School Science Lessons
2024-04-18
Yam Project
(ProjYam)
Edible Dioscorea species have opposite leaves, but poisonous species have alternate leaves.
Contents
Preface
1.0 Importance and distribution
2.0 Climate and cultivation conditions
3.0 Leaves
4.0 Stems and roots
5.0 Tubers
6.0 Bulbils
7.0 Flowers
8.0 Planting and growth period
9.0 Recording growth
10.0 Staking
11.0 Weeding
12.0 Fertilizing
13.0 Pests and diseases
14.0 Harvesting and handling
15.0 Storing
16.0 Chemical composition
17.0 Dioscoreaceae, the yam family
18.0 Dioscorea genus
19.0 Yam species
20.0 Global Initiative underway to preserve yam biodiversity
1.16 Records
13.0 Pests and diseases
13.0 Pests and diseases
13.11 Butterfly pests
13.10 Mealybugs
13.1 Yam anthracnose disease
13.2 Yam leaf spot
13.3 Yam mosaic disease, (YMV)
13.4 Yam nematodes
13.5 Yam rusts
13.6 Yam scale
13.9 Yam beetles
1.0 Importance and distribution
Yams are important in the lives of some of the people for yam festivals at harvest time and food.
In some places a man is important if he can grow the biggest yams.
Some people put special stones near the planting place, because they believe "magic stones" will help the yams to grow.
West Africa is the most important cultivation zone, producing about 93 per cent of the world's edible yams, but the crop is also of importance in parts of eastern Africa, the Pacific area (including Japan), the Caribbean and tropical America.
Yams are very important in all parts of Melanesia, Papua New Guinea, the Solomon Islands, New Hebrides and New Caledonia.
They are grown mostly by the coastal people, not by people living inland or on coral islands.
Yams are also important in the high islands of Micronesia, in Ponape, Kusaie, Truk and Yap.
Yams are less important in most parts of Polynesia, but some are grown in Tonga, the Cook Islands, French Polynesia, Samoa and also in Fiji.
2. Some yam varieties have needle-shaped crystals of calcium oxalate raphides, just under the skin of the tubers, so they should be peeled thoroughly, but the needle-shaped crystals cab cause hand irritation.
Similarly leaves should be double boiled before being consumed.
Some varieties contain | Dioscorine |, a toxic alkaloid that must be removed by slicing and leaching in water.
Many Dioscorea species contain the steroid saponin | Disogenin |.
Dioscorea polystachya, Chinese yam, Daleys Fruit Tree Nursery
2.0 Climate and cultivation conditions
Edible yams cannot withstand frost, except Dioscorea japonica and Dioscorea opposita have poor growth below 20oC, and optimum growth at about 30oC.
Very high temperatures are bad for yams.
Yams require adequate moisture between the 14th and 20th weeks of growth.
Yams grow well if there is a dry season lasting 2 - 5 months followed by a wet season with at least 1150 mm of rain.
Good soil drainage is essential and for optimum yields a deep well-drained sandy loam is required.
Most yams are grown on land after it has been cleared from bush, so fallow mulching is often practised.
Most yams can be grown successfully at low or medium elevations, not above 900 m.
Day length greater than 12 hours favour the growth of the vine, tuber development occurs short-day conditions, 10 hours of daylight.
3.0 Leaves
See diagram 63.3: Yam leaves.
See diagram: Dioscorea Leaves.
1. The large round leaves on strong petioles vary in the shape of the leaf, the length and thickness of the petiole.
The leaves are in pairs opposite one another, or are alternate (first one side then the other).
Leaves have strongly marked reticulate veining (unusual for a monocotyledon), sometimes lobed or palmate.
Leaves are usually green, but young leaves may be purple.
Petiole base of the long petiole where attached to stem is sometimes modified to be winged or spined, usually twist so leaf is towards the light.
2. Activity: Use leaves of 2 or 3 different shapes to name the parts of the leaf and label leaf stalk, veins and lamina.
Trace the shape of two different leaves, the small veins and the parts of the leaves.
4.0 Stems and roots
See diagram 63.4: Yam twining.
1. Yams have weak stems, cannot stand up by themselves, need to have something to climb up to reach the sunlight, circumnavigates to find support by twining.
2. The first adventitious roots form from the base of stem, up to 3 mm thick, grow mainly in top 30 cm of soil as the main feeding roots.
3. Late, thin, short roots form from tuber.
Roots may have spines.
Stems grow for several metres before branching.
4. Activity: Draw a stem that twines to the right and one that twines to the left.
Look at how leaves are arranged on the stem.
