Yam (Dioscorea spp.).
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).
Table 1: Yam - Area and production in selected countries
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 |
|
|
|
Africa |
|
|
|
North and Central America |
10.04 |
11.12 |
|
South America |
|
|
|
Asia |
|
11.35 |
|
Oceania |
14.84 |
13.51 |
|
All developed countries |
17.91 |
17.92 |
|
All developing countries |
|
|
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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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.
Bitter yam (Dioscorea dumetorum)[edit | edit source]
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.
Chinese yam (Dioscorea opposita)[edit | edit source]
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).
Cush-cush yam (Dioscorea trifida)[edit | edit source]
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)
Greater yam (Dioscorea alata)[edit | edit source]
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.
Intoxicating yam (Dioscorea hispida)[edit | edit source]
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.
Lesser yam (Dioscorea esculenta)[edit | edit source]
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
