Dioscorea esculenta (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Dioscorea esculenta (Lour.) Burkill

Protologue: Gard. Bull. Straits Settl. 1: 396 (1917).

Family: Dioscoreaceae

Chromosome number: 2n= 40, 90, 100

Synonyms

• Dioscorea aculeata L. (1754, not of Sp. pl., 1753),
• Oncus esculentus Lour. (1790),
• Dioscorea tiliifolia Kunth (1850).

Vernacular names

• Lesser yam, Chinese yam, Asiatic yam (En).
• Igname des blancs, igname de Chine (Fr)
• Indonesia: ubi aung (West Java), ubi gembili (East Java), kombili (Ambon)
• Malaysia: ubi torak, kembili, kemarung
• Papua New Guinea: mami (Pidgin), taitu (Motu), kalak (Yatmei), diba (Hanuabada)
• Philippines: tongo (Tagalog), aneg (Ibanag), baribaran (Bikol)
• Cambodia: dâmlô:ng sya
• Laos: hwà katha:d, man 'o:nz
• Thailand: man-musua (central), man-chuak (northern)
• Vietnam: củtừ, khoai từ, từgai.

Origin and geographic distribution

D. esculenta is native to Thailand and Indo-China, and may have originated there. It also grows wild in northern India, Burma (Myanmar) and New Guinea, but it is not known whether these are escapes from ancient cultivation or real relicts of its natural distribution area. Within South-East Asia the main direction of prehistoric spread has been out of the continent of Asia through the Philippines, thence south and south-east and ultimately towards the south-west. After 1500 it spread throughout the tropics. At present its cultivation is most important in South-East Asia (especially in New Guinea), Oceania, the Caribbean and China.

Uses

The tubers are eaten as a starchy staple, after cooking or roasting, and their taste is sweet and pleasant. Flour and starch are also extracted. Starch grains are small, and more easily digested than those of other yams. D. esculenta is therefore used in special diets for persons with alimentary disorders. Grated raw tubers are applied medicinally as a poultice on swellings, especially on the throat.

Production and international trade

No separate production statistics are available for D. esculenta. Production and trade is mainly local and very little of this yam enters into international trade. In South-East Asia, D. esculenta comes second in importance after D. alata L.

Properties

The proximate composition of the tuber per 100 g edible portion is: water 70-80 g, protein 1.3-2.1 g, fat 0.1-0.3 g, carbohydrates 26-36 g (starch 25 g, sugars 1-11 g), fibre 0.2-1.5 g, ash 0.5-1.2 g, vitamin A 0.017 mg, vitamin B₁0.08 mg, vitamin B₂0.02 mg, and vitamin C 20.3 mg. The starch is mainly amylopectin, and the starch grains are small (0.5-2.0μm) and angular, with a gelatinization temperature of 69.5-80.5°C. The sugar content is higher than for most other yams, and this gives it a sweet taste. Tubers of some Philippine cultivars reportedly contain up to 0.7% diosgenin. Lysine is the limiting amino acid.

Description

• A herbaceous, pubescent, often prickly, climbing annual.
• Roots thorny in wild plants, often thornless in cultivated plants, fibrous, mostly confined to the top 1 m of the soil.
• Tubers 4-20 per plant, thrust downwards from a corm, on stolons 5-50 cm long; mature tubers shortly cylindrical, sometimes lobed, 8-20 cm × 2-5 cm; skin brown or grey-brown, thin, often rough with indurated bases of rootlets; flesh white.
• Stem terete, twining to the left, prickly at the base and less so upwards.
• Bulbils absent.
• Leaves alternate, simple, cordate, 10-15 cm × 10-17 cm, acuminate, 9-13-nerved, secondary veins regular but not conspicuously ladder-like; petiole 1-1.5 times as long as the blade, often with 2 prominent spines at the base.
• Inflorescence unisexual; male inflorescence solitary in distal leaf axils, usually carrying one flower at a time along the axis but up to 70 or more in total; female inflorescence on downcurved spike-like racemes, solitary from upper leaf axils, up to 40 cm long.
• Fruit (very rare) a reflexed capsule, 27 mm × 12 mm.
• Seed winged all round.

Growth and development

Because of limited tuber dormancy, the planted tuber sprouts readily, and roots then proliferate. The vine then grows vigorously, and at 2-4 months after planting, underground stolons are produced. The tips of the stolons eventually give rise to the tubers. Mycorrhizal associations aid in nutrient absorption. Flowering is rare in cultivars, but in Papua New Guinea male flowers commonly occur. The plant senesces after 6-10 months.