Draw the twining of the stems in two kinds of yams.
Look at the arrangement of leaves on the stems and draw one leaf attached to the stem.
Cut across the stem of a winged yam and draw the shape of the stem.
Stems may have leaves in opposite pairs or leaves alternate on the stem.
5.0 Tubers
See diagram 63.5: Yam tubers.
1. Tubers consist mainly of starchy tissues covered by a suberized layer forming a skin.
There is great variation in the size, form and colour of tubers, in their texture, flavour, thickness of skin, and in storage behaviour.
The principal economic species are the Enantiophyllum yams which usually produce one to three tubers, which may be globular, cylindrical or elongated, branched or lobed, weighing from 3 to 15 kg.
The Lasiophyton yams form several medium size tubers, sometimes fused into an irregular clusters.
Asian Combilium yams and the American Macrogynodium yams produce a large number of small spindle-shaped tubers, similar to sweet potatoes.
Tubers are storage organs and often grown to a considerable size, they produce short, fibrous adventitious roots and annual shoots, which are twining, (except in dwarf species), the direction of twining being specific.
Each yam plant may make one large tuber, but some varieties make several smaller tubers.
2. Activity: Observe yam tubers with different shapes.
Note the shape of the whole tuber, the colour of the flesh, the colour just under the skin, the appearance of the outside skin.
Draw some tuber shapes.
The colour of the flesh and the taste of the flesh may be different.
The colour of the flesh just under the skin may vary in different varieties.
6.0 Bulbils
See diagram 63.4: Bulbils.
Many yam species also produce bulbils in the axils of leaves, which may become similar to underground tubers, but smaller.
In a few species, e.g. Dioscorea bulbifera (Opsophyton), the bulbils are the main storage organs.
Bulbils are formed in the axils of the leaves, are like grey to dark brown condensed stems, may be large and tuberous, and can be eaten.
They can sometimes make shoots and roots if a stem cutting is taken with the bulbil.
Bulbils usually form when the plant has been growing for a long time and is near the end of its growth for that year.
Bulbils can be used as planting material.
7.0 Flowers
See diagram: Elephant's foot flowers.
Yams do not usually make flowers.
The male and female flowers are formed on different plants, i.e. yams are dioecious.
Male flowers are small, borne in panicles from leaf axils, 3 sepals, 3 petals, 3-6 stamens.
Female flowers are larger, borne in spikes from leaf axils, 3 sepals, 3 petals, 3 stigmas, inferior ovary has 3 locules each containing 2 ovules, and are pollinated by insects.
Capsules are dry, dehiscent, usually trilocular, with 6 seeds, usually winged for wind dispersal, many of the cultivated forms sterile.
Seeds are dormant for months after plant senescence then germinated to produce small seedlings with no economic significance.
8.0 Planting and growth period
See diagram 63.7: Planting material.
Plant yams at the end of the dry season and before the tubers begin to make long sprouts.
1. After the yams have made tubers, the plants lose some of their leaves.
The tubers have a resting time usually in the dry part of the year that lasts for 3 to 4 months, then the tubers will start to make sprouts.
Plant the tubers before sprouting, because the sprouts may be broken off the tuber during planting.
Early varieties of yams are usually planted in August.
The late varieties are usually planted before the end of September.
2. Planting material may be any of the following:
* Whole tubers.
If the tubers are small, they are not cut, but planted whole.
* Cut pieces of tuber.
The tubers may be cut into three pieces, called heads, middles and tails.
* The heads are the pieces that grow into the best plants.
There are many buds in this part of the tuber.
3. Healing the cuts
After the tubers have been cut into pieces, it is best to leave them for a week so that the cut part can heal and get hard.
To stop the cut pieces of tuber from rotting, put fungicide on the cut surfaces.
4. Yam garden
To prepare the yam garden, dig a long trench and put some compost into it.
Then cover the compost with topsoil and heap it up to make a mound.
Plant the yams in this mound.
Make the mounds about 1 metre apart and plant the yams about 45 cm apart along the ridge with the tops is 5 cm below the surface.
Cover the place where the yam was planted with dead grass to keep the soil moist around the yam piece.
5. Planting
Planting is done with small tubers (seed yams), cuttings off the tubers, setts (all types of vegetative planting material, pre-sprouted tubers, pieces of tuber), or bulbils, but tuber production by vine cuttings is uneconomic.
The best planting material is the small whole tuber and aerial bulbils can be used.
For most food yams, setts cut from the tuber are used, called tops or 'heads' (proximal), middles, and bottoms or 'tails' (distal).
The larger the sets, the earlier and greater is the rate of germination.
