Dioscorea alata (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Dioscorea alata L.

Protologue: Sp. pl.: 1033 (1753).

Family: Dioscoreaceae

Chromosome number: 2n= 20, 30, 40, 50, 60, 70, 80

• Dioscorea globosa Roxb. (1832),
• D. purpurea Roxb. (1832),
• D. vulgaris Miquel (1859).

• Greater yam, water yam, ten-months yam (En).
• Igname de Chine, pacala, igname ailée (Fr)
• Indonesia: huwi (Sundanese), uwi (Javanese), lame (Sulawesi)
• Malaysia: ubi kipas, pokok ubi, ubi tiyang
• Papua New Guinea: yam tru (Pidgin), nyaing (Yatmei), kolpur (Melba)
• Philippines: ubi (general), kinampay (Bisaya)
• Cambodia: dâmlô:ng chiem moan, dâmlô:ng phluk
• Laos: man hlièmx
• Thailand: man-liam (northern), man-sao (central), man-thu (southern)
• Vietnam: củcái, củmỡ, khoai vạc.

D. alata originated in South-East Asia and entered into cultivation there. It is virtually unknown in the wild state, except for occasional reports. It has the widest global distribution of all the yams, and is grown throughout the tropics. Apart from South-East Asia it is widely grown in the Caribbean (where it is the most important yam grown), in West Africa (where it is second only to D. rotundata Poiret), and in Oceania. In South-East Asia, it is the most important species and is grown in virtually all countries of the region, especially in Indonesia, Malaysia, Papua New Guinea, the Philippines, and Vietnam.

The tubers and larger bulbils of D. alata are consumed as a starchy staple, after cooking in various ways. They can be processed into yam flakes or yam flour. Their use as a source of starch is minor. The cultivars with purple flesh are used in the manufacture of ice-cream and other confectioneries. D. alata is also used for ceremonial purposes in Papua New Guinea.

Most available production data do not separate the statistics on D. alata from those of the other yams. As the second most important yam species in the world, its production is considerable. Most of it is traded and consumed locally in the country of production. There is some export trade in D. alata from the Caribbean to North America and Europe.

Tuber texture is looser than that of other edible yams. The approximate composition of the tuber per 100 g edible portion is: water 70 g, protein 1.1-2.8 g, fat 0.1-0.3 g, carbohydrates 28.5 g (starch 28 g, sugars 0.5 g), fibre 0.6-1.4 g, ash 0.7-2.1 g, vitamin A 0.18 mg, vitamin B₁0.09 mg, vitamin B₂0.03 mg, and vitamin C 5-28 mg. The starch is mainly amylopectin. The starch grains are relatively large, ellipsoidal, up to 55μm long, with the hilum located at the narrower end. Gelatinization temperature is 69-88°C, and viscosity 100-200 Barbender units. Unlike most other yams, starch from D. alata has a high gel strength. Extracted starch from white-fleshed cultivars has 13.3% moisture, 21% amylose, 0.24% reducing sugars, 0.15% protein, 0.23% ash, pH of 7.3, and iodine absorption of 5.5. Purple-fleshed cultivars yield starch of 13.9% moisture, 21.1% amylose, 0.12% reducing sugars, 0.13% protein, 0.20% ash, pH of 6.9, and iodine absorption of 5.5. The anthocyanins from the purple-fleshed types are cyanidin glycosides. Lysine is the limiting amino acid. Per 100 g dry matter the tuber also contains: oxalate 50 mg, cyanide 1.08 mg, and tannin 75 mg.