Other botanical information

Wild plants of D. esculenta usually have more vigorous foliage, larger leaves, longer stolons, more fibrous tubers and more thorny shoots and roots than cultivated plants. In the literature 2 botanical varieties are distinguished: var. spinosa (Roxb.) Prain & Burkill for plants well provided with thorny roots, and var. fasciculata (Roxb.) Prain & Burkill for plants with few thorny roots. The type specimen of D. esculenta is too poor to determine which variety should become var. esculenta. It would be more useful to classify the cultivated plants into cultivar groups and cultivars. South-East Asian cultivars include "Asakua", "Mart", "Saikidi", and "Kuali" from Papua New Guinea.

Ecology

The natural habitat for D. esculenta is the humid and subhumid tropics. The plant does best where there is well-distributed rainfall of 875-1750 mm/year. Those parts of South-East Asia without a dry monsoon are rather too humid for it. The average minimum temperature for good growth is 22.7°C. Temperatures of 35°C and above reduce sprouting of the planted set. The plant is essentially a lowland crop, but grows successfully at an altitude of 900 m in the Himalayas. The maximum elevation for D. esculenta cultivation in Papua New Guinea is 900 m. Tuber formation is promoted by short-day conditions. Light, well-drained soils of pH 5.5-6.5 are best.

Propagation and planting

Small whole tubers are used as planting material. The tubers have a very short dormancy period. They are planted on mounds (e.g. in Papua New Guinea), ridges, or on the flat. Intercropping with other food crops is common, with variable spacing between D. esculenta plants. For sole cropping, spacing is 100 cm × 50 cm.

Husbandry

Plants are usually, but not always, staked soon after emergence. Manual weeding 2-3 times in the season is necessary. Herbicidal weed control, using atrazine and paraquat, is also practised.

Diseases and pests

Leaf spots and leaf necrosis are caused by Cercospora spp. and Glomerella spp. 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. The yam scale (Aspidiella hartii) and mealy bugs may infest the tubers, especially during storage. Nematodes, especially the root-knot nematode (Meloidogyne spp.) and the yam nematode (Scutellonema bradys) attack the growing plant and cause wartiness of the harvested tubers. Wild animals, such as pigs and rodents may destroy the crop in some locations.

Harvesting

The crop matures in 6-10 months (8-9 months in Malaysia) when the vines turn yellow and begin to senesce. Harvesting should not be delayed, since if left in the soil the tubers soon begin to sprout because of their short dormancy period. The tubers do not lie deep in the soil, so simple hand tools are used to lift them.

Yield

The following yields have been reported: 24.6 t/ha in Malaysia, 20-30 t/ha in the Philippines, 70 t/ha in Irian Jaya, 10-20 t/ha in Papua New Guinea. Tubers may weigh 0.1-3 kg.

Handling after harvest

In commercial production, the tubers are graded in the field, and packed for the market in well-ventilated boxes. They are not packed in sacks because of the greater risk of bruising and rotting. Because the skin of the tuber is thin, tubers are easily bruised and storage is difficult.

Genetic resources

Germplasm is maintained at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria, and the United States Department of Agriculture (USDA) Agricultural Experiment Station, Mayaguez, Puerto Rico. In South-East Asia, some lines are kept at the Laloki Experiment Station in Papua New Guinea.

Breeding

Breeding is difficult because female flowering is very rare and fruiting even more so. The main breeding objectives are resistance to diseases, higher yields, amenability to production with reduced labour, and greater storability.

Prospects

D. esculenta is well established as a staple in South-East Asia and elsewhere. The prospects remain good that it will retain this position. Research priorities are breeding of improved cultivars and improvement of cultural practices.

Literature

• Bourke, R.M., 1982. Root crops in Papua New Guinea In: Bourke, R.M. & Kesavan, V. (Editors): Proceedings 2nd Papua New Guinea Root Crops Conference, Part 1. Department of Primary Industry, Port Moresby, Papua New Guinea. pp. 51-63.
• King, G. & Hackett, C., 1986. Tabular descriptions of crops grown in the tropics. 14. Lesser yam (Dioscorea esculenta L.). Technical Memorandum 86(15). Commonwealth Scientific and Industrial Research Organization (CSIRO), Canberra, Australia. 52 pp.
• Martin, F.W., 1974. Dioscorea esculenta. In: Martin, F.W., Sadik, S. & Degras, L. (Editors): Tropical yams and their potential. Part 1. Agriculture Handbook No 457. United States Department of Agriculture (USDA), Washington, D.C., United States. 18 pp.
• Onwueme, I.C., 1978. The tropical tuber crops. Wiley, Chichester, United Kingdom. pp. 3-106.

Authors

I.C. Onwueme