The body of the yam may be cut off and the head left in the soil to grow and produce seed yams for propagation, called 'topping' or 'milking'.
Most yams have a definite period of dormancy, but this may be broken by the use of ethylene chlorohydrin, C2H5ClO.
Yams are usually intercropped with maize and vegetables, such as cucurbits, pumpkins, peppers and okra.
6. Planting systems
Three types of planting systems:
* Setts planted on the flat, in areas such as river flood plains, where the soil is deep and soft.
* Setts planted in trenches or holes, just below the soil surface.
* Setts planted on mounds, along the sides of ridges or raised beds, the most widespread method.
Setts are planted deep to avoid drying out of the young shoots and so the head of the sett is also placed downwards.
For most large tuber yams, 10 000-15 000 / ha are used, requiring at least 2.5 t / ha of setts.
7. Mounds
Mounds vary from about 50 cm high and 100 cm wide at the base for planting one sett, to 100 cm high and 300 cm wide at the base for planting three or more setts.
Larger mounds are preferred and the setts can be planted in holes dug in the sides near the natural ground level.
8. Support
Support is necessary for good plants and tuber development, using stakes, trellises, strings attached to horizontal ropes, corn stalk.
Dioscorea alata and Dioscorea esculenta, are adapted to trailing on the ground without support, and other species may yield without support.
9. Time of planting
Planting is normally by hand, then the yams must be kept free from weeds for the first three months of growth.
Where the rains last 8-10 months, planting normally takes place just before or at the beginning of the rains.
Where the rains last less than 8 months early planting, up to 3 months before the rains, can give increase in yield.
The wider the spacing, the lower the yield, common spacings are 1.2 x 1.2 m.
10. Growth period
Most edible yams normally reach maturity 8-11 months after planting, but some first harvest after 5 months.
The growth period ends at the end of the rainy season.
9.0 Recording growth
See diagram 63.8.1: Yam growth.
Keep good records of the growth of the yams.
Make a table to include the dates when yams were planted and shoots first came, how high the plant was each week, how long the plant grows each day.
Choose one planted yam and make notes on its growth.
Put a stake in the ground for the yam to grow up.
Every day for one week make a mark on the stake showing how high the yam has grown.
In the diagram see the growth of one yam.
You will learn more about yams if you keep careful records.
Make a table for keeping records.
Make the first entry of the date yams were planted.
Use growth records to calculate the average daily growth of the yam shoot.
The yam in the diagram grew nearly 8 cm a day in wet weather, but only about 4 cm a day when the weather was dry.
In some countries yams grow very much faster than this.
10.0 Staking
See diagram 63.9: Yam staking.
Supports for yams:
1. Push long pieces of branches, stakes, into the soil near the planting piece.
2. Put pieces of dead bushes on the soil over the planting place so the yams can grow over these bushes.
However, this kind of support is that it is not high enough, and the yam leaves may not get enough sun.
3. Make "A frames", posts or pieces of bamboo or other stems pushed into the soil so they lean towards each other.
They are tied together at the top, and other long pieces are tied across the tops of the A frames to keep them still in the wind.
4. Wire supports.
Plant the yams in ridges, then bury strong posts 120 cm in the hollows between two ridges.
Tie wires to the end posts, at the top and at the bottom, and use wires and pegs to stop the end posts from moving.
11.0 Weeding
It is very important for the yam garden to be weeded, especially in the early stages when the yams are quite small.
Weeds stop the growth of the yams by stealing water, light and plant foods in the soil.
In small gardens, it is best to weed the yams by hand.
Yam foliage should become thick enough to cover the ground and eliminate weeds by shading, especially when the vines are unstaked.
12.0 Fertilizing
Yams planted in very good soil or soil that has just been cleared from bush usually do not need any fertilizers.
If fertilizers are used, sometimes the yams grow much better, but sometimes it is hard to see if the fertilizers have increased yield.
If the soil is poor, or if it has had yams or other crops growing in it before, add fertilizers on the soil.
NPK formulations 11-11-13, 10-10-20, 12-12-18 have been used.
Eight weeks after planting, when the yam shoots are growing up the stakes, then nitrogen fertilizer should be used at the rate of 100 kg per hectare to increase leaf area.
If heavy rains cause leaching of fertilizers then the applications should be split, first application about one month after emergence and second application up to nine weeks later during the tuber bulking period.
Apply fertilizers about 15 cm from the plant in mounds or in a continuous band between ridges if flat planting.
14.0 Harvesting and handling
At the start of the dry season yam plants die back and tubers are ready for single harvesting, from one month before shoot senescence to two months after senescence, but after that, tubers deteriorate in the soil.