• Dioecious, annual, glabrous herb, climbing up to 10 m. Root system fibrous, shallow, mostly confined to the top 1 m of the soil.
• Tubers usually single, varying in size and shape, often very large; cylindrical or clavate in shape (often found as deep as 1.5 m) or globose, stout and short, pyriform, often variously lobed or fingered and fasciated or curved; skin brown to black; flesh white, cream or purplish (superficially or throughout).
• Stem twining to the right, unarmed but sometimes rough or warty at base, quadrangular and usually conspicuously 4-winged, green to purplish.
• Axillary bulbils abundant but number varies with cultivar.
• Leaves simple, alternate at the very base, thereafter opposite, subsagittately or subhastately ovate, 10-30 cm × 5-20 cm, acuminate, 5-nerved, bright green or sometimes tinged purplish; petiole up to as long as the blade, sometimes marginally frilled, auriculate at base.
• Male inflorescence a spike, 1-2 together, aggregated on leafless axillary branches up to 30 cm long, axis sometimes slightly zigzag.
• Female inflorescence a solitary, axillary spike, up to 60 cm long. Some cultivars do not flower, many remain sterile.
• Fruit a transversely elliptical, 3-winged capsule, about 2.5 cm × 3.5 cm.
• Seed orbicular, winged all round.

As with most other edible yams, the growth and development of D. alata falls roughly into 4 phases. The first phase spans from 0-6 weeks after emergence, and is characterized by extensive root and vine growth, but very limited leaf expansion. The second phase, lasting from 6-12 weeks after emergence, is characterized by limited root growth, extensive shoot and leaf growth, and the onset of tuber formation. The third phase, lasting until the end of the season, is characterized by tuber bulking and maturity. Root and shoot growth are very limited in this phase. The total growing period is 8-10 months. In the fourth phase, the shoot senesces and dies back, and the tubers enter a dormant period lasting 2-4 months before sprouting again.

D. alata is the most important yam for South-East Asia and is easily recognized by its quadrangular winged stems. The variability of the tubers in form, size, weight, colour and flesh is so great that in every country where it grows and is used, many forms, selections and cultivars exist, all with their own vernacular names.

Because D. alata is only known from cultivation, a classification into cultivar groups and cultivars is most suitable, but there is no satisfactory worldwide system. In a study of 235 different cultivars from all over the world, 15 cultivar groups could be distinguished. The greatest variability was present in materials from New Guinea, Indonesia and the Philippines. Relevant groups for South-East Asia include:

• cv. group Purple Compact. Plants with a large content of anthocyanin, especially in the petiole; well developed, ruffled wings; tubers short with coloured flesh and cortex. Principal region: the Philippines. Cultivars include "Morado", "Vino Violeta", "Kinampay".

• cv. group Primitive Purple. Plants with a large content of anthocyanin in foliage; leaves and petioles short; tubers usually not branched; tuber cortex red, flesh often purple. Principal region: New Guinea. Some cultivars: "Buster", "Kiubu", "Aupik".

• cv. group Primitive Green. Plants with light green leaves, foliage almost free of anthocyanin; tubers long with prominent neck; cortex and flesh very white, cooking qualities poor. Principal regions: New Guinea, Indonesia. Cultivars include "Masi", "Lanswa", "Suabab".

• cv. group Compact. Plants with foliage tinged with anthocyanin; tubers short, wide; little coloration of cortex and flesh, very good cooking qualities. Principal regions: New Guinea and the Philippines. Some cultivars: "Uhbisi", "Toma", "Kabusak".

• cv. group Poor White. Plants with solitary tubers; cortex coloured but flesh whitish; pronounced gradient of colour and texture. Principal regions: New Guinea, India. Cultivars include "Baron", "Toa", "Goarmago".

D. alata is a plant of the subhumid to humid tropics. An annual rainfall of 1000-1500 mm, distributed over at least 6-7 months, is required for its cultivation. It is mostly a crop of the lowlands, though it occurs at elevations of up to 2500 m in India. Tuberization is promoted by short days (less than 12 h). It tolerates poorer soils than most other cultivated yams. D. alata is sensitive to aluminium toxicity in the soil.

Tuber dormancy lasts for 2-4 months after harvest, but can be broken with ethylene chlorohydrin. Small intact tubers ("seed tubers") or tuber pieces weighing 50-500 g are used for planting on mounds, ridges, or on the flat. Intercropping is the most common practice, with the distance between D. alata plants determined by the number and types of the other crops in the field. Where sole cropping is done, rows are 1 m apart, with intra-row spacing of 50-100 cm. In Papua New Guinea, shifting cultivation is a common practice.

