cayenensis complex, and this approach is taken here. Much of what follows is common to both 'species'; differences between the extremes of the complex will be noted under the 'specific' names.

The plants grow vigorously and can climb to a height of 10-12 m. The stems are cylindrical or slightly striated and are usually spiny, though sometimes completely smooth. The leaves are extremely varied, from deeply cordate to almost orbicular, 4-20 cm long, opposite or alternate. Some varieties have purplish leaf veins and stems. Male flowers are small (1-3 mm in diameter), borne on spikes; female flowers are much less frequently borne and the production of seeds is somewhat rare. Tubers are large (commonly 2-5 kg in weight, sometimes up to 25 kg or more), generally cylindrical but sometimes distorted, and thick skinned. The flesh ranges from white to yellow. More than 200 cultivars are known. The commonest differences between the two extreme types are:


D. rotundata

D. cayenensis

Tuber colour (flesh)

White

Yellow

Leaf shape

Narrowly ovate

Broadly ovate

Climatic preference

Intermediate rainfall

High rainfall

Growing season

7-8 months

10-12 months

Number of harvests

2

1

Possible time of harvest

Limited: late summer to

Almost year round winter

(Source: Martin and Sadik, 1977.)

Origin and distribution

West Africa appears to be the centre of origin, with initial domestication from the Ivory Coast to Cameroon, and in this African 'yam belt' these are the most important of all the yams. From West Africa they spread to Brazil and the Caribbean (presumably by way of the slave ships), and are important in Jamaica, Puerto Rico and the French West Indies, but not in the other islands. They were introduced into New Caledonia by the French.

Cultivation conditions

A warm tropical climate is required, but while D. cayenensis needs a long rainy season (about 10 months), D. rotundata cultivars can be grown with only 6-7 months of rainfall (100-150 cm evenly distributed) and thus can be grown further away from the equator where dry seasons are longer; also, because of its greater tolerance of drought, this type is adapted to the Caribbean region, though D. cayenensis is also grown there. D. cayenensis is relatively tolerant of sandy soils: D. rotundata thrives best on heavy soils even with a high clay content. Responses to organic matter (FYM or heavy mulch) are good. Nitrogen appears to be especially important (in Ghana 67 kg nitrogen applied after tuber reserves were exhausted gave a 22 per cent increase in yield; phosphorus gave a small response but potassium none), but fertiliser requirements vary from place to place.

Planting procedure

Material-usually small whole tubers, crowns or mid-section cuttings of large tubers (in 100-150 g pieces), dried for a few days before planting and preferably treated with wood ash to protect the pieces from fungal infection. Propagation of D. rotundata by stem cuttings is possible but does not yet appear to be commercially developed. Planting material must be disease-free.

Method-in Africa, yams are normally planted in land freshly-prepared by 'slash and burn', in mounds which are usually large enough for one plant, but sometimes for several. The pieces are planted at a depth of 5-15 cm, sometimes with the stem end down. The mound may be mulched with dried grass. In the Caribbean, planting in ridges 30-50 cm high is the usual practice, the seed tubers being placed 10-15 cm deep, by hand or by machine. In some areas, where soil drainage is naturally good, planting on the flat is practiced. Staking appears to be essential.

Field spacing-mounds are 1-2 m apart, and ridges also 1-2 m apart. Maximum yields are obtained by spacings of 1 x 1 m (10 000 ha).

Pests and diseases

Weeds-control is as described for Yam.

Pests-there seem to be few serous insect pests, but nematodes, especially Pratylenchus spp., Meloidogyne spp. and Scutellonema spp. are common, affecting not only the growing tuber but being associated with dry rot in the stored tuber. It has been stated that seed pieces can be treated with hot water at 50- 60°C for 30- 60 minutes; treatment of the soil with dibromochloropropane has been effective. In Africa, the greater yam beetle Heteroligus meres may attack the tubers and termites can cause severe damage. In the Caribbean, the white grub, Lachnosterna sp., and the sugar cane root borer, Diaprepes abbreviatus, often damage tubers. Field sanitation and avoidance of infected planting material are important aspects of control.

Diseases-fungal diseases are seldom serious, but anthracnose (due to Colletotrichum gloeosporioides) can be sporadically severe; resistant cultivars are available. More serious is the green banding virus (called mosaic in Puerto Rico, and shoestring disease in Africa) which, when severe, decreases yields significantly. Again, the use of planting material from healthy plants is essential.