Dig around the tuber to loosen it from the soil, lift it still attached to the vine, cut off the vine, taking care to avoid damaging the tubers.
In Dioscorea rotunda and some other species, the tuber is carefully cut below the head and removed, called "topping", leaving the top to grow again and produce another tuber, called double harvesting.
Aerial tubers or bulbils can be plucked by hand from the vine as required.
Yam tubers unaffected by pests, diseases or damage, may be stored at normal temperatures and with good aeration until their natural period of dormancy is broken.
15.0 Storing
See diagram 63.17: Yams sprouting.
Yams keep better than any other root crop and sometimes yams can be stored for 3 or 4 months.
The yams must first be dried by spreading them out under a house, then they can be stored in any cool dry place.
In some countries, farmers keep the yams in a special yam house.
Do not store damaged yams with good yams.
Damaged yams can be eaten by people or pigs.
Store tams until they start to sprout, when they should then be planted.
Sprouts should not get too long and can not be are broken off, because another sprout will not grow from this place.
Stored tubers may lose up to 40% cent of their weight.
Store yams in heaps or in a vertical framework for ventilation or in thatched huts with a raised platform.
Most yams, except of Dioscorea trifida, suffer chilling injury at temperatures below 12oC.
Insect damage to yams during storage is usually not serious, but lesions caused by insects and cuts and bruises during harvesting may allow entry of fungi or bacteria.
Prolongation of dormancy by chemical methods is not usually successful.
16.0 Chemical composition
Dioscorea alata tubers: Moisture 70%, starch 28%, sugars 0.5%, fat 0.1%, crude protein 1-3%, crude fibre 0.5-1.5% vitamin C 6-12 mg per 100g
Dioscorea esculenta has similar percentage compositions, but also is reported to contain vitamins A, B1 and B2.
17.0 Dioscoreaceae, the yam family
18.0 Dioscorea genus
19.0 Dioscorea species
Dioscoreaceae, the yam family
Twining habit, dioecious, pubescent or hairless herbs.
Stems annual, arising from tubers, often prickly especially below.
Leaves alternate or opposite, with petioles, often ovate or heart shape, usually entire, sometimes deeply or shallowly lobed, occasionally compound with 3 to7 leaflets.
Tubers, aerial tubers sometimes occur, arising in leaf axils.
Perianth suberect or spreading, with an oblong 3-locule ovary beneath it.
Styles 3, short.
Ovules 2 per locule.
Stamens 6, all fertile, or 3 reduced to staminodes, inserted at base and shorter than the perianth.
Fruit, capsule rigid, deeply 3-lobed or triangular-ellipsoid, dehiscing into 3 valves.
Seeds up to 2 per locule, variously winged or rarely wingless.
The yam family, monocotyledons, are twining plants.
The stem is not strong so it needs to have something strong that it can twist around to climb up.
There are many different kinds (species), of yams and each kind of yam has many varieties.
18.0 Dioscorea genus
Dioscorea is a largest genus of the Dioscoreaceae with over 600 species with subterranean tubers or rhizomes.
The basic chromosome number, n =10, but also polyploidy in some varieties.
Genus Dioscorea is divided into a number of taxonomic sections.
About 60 species of Dioscorea have been used for food, but most are of little importance.
Dioscorea species lived long ago southern Asia, Africa and South America and domestication of the different species appears to have been done by aboriginal man so that wild yams and domesticated cultivars occur throughout the tropical and subtropical world.
