Yam (Dioscorea sp) is rich source of carbohydrate & protein & an important source of food in Tropical Asia & America. The processing technology of this crop is conventional method which involves boiling, roasting & frying. About 50-60 species of Dioscorea are cultivated to a limited degree or gathered as famine ford. Following are some species mainly used for food purpose.
- alata – greater yam (Asiatic yam)
- esculenta – Lesser yam (Asiatic yam)
- rotundata – white yam
It is believed to be native of South East Asia, probably Burma or Assam (India)
Dioscorea tuber contains Diosgenin, an alkaloid that is used in preparation of contraseptive drugs that are used in family planning.
There are several local varieties of yam cultivated in Nepal. Major exotic varieties are
- Sree keerthi
- Sree Roopa
- Sree subhra
- Sree latha
- Sree kala
- Sree subhra
- Sree Priya
- Sree dhanya
Yam requires a fertile, well drained soil & grows best in sandy loam soil. High rainfall is required for maximum production. Temperature in the range of 20-30 degree Celsius promote better growth.
The soil should be dug deep or ploughed to a depth of 10cm. On sandy soil, yam is planted in flat beds. Raised beds and trenches are also used for planting. The seed pieces are planted at the depth of 15 cm in the soil & planting deeper than this has been found to give lower yield.
Manuring & Fertilization:
Apply 10 tons/hector well decomposed FYM during field preparation. Apply 80-120: 60: 80 kg NPK/ha is recommended fertilizer for this crop. Nitrogen & potassium is given in split doses where first half at 1st interculture & half of them is applied 1 month after planting at 2nd interculture.
Yams are propagated vegetatively & tuber pieces are normally planted. For propagation, whole tuber is cut into seed pieces consisting of tops (proximal), middles & bottom (distal). The seed tuber yams & tops are preferred for their earlier & uniform sprouting. The tuber pieces may be dipped in cow dung slurry & allowed to dry under shade before planting. A seed tuber weight of 200-250 gram is ideal for optimum production in D. alata. For D. esculenta a seed tuber yam weight of 100-150 gram is optimum.
Different spacing is maintained for different species of yam.
- esculenta: 75 cm * 75 cm
- alata & D. rotundata: 90 cm * 90 cm
Mulching immediately after planting is beneficial for increasing the tuber yield. Staking exposes the leaves to sunlight & the yields of the staked plants are higher than those of unstaked plants. Yams generally take about 30-40 days for sprouting & about 2 months to develop a vegetative cover on the ground. The first weeding may be done 1 week after 50% sprouting and the second a month later.
Harvesting & Yield:
Yams are generally, harvested at 8-9 months after planting when they attain maturity. The tuber yields depend upon the species & cultural practices followed. Tuber yields varies from 20-40 ton.ha in alata & rutundata whereas 10-30 tons in esculenta.
There are many insects pest attacking yam. Spotted beetle, tobacco caterpillar are major which cause huge defoliation & damage to plant system. Damage symptoms with control measures are described below :-
Spotted beetle (Galerucida bicolor)
The grubs and adults feed on tender leaves & stem of plants. It is best controlled by following ways: –
- Spray the crop with malathion @ 0.06%.
Tobacco caterpillars (Spodoptera litura)
the caterpillar feeds on the leaves resulting into serious defoliation.
- Spray the crop with malathion @ 0.06% at 10-15days interval.
Hairy caterpillar (Pericallia ricini)
The caterpillars cause damages on non trained plants. The larva defoliates the plant.
- Spray the crop with malathion @ 0.06% at 10-15 days interval.
Scale insect (Aspidiella hartii)
The tiny nymph and female feeds on vines & tubers.
- Use insect free tuber for planting.
Root knot nematode (Meloidogyne sp.)
It forms galls around feeding site on the roots.
- Deep summer ploughing & crop rotation.
- Grow resistant varieties.
Mainly two major diseases are persistent in yam which can cause huge economic & yield loss. Control measures & damage symptoms are described below:
Blight & dieback (Cercospora sp. and Colletotrichum sp)
Initially brown to black spots are formed on the leaves which gradually coalesce. The affected leaves blighted. In severe defoliation, death of the plant also occurs.
- Spray Dithane M-45 @ 0.5% at 15 days interval.
The characteristic symptoms of mosaic are seen as development of dark and light green areas on the leaves, crinkling of leaves, vein banding and stunting of plants.
- Local resistant / tolerant varieties.
- Rogue out the affected plants from the field.