|By goGreen | October 3, 2011|
LESSER YAM, Asiatic yam, Lesser Asiatic yam.
Dioscorea esculenta (Lour.) Burk. (Combilium).
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.).
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.
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.
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.
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.
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.
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.
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).
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.
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.
Tropical Development and Research Institute
127 Clerkenwell Road, London EC1R 5DB
Overseas Development Administration