Plants are usually staked soon after emergence. Unstaked cultivation also occurs, which suppresses weeds better but gives lower yields. Weeding is done 3-4 times during the season, using hand tools. Most production in South-East Asia takes place without irrigation, mechanization or even chemical fertilizer. In general, yam is the first crop in the rotation after fallow.

Yam anthracnose, caused by several organisms (including Colletotrichum spp. and Glomerella spp.), is a serious disease which results in blackening and die-back of the leaves. The disease is more severe on D. alata than in other edible yams. The best control measure is the use of resistant cultivars such as "TDA 291" or "TDA 297" (International Institute for Tropical Agriculture). Phenol content is reportedly higher in resistant cultivars than in susceptible ones. Tuber rots caused by Fusarium spp., Penicillium spp., and Rosellinia spp., all afflict D. alata tubers especially in storage. The yam virus complex, of which very little is known, also attacks D. alata and depresses yield.

The yam beetle (Heteroligus spp.) attacks the tubers, and can be controlled with insecticidal dusts, or by planting as late as possible in the season. Scale insects and mealy bugs may infest the tubers, especially during storage. Nematodes attack the plant, resulting in warty appearance of the tuber. Wild animals, such as pigs and rodents may destroy the crop in some locations.

Harvesting occurs 7-10 months after planting. Hand tools are used to dig up the tubers.

Yields range from 8-30 t/ha. Individual tubers usually weigh 5-10 kg, although up to 60 kg have been reported. A yield of 42 t/ha has been reported in Malaysia.

Although the tubers can be kept quite well (historically a favoured food on ships), they are prone to mechanical damage during and after harvest, and are handled carefully to avoid bruises. They are stored in cool shady conditions, but are subject to chilling injury if stored below 12°C. Tubers can be processed into yam flakes or yam flour, or marketed fresh.

Germplasm collections of D. alata are held at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria, the USDA Agricultural Experiment Station, Mayaguez, Puerto Rico, and the Philippine Root Crops Research & Training Centre, at VISCA, Baybay, Philippines.

The main breeding objectives are resistance to diseases, especially yam anthracnose and the yam virus complex, higher yields, amenability to mechanization and unstaked cultivation, and better storability.

D. alata is already well established as the most ubiquitous of all edible yams. The prospects remain good for its continued use as food, especially if the high labour requirements to produce it can be substantially reduced.

• Alozie, S.O., Nwankiti, A.O. & Oti, E., 1987. Source of resistance in anthracnose blotch disease of water yam (Dioscorea alata) caused by Colletotrichum gloeosporioides Penz. 2. Relationship between phenol content and resistance. Beitrage zur Tropischen Landwirtschaft und Veterinarmedizin 25(1): 55-59.
• King, G. & Hackett, C., 1986. Tabular description of crops grown in the tropics. 13. Greater yam (Dioscorea alata L.). Technical Memorandum 86(4). Commonwealth Scientific and Industrial Research Organization (CSIRO), Canberra, Australia. 53 pp.
• Martin, F.W., 1976. Dioscorea alata. In: Martin, F.W., Sadik, S. & Degras, L. (Editors): Tropical yams and their potential. Part 3. Agriculture Handbook No 495. United States Department of Agriculture (USDA), Washington, D.C., United States. 40 pp.
• Martin, F.W. & Rhodes, A.M., 1977. Intra-specific classification of Dioscorea alata. Tropical Agriculture (Trinidad) 54: 1-13.
• Onwueme, I.C., 1978. The tropical tuber crops. Wiley, Chichester, United Kingdom. pp. 3-106.
• Udoessien, E.I. & Ifon, E.T., 1992. Chemical evaluation of some antinutritional constituents in four species of yam. Tropical Science (UK) 32(2): 115-119.

I.C. Onwueme & Z.N. Ganga