FARMER'S BOOKSHELF

An information system of tropical crops in Hawaii
Department of Tropical Plant & Soil Sciences
University of Hawaii at Manoa



Yam

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Climate



White yam (Dioscorea rotundata Poir), a native of West Africa, is grown in greater acreage than any other yam species in the world. White yam is the most favored yam species in W. Africa because it posseses a highly viscous starch. This commends its use in the pounded yam preparation, which is one of themost popular and prestigious foods of West Africans (Mozie and Okoro, 1990).

  1. D. rotundata. White yam, white guinea yam. Grown in West Africa, West Indies, and in the Caribbean. Compact spherical or cylindrical tubers are white, cream, and yellow fleshed.

  2. D. alata. Water yam, greater yam. Grown in SE Asia and in the Caribbean. Cylindrical, white purplish with loose watery texture.

  3. D. esculenta. Lesser yam, potato yam. Grown in China, SE Asia, South Pacific and Caribbean. Forms several tasty flavor-rich tubers with smooth thin yellow-brown skin, and white, sweet-fleshed grained starch.

  4. Others: D. Cayensis (yellow guinea yam); D. dumetorum (bitter yam, trifoliate yam); D. bulbifera (aerial yam).


Culture


Post Planting Treatments

Irrigation: (water requirements of the plants). Requires 100 cm of water well distribued throughout the growing season.

Mulching. Mulch after planting to protect plants from excessive heat and desiccation. Drastic reduction in yields result if mulching is not performed (results in reduced emergence percentage), especially in areas where hot temperatures and dry weather i s common.


Description


Name: Yams, Dioscorea spp.


Diseases


Tuber roots (Botryodiplodia theobromae, Rhizopus nodosus, Fusarium oxysporum, F. solani). Control through careful selection of tubers for planting. Cut seed should be dipped in Bordeaux mixture or with a systemic fungicide. Also controlled with crop rotation.


Weeds

* Weed control is important during the first 2 to 3 months after planting.


Fertilizer


  1. Type of soil. Prefers deep, rich, permeable soil. Loosed textured soil is necessary for tuber development and expansion.

  2. Drainage requirements. Heavy waterlogged soils should be avoided. Good drainage may be provided by building the soil into ridges or mounds 30-45 cm high.

  3. Nutritional Profile of Soil. Soil should be limed to about pH. 5.5 with essentially no exchangeable Al3+ present.

  4. Land Preparation. Soil at planting-time should be loose and well-worked. Planting is thus preceded by ploughing and harrowing. After that the yams are planted in rows. Sub-soiling or very deep ploughing before planting may improve root growth and yields. Alternatively, after ploughing and harrowing, shallow trenches running the lenght of the field are can be dug with machines. The yam setts are put in the trenches, which are then covered up. The opening of the trenches, placing the sett, and closing of the trench can all be done mechanically and in one operation.


Nutrient needs

* Nutrients (fertilizers used and application quantity and methods). It is best to apply the fertilizer in a continuous band along the row, about 10 cm away from the plants. Fertilizer application should be split to minimize fertilizer leaching: the first application is made about one month after emergence and the second is made about 7-9 weeks later when tuber bulking is in progress. A total application of 400 kg/Ha of 11-11-13 compound fertilizer is recommended.


Handling


  1. USDA treatments and requirements. Tubers can remain in the ground until needed. Under normal conditions after harvest, tubers remain dormant for a period of 10-15 weeks and then start sprouting. Prestorage curing is necessary. Tubers are usually cured before storage by being spread out evenly in the shade for 2-3 days after harvest. Gibberellic acid (GA3) applied immediately after harvest or before curing starts, extends the dormant period and prolongs tuber shelf life of many yam cultivars. T he response to GA3 is cultivar-dependent. Standard curing practice: Maintain at 29-32C (84-90F) at 90-95 RH for 4-8 days.

    Recent studies show that prior to storage, yams should undergo curing. This is because of the wounds that are inevitably inflicted on tubers during harvesting and subsequent transportation and handling. Curing refers to the healing of these wounds by controlled processes during the first week after harvest. The curing process normally involves exposure to the freshly harvested tubers to a controlled temperature range of about 29-35C and RH 90-95 for 5-7 d before storage in the shade. Curing tends to prolong the life of the tubers during subsequent storage by drastically reducing the spread of pathogenic organisms through the tissue of the tuber. In their study curing (peridem layer formation) improved with yellow (580 nm), green (520 nm) and blue ( 490 nm) tungsten lamps for 7 d at 27C in well-ventilated chambers, but not with red (660 nm) (Mozie and Okoro, 1990).