19.0 Dioscorea species
21.0 Dioscorea alata, water yam
Dioscorea astericus, South-Central Africa
Dioscorea batatas (D. polystachya), Chinese yam, cinnamon vine, China
22.0 Dioscorea bulbifera, potato yam
Dioscorea cayenensis, yellow guinea yam, twelve-months yam, Lagos yam, vegetable, Africa
White yam, (Dioscorea cayennensis subsp. rotundata), yam chips, steroid saponin disogenin, West Africa, Dioscoreaceae
Dioscorea composita (D. tepinapensis), yam, barbasco, Mexico
Dioscorea dregeana, wild yam, similar chemical properties to D. villosa, southern Africa, Dioscoreaceae
23.0 Dioscorea dumetorum, bitter yam
24.0 Dioscorea esculenta, lesser yam
Dioscorea floridana, Dioscorea villosa subspecies, Florida yam
Spear-leaved Dioscorea, (Dioscorea hastifolia), warrine, warram, warrany, native yam, climbing vine with slender twining stems, up to 3 m, separate male and female plants, spearheaded leaves, (Latin hasta spear), yellow flowers, green triangular fruit, long white edible tubers which persists during summer when the rest of the plant has dies, Australian native food, lives in woodland and shrub communities, Australia, Dioscoreaceae
Dioscorea hastifolia, Warrine, Tucker Bush
Dioscorea hispida, intoxicating yam, grows wild in India, Philippines, Papua New Guinea, need to detoxify before eating
25.0 Japanese mountain yam, (Dioscorea japonica), East Asian mountain yam
Dioscorea nummularia, "Palai", very spiny stem and climbs from left to right, vegetable, Western Samoa
Dioscorea opposita, Chinese yam, bulbils, frost-tolerant, long thin vertical tubers, vegetable, yam flour, tryptophan, serine, China, Japan
Dioscorea pentophylla, five leaf yam, leaf with 3-5 lobes, spiny stem and climbs from right to left, vegetable
Dioscorea praehensilis, bush yam, forest yam
28.0 Dioscorea rotundata, white yam
Dioscorea strydomiana
29.0 Dioscorea transversa, pencil yam
30.0 Dioscorea trifida, cush-cush yam
31.0 Dioscorea villosa, yellow yam
Dioscorea wallichiii, kruo
See diagram: Dioscorea wallichii leaves.
Dioscorea strydomiana
Wild yam, (Dioscorea villosa), herbal medicine, root saponins for different disorders. but no evidence for anti-cancer function, North America, Dioscoreaceae
Purple yam, (Dioscorea alata)
(Dioscorea alata), purple yam, greater yam, winged yam, water yam, "common yam", white yam, Guyana arrowroot, ten months yam, name-de-Agua, greater Asiatic yam, Lisbon yam, white Manila yam, common yam, "yam" (Enantiophyllum group), Dioscoreaceae.
It is a perennial, deciduous, hardy plant, a large climber with winged stems from a large single violet-purple tuber, climbs from left to right, herbal medicine, culinary uses, large tuber in ground and bulblets that form on vine, eaten cooked, most grown cultivated yam, Southeast Asia,
See diagram 63.2: Winged Yam.
Climber, large, hairless, quadrangular winged stems, up to 15 m in height, twining anticlockwise to the right, large single tuber.
Stem, square, twining, four or more thin pieces of stem or "wings" at each corner, also on petioles, forming auricles at junction, usually green, but purple-red if anthocyanins present, twines to the right, no spines
Leaves, opposite or the lower alternate, hairless, petiole up to 10 cm, long, blade ovate with widely heart-shaped base apically narrowed to an acute acumen, up to 13 cm long and 10 cm broad.
Tubers, usually single, shape varies in different varieties, generally cylindrical, but may be long and serpentine to almost globular, and are often branched or lobed, or even flattened and fan-shaped.
Tuber replaced annually, normally cylindrica,, up to 6 cm in diameter, descending vertically, but in some cultivars very diversely shaped, branched or expanded above, sometimes with lobes curved or spreading horizontally.
Tubers, aerial tubers subglobose or irregularly and narrowly ovoid, up to 12 cm long.
Tuber weight is usually 5-10 kg though special cultivation can produce giant tubers of 60 kg or more.
Tuber flesh white to purple, loose texture.
Inflorescence hairless.
Flowers, rarely, usually reproducing vegetatively, bulbils developed in some forms.
Flowers male, 2 in the leaf axils or forming axillary terminal panicles in the axils of bracts, spreading, axis zigzag, with the sessile flowers directed forwards and outwards, perianth subglobose, not opening widely, 1.5 mm across.
Flowers female 1 per leaf-axil, up to 35 cm long, perianth triangular-subglobose, 5 mm across.
Ovary hairless.
Perianth triangular, 5 mm across.
Fruits, capsule, up to 3.5 cm in diameter, hairless.
Seeds winged all round, few cultivars produce fertile seed and most are completely sterile.
Bulbils are sometimes formed in leaf axils.
Dioscorea alata is not known in the wild state, but is cultivated throughout the tropical world.
Native species originated in the Assam-Burma region and now in the eastern Caribbean and in the Pacific is the most popular yam.
Cultivation conditions:
Rainfall for optimum yields is 150 cm evenly distributed over 6-7 months.
Dioscorea alata will tolerate poorer soils than most other species of yam, but it responds to fertilizers about 10 weeks after planting, when the plant is completing its dependence upon the parent sets.
Cultivate it at low or medium elevations with day length of less than 12 hours required for suberisation.
Plant setts with two or three sprouts, or small whole tubers or small pieces of approximately 100 g cut from stored yams and dried for several hours before planting.