Growth period

D. cayenensis types mature in 10-12 months and D. rotundata in 7-8 months. The early maturing of the latter permits double harvesting, as the early tuberisation results in large, though immature, tubers being present after 4 months. Some may be harvested then (and stored if so wished) while the remainder are harvested after 7-8 months. This practice is quite common in Africa where the mound permits tubers to be removed with little disturbance to the other tubers.

Harvesting and handling

Normally harvesting is by hand, though recent developments suggest that there may be possibilities for mechanically harvesting material planted in high ridges. Storage is as described for Yam. D rotundata tubers store better than those of D. cayenensis: under tropical conditions the storage life of sound tubers can be up to 4 months and 2 months respectively, although sprouting and excessive desiccation could be a problem.

Primary product

Tubers-usually only one large tuber per plant, though sometimes more are produced. A single tuber normally weighs 2-5 kg (but can be 25 kg or more). The skins are brown and thick, the flesh white in D. rotundata types to yellow in D. cayenensis.

Yield

In Africa, 8-18 t/ha and in the Caribbean, 15-25 t/ha are typical yields.
Over 67 t/ha has been reported from Puerto Rico.

Main use

Both types are eaten boiled, mashed, fried, etc. In Africa D. rotundata types are used in large quantities for the preparation of 'fufu'.

Subsidiary uses

Instant yam flakes can be made from suitable cultivars, though D. cayenensis yams are not favoured because of their yellow colour. Yam flour is also prepared.

Special features

Approximate composition of the edible portion of mature tubers has been quoted as: energy 439 kJ/100 g; water 58-33 per cent; protein 1.02-1.99 per cent; fat 0.05-0.12 per cent; carbohydrate 15-23 per cent; fibre 0.35-0.79 per cent; ash 0.53-2.56 per cent.

Higher moisture contents occur in immature tubers. Ascorbic acid is quoted as 6.5-11.6 mg/100 g in D. rotundata and 4.5-8.2 mg/100 g in D. cayenensis. Differences in the nature of the starch are reported: D. rotundata granules are large (10-70 microns) but those of D. cayenensis are smaller (3-25 microns). No commercial use has been made of the processed starch.

Processing

See Yam.

Production and trade

The major part of the total African production of yams (19 million t) is of the D. rotundata/cayenensis complex, and there is substantial production (though small by comparison with Africa) in Brazil and the Greater Antilles. There is a small trade from Brazil and Jamaica (estimated at about 9 000 t annually) to the UK, which also imports a small quantity from West Africa.

Major influences

Although apparently threatened by less labour-intensive and therefore cheaper carbohydrate crops, such as cassava, and by imported rice, African yam production appears to be holding its own. There seems to be little scope for expansion of exports or of processed products, mainly because of the high cost of the raw material.

Bibliography

AYENSU, E. S. and COURSEY, D. G. 1972. Guinea yams. Economic Botany, 26, 301-318.

COURSEY, D. G. 1961. The magnitude and origins of storage losses in Nigerian yams. Journal of the Science of Food and Agriculture, 12, 574-580.

COURSEY, D. G. 1967. Yams. London: Longmans, Green and Co. Ltd, 230 pp.

COURSEY, D. G. 1976. The origins and domestication of yams in Africa. World Anthropology (Harlan, J. R., de Wet, J. M. J. and Stemler, A. B. L., eds), pp. 385-408. The Hague, Netherlands: Mouton, 498 pp.

COURSEY, D. G. 1983. Yams. Handbook of Tropical Foods (Chan, H. C. (Jr.), ed.), pp. 555-601. New York: Marcel Dekker, 639 pp.

HUTTON, D. G., WAHAB, A. H. and MURRAY, H. 1982. Yield response of yellow yam (Dioscorea cayenensis) after disinfesting planting material of Pratylenchus coffeae. Turrialba, 32, 493-495. (Current Advances in Plant Science, 15(9), 13488).

IRVINE, F. R. 1969. Yam (Dioscorea spp.). West African Agriculture, 3rd edn, Vol. 2. West African Crops, pp. 160-173. London: Oxford University Press, 272 pp.

JARMAI, S. and MONTFORD, L. C. 1968. Yam flour for the production of fufu. Ghana Journal of Agricultural Science, 1, 161-163.

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