    Pre-cooling: cold room. Sensitive to cold temperatures below 13C (55F), to decomposition at high humidities, and also sensible to ethylene exposure. Transport through highway and piggyback trailers, and van containers. Life-time during transport and storage= 6-7 months.

  2. Packing (no. lbs/box; box type). Full telescoping fiberboard cartons with paper wrapping or excelsior to reduce bruising, loose pack. Boxes of 11 kgs (25 lbs), 23 kgs (50 lbs). Hand loaded or unitized on pallets.

  3. Cleaning (methods and requirements). Clean tubers in the field, after harvest.

  4. Waxing (type of wax and quantity/methods). Not-applicable

  5. Vacuum packing. Not-applicable.

  6. Chemical treatments (fungicides etc.). Fumigate with methylene bromide before storage. Dipping with benlate, captan, thiabendazole (250 ppm), or limewash can help to reduce infection and storage rots.

  7. Hot water treatments, if necessary. Not applicable, since it damages the roots.

  8. Refrigeration requirements (after harvest and during treatment). Store at 15C. Chilling injury occurs below 13C (55F).

  9. Humidity requirements. Maintain in storage at 70-80 RH. Decay occurs at higher RHs.


Harvest


Expected yield per acre (based upon specific planting density). 40 MT at 10,000 plants per hectare harvested at 8 months after planting.

* Harvesting can be done at any time of the year. Harvesting may be done at any time after large-scale leaf yellowing sets in. Since little or no more tuber material is added during the last month before vine death, the time of harvesting is not very critical. Most species produce only 1-3 tubers per plant. Harvest can be conducted at 6 to 10 months after planting (MAP). An alternative is to harvest first at 4-5 MAP and a second harvest at 8-10 MAP. Harvesting involves digging around the tuber to loosen it from the soil, lifting it, and cutting its attachment to the vine. Roots should preferably be dug on a sunny day to avoid wetting and subsequent decay of yams.


Harvest

Hand harvested. Harvesting large tubers is a time-consuming process because the tubers must be excavated from considerable depths without bruising them.

  1. Photos or diagrams of a typical packing plant

  2. Flow chart of the packing operation


Insects


Yam beetle (Heteroligus meles). Control with Aldrin, endosulfan or grammalin dusting of holes or soil surface.

Chrysomelid beetle (Lilioceris livida). Control by removing larva by hand or by spraying with carbaryl.

Mealybugs (Planococcus citri). Control by using a clean planting stock.

Yam nematode (Scutellonema bradys). Control with rotation and fallowing.


Market/Uses


Availability.

Year-round from the Caribbean, and Central America.

* Greater than 90% of world production is concentrated in West Africa. Total world production is estimated at 25 million MT per year. The major producing countries in West Africa include Nigeria, Ivory Coast, Indonesia, Ghana, and Togo.


Planting


Planting material. Seed pieces 150-300 g in weight, from mature tubers or bulbils. Around 25-30% of the yams produced are usually reserved for planting. The best planting material, however, are small tubers, resulting in improved germination percentage s and in less tuber rotting.

Preplanting treatment. Cut seeds pieces should be cured for a few days before planting to allow wounds to heal. Dipping in pesticidal solutions of benlate, captan, limewash or bordeaux mixture offers some protection against fungi. Dusting the sett with 2.5% aldrin or "grammalin A' dust protects the planting sett and the subsequent tubers against insect attack.

Diuron (3 kg/Ha), metribuzin, linuron (3 kg/Ha), simazine (3 kg/Ha), TCA, alachlor, and atrazine (3 kg/Ha) can be utilized as pre-emergence herbicides.

Recent experiments with plastic mulch resulted in greater yields, greater moisture and nutrient retention, reduced weed growth, temperature regulation, and it eliminated the need for staking.

Planting of yams on the flat is preceded by ploughing and harrowing. After ploughing and harrowing, shallow trenches running the lenght of the field are dug with machines. The yams setts are put in the trenches, which are then covered up. The opening o f the trenches, placing of the sett and closing of the trench can all be done mechanically and in one operation. Sub-soiling or very deep ploughing may improve yields of flat planting of yams (as compared to the more traditional ridge or mound plantings) .

Depth of Planting. 10 cm.

Timing of planting (crop cycle). Preferable at the start of the rainy season.

Planting density (expected germination). 10,000 pls/Ha. Space at 1 m between plants and 1 m between rows. Germination percentage depends on dormancy which lasts from 1-4 months. Active buds should be present in each seed piece. Emergence may begin ab out 2 weeks after planting and continue for 2-3 months. This characteristic makes it hard to choose and use herbicides efficiently because timing of the applications is difficult.