Plant setts by hand on mounds or ridges, in holes 5-10 cm deep.
Keep the crop weed free for the first 3 months.
If not staked, complete ground cover occurs 3-4 months from sprouting and weeds are eliminated by shading.
Field spacing if monoculture, plant on ridges 1.7 m apart, with 0.75-1 m, so that vines need not be staked, but closer spacing can be used in areas of low rainfall.
Seeding rates vary from 650 kg / ha to 1 400 kg / ha of setts.
Maturity is normally reached after a growth period of 9-10 months, but some early varieties can be harvested at 6 months.
Harvesting is done manually by forking, but the size and irregular shape of the tubers causes up to 25% damaged tubers.
Storage under tropical conditions is normally for 4-6 months.
If tubers are sound, storage is terminated by the breaking of dormancy, but if sprouts are removed, storage may extend to 8 months.
Tubers weigh 5-10 kg, and are usually cylindrical, but extremely variable.
Tuber skin are thick and dark and the flesh may be white, pink or purple.
Dioscorea alata tubers have a definite period of dormancy of 2-4 months, which may be broken with ethylene chlorhydrin.
Average farm yields have been reported as Malaysia 42.5 t / ha, Trinidad 46.8 t / ha, St. Vincent 57.5 t / ha, Fiji 25.2 t / ha, Barbados 5-6 t / ha
Used mainly as a vegetable, similarly to the potato, French fries and chips, processed products, e.g. yam flakes, yam powder, and coloured cultivars are used for ice cream colouring and flavouring.
Damaged tubers may be fed to pigs.
Analysis of tubers: water 65-73%, protein 1.1-2.8%, fat 0.03-0.3%, carbohydrate 22-29%, fibre 0.7-1.4%, ash 0.7-2.0%. Ascorbic acid 4.9 to 8.2 mg / 100 g.
South Pacific cultivars contain 6 mg / 100 g of carotene.
Dioscorea alata var. atropurpurea contains cyanidin glycosides.
Prepare yam flakes by hand peeling, slicing into 1 cm thick pieces, cooking in water or steam until soft, push through a potato-rice mix to a slurry and dehydrate on drum dryer.
Prepare yam powder, by cooking unpeeled tubers, then peeling, grating and drying at 50oC to 10% moisture.
Dioscorea alata is the world's most popular yam after the Dioscorea rotundata / cayenensis complex, Australian native food, Dioscoreaceae.
Air yam, (Dioscorea bulbifera)
(Dioscorea bulbifera), air yam, aerial yam, wild yam, bulbil yam, potato yam, air potato, round yam, aerial potato, "wild yam", perennial, deciduous is a climber with round stems and has edible aerial tubers (bulbils), in the leaf axils, Dioscoreaceae.
Potato and root is peeled and diced, soaked well, then cooked or roasted.
Climber with round stems bearing edible aerial tubers called bulbils in the leaf axils.
Tubers, many tubers up in the stem above the ground and some tubers under the ground.
Tubers, aerial, globe-like or angular, up to 7 cm in diameter, brown.
Plant hairless or some inconspicuous hairs at the beneath the base of the leaf blades.
Stem, twining, up to 12 m long, twists to the left.
Leaves as in 3, D. asteriscus.
Inflorescence, flowers are sessile and directed downwards, towards apex of inflorescence.
Male, perianth lobes of flower not spreading, tepals lanceolate, up to 2 mm long.
Flowers Perianth lobes of ? flower directed towards apex of inflorescence, up to 2 mm long.
Fruits, capsule and seeds similar to those of D. asteriscus, but capsule only 2 cm long and 1.2 cm in diameter.
Australian native food, Dioscoreaceae.

Djitama, (Dioscorea bulbifera var. rotunda), bush yam, round root, (Yirritja), mankinjdjek, “cheeky yam”, “wild potato”, climbing plant, large perennial tuber, in monsoon vine forests. toxic unless cooked correctly, i.e: boiled, grated, and left overnight, Australian native food, Dioscoreaceae, Australia
Bitter yam, (Dioscorea dumetorum)
Dioscorea dumetorum, bitter yam, cluster yam, trifoliate yam, soft texture, wild forms poisonous, Dioscorea hispida is similar to D dumetorum, West Africa.
Dioscorea elephantipes, elephant's foot, Hottentot bread, vegetable, Dioscoreaceae
Lesser yam , (Dioscorea esculenta)
(Dioscorea esculenta), lesser yam, Asiatic yam, Lesser Asiatic yam, Chinese yam, Dioscoreaceae.
