GREATER YAM, Greater Asiatic yam, Ten months yam, Water yam, Winged yam.
Dioscorea alata L. (Enantiophyllum).
Ambi (N. Guin.); Avase (Togo); Batatilla (Ang.); Bobayassi (W. Afr.); Cabeza de negra (Col.); CarÃ¡ de Angola, CarÃ¡ branco, CarÃ¡ cultivado, CarÃ¡ inhama (Braz.); Couche couche (Lat. Am.); Cucam, Cucui-mo (Viet.); Dandaba (Sen.); Gbara-guÃ© (Guin.); Goradu (Assam); Huwi (Sud.); Igname aileÃ¡ (Gab.); Igname de Chine (Fr.); Kachil, Katula, Khanulu (Ind.); Khoai-mo (Viet.); Kiseba (Ug.); Kuvi (Pacif. Is.); Lisbon yam (W.l.); Name asiatico (Venez.); Ã‘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.).
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.
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.
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).
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.
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.
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.
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'.
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.
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.
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.
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.
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