It is a small climber producing more than 1 small tuber, which climbs from right to left, round thin stem with big spines.
It has a round spiny stem that always twists to the left, i.e. climbs from right to left.
The tubers are small and rounded with groups of them to each plant.
The leaves are alternate.
Dioscorea esculenta is domesticated as a staple food in southern China and today it is widely distributed throughout the tropics.
The delicate and perishable Dioscorea esculenta tubers are normally traded only within a community or village.
It grows best at high temperatures with best yields with high rainfall (175 cm), but dry periods more than 2 months may cause death.
Good drainage and high organic matter improves growth, but sandy soils are not suitable, and heavy clays can cause misshapen tubers.
Dioscorea esculenta is a vine, climbing up to 3 m, with thin stems, 1-3 mm in diameter, which are smooth to prickly, twine clockwise, (to the left), in climbing, no bulbils.
Leaves are alternate, almost round, but pointed at the tips and deeply lobed at the base, finely hairy, 10 cm in diameter, with petioles thickened at the base with four sharp prickles.
Flowers are rare in most cultivars.
Roots are fibrous, often prickly.
Tubers, numerous (up to 40), stalked edible tubers independent of each other.
Tubers are swollen ends of stolons from crown of the plant, with each stolon bearings only one tuber.
Tubers are like long and narrow, 5-20 per plant, but Papua New Guinea cultivars produce very large tubers weighing up to 3 kg.
Fertilizer recommended is 400 kg / ha of an 11:11:33 NPK mixture applied 6-8 weeks after planting.
Plant with small whole tubers of 55-85 g weight, about 10 cm below the surface, in mounds 90 x 90 cm, ridges 90 to 130 cm 1 m apart using 2, 000 kg / ha of seed tubers, and staking may increase the yield.
Growth period when crop is mature at Fiji 6-7 months, Malaysia 8-9 month, West Indies 10 months.
Tubers are thin skinned and succulent, and easily damaged during harvesting lifting is normally done by hand.
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 before fungi cause rotting.
Uninjured tubers can be stored for 4 months in well-ventilated conditions at tropical temperatures.
Storage and sprouting causes loss of dry matter, shrivelling, increased sweetness, less palatability.
Tubers are thin skinned with yellow flesh, and appear pale yellow before the skin is removed.
Tuber surface is smooth, except for adventitious roots and damage depressions like the eyes of a potato.
Tuber flesh is floury to succulent, crisp, with little fibre and a bland, but sweet flavour.
Tuber yields in pure stands can be more than 25 t / ha.
Tubers are boiled in their skins or after peeling for up to 10 minutes, or baked in their skins, or fried as slices or as chips (French fries).
Tuber composition: water 67-81%, protein 1.3-1.9%, fat 0.04-0.3%, carbohydrate 17-25% (mainly starch with sugars 7-11%).
Dioscoreaceae
Japanese mountain yam, (Dioscorea japonica)
Dioscorea japonica, Japanese mountain yam, , East Asian mountain yam "wild yam, shan yao | Paeonol | Japan, China, Korea.
Dioscoreaceae.
Pacific yam, (Dioscorea nummularia)
(Dioscorea nummularia), has a very spiny stem and climbs from left to right, widely cultivated edible root, grows in Pacific Islands, Dioscoreaceae.
Fiveleaf yam, (Dioscorea pentophylla)
(Dioscorea pentophylla), has leaf with 3.5 lobes, spiny stem, and climbs from right to left, Dioscoreaceae.
White yam, (Dioscorea rotundata)
Dioscorea rotundata, white yam, white Guinea yam, has greatest production in the world.
Stem, wingless, circular cross section, several metres long, spiny, white bloom, hairless, twines to the right.
Leaves, heart- shaped, pointed tip, opposite arrangement.
Tuber, cylindrical, brown, smooth skin, white flesh.
Important food source, yam chips, steroid saponin disogenin, origin West Africa.
Dioscorea rotundata-cayanensis, African yam
Native yam, (Dioscorea transversa)
(Dioscorea transversa), native yam, pencil yam, long yam, long yam midiny, , stems twine anticlockwise, heart-shaped shiny leaves with 5-7 prominent veins, rounded seed pods are green or pink before drying, slender edible tubers, some varieties have bulbils and tubers, Australian native food, tubers eaten raw, Australia, Dioscoreaceae.

Karrbarda, (Dioscorea transversa), long yam, similar to “cheeky yam” (Dioscorea bulbifera var. rotunda), Australian native food, Dioscoreaceae, Australia
Indian yam, (Dioscorea trifida)
Dioscorea trifida, Indian yam,, cush-cush yam, aja, yampi, mapuey, equatorial growth, thin skin tubers, good taste, South America. Dioscoreaceae.
Wild yam, (Dioscorea villosa)
See diagram: Dioscorea villosa
Dioscorea villosa is a yellow yam, "wild yam", perennial, deciduous, climber, herbal medicine, pre- and post- menopausal women, skin cream, contains | Diosgenin | herbal medicine, act on the contractile force of cardiac muscle, cardiotonics, antiarrhythmics root saponins used for different disorders, but no evidence for anti-cancer function, North America
Wild yam (scientific name Dioscorea villosa) is a plant native to North America, Mexico, and part of Asia.
The root and bulb of the plant have long been used for traditional medicine.
In recent years, the phytosteroid diosgenin has been identified as the key medicinal component.
Phytosteroids are plant-based steroids that are similar to the steroids produced in the human body.
Diosgenin from wild yam is used to make a variety of steroids for medical use, including progesterone, cortisone, and dehydroepiandrosterone (DHEA).
Wild yam is also known as American yam, Chinese yam, Colic root, Devil's bone, Four-leaf yam, Mexican yam, Rheumatism root, Shan Yao (Chinese medicine), Yuma.
Wild yam is sold in the United States as a dietary supplement to treat hormone imbalances, e.g. morning sickness, premenstrual syndrome (PMS), hot flashes, menstrual cramps, vaginal dryness, low libido.
Wild yam is NOT a "natural estrogen" or a "phytoestrogen".
The National Institutes of Health has stated that there is "insufficient evidence" that wild yam can aid the treatment of PMS, infertility, painful periods, low libido, or vaginal dryness.
Wild yam was called "rheumatism root" in the 19th century, because it was used to treat joint and muscle pain, which were called "rheumatism."
Wild yam may aid in the treatment of the chronic inflammation of osteoarthritis ("wear-and-tear arthritis").
Wild yam supplements are sold in capsules, tablets and ointments.
Dried herb sold as root and root powder.
20.0 Global Initiative underway to preserve yam biodiversity, 16 September 2010
Crop Trust
World yam collection in Nigeria provides rescue for African yam diversity in an initiative to conserve critical crop collections backed by the Global Crop Diversity Trust.
Farmers and crop scientists world wide are engaged in an ambitious new effort to add 3, 000 yam samples to international gene banks to save the diversity of a crop consumed by 60 million people on a daily basis in Africa alone, according to an announcement today from the Global Crop Diversity Trust.
In most countries of the African yam belt, many potentially important yam varieties are preserved only in fields, where they are in danger of being picked off by pests or diseases and more common disasters like fire or flooding.
Yam varieties gathered from West and Central African countries through the project are being sent to the International Institute for Tropical Agriculture (IITA), in Ibadan, Nigeria, where tissue samples of the crop will eventually be frozen at ultra low temperatures in liquid nitrogen, a technique known as cryoconservation, which offers the most secure form of long term storage currently available.
Most of the world's crops can be conserved over long periods simply by drying the seeds and storing them under cold, dry conditions.
However, most crops, including yams, cannot be stored so easily and must be conserved as vegetative material in tissue culture.
Farmers in West Africa's "yam belt", which includes the countries of Nigeria, Côte d'Ivoire, Ghana, Benin and Togo, produce more than 90% of the world's yams.
The project, however, will also include yam varieties collected in the Philippines, Vietnam, Costa Rica, the Caribbean and several Pacific nations.
It is the first world wide effort to conserve yam species and cultivars.
The project is funded with support from the UN Foundation and the Bill and Melinda Gates Foundation.
According to farmers' reports, many traditional varieties are disappearing in their production zones, because of high susceptibility to pests and diseases, poor soil, soil moisture content, weeds and drought, which make them less productive or more costly to grow compared to other crops such as cassava.
13.0 Pests and diseases
Pests
Absence of host plants and a fallow period are recommended, and care must be taken to avoid planting infected material.
Only clean and healthy material should be planted, (the dusting treatment recommended for yam beetles should be used), and if aerial parts of the plant are affected, spraying with malathion or (malathion + an oil emulsion) is recommended.
Dusting the plant setts with 2 per cent aldrin or 0.5 per cent gamma-HCH will normally prevent attack.
Diseases
* Do not keep the tubers from diseased plants to use as planting material.
* Do not continue planting yams in this soil, but use a rotation, plant another crop next year in this place.
* Use varieties of yams that do not get this disease.
* Plant the yams at a time when there may not be many heavy rain storms.
* The danger of attack is less once the yams have grown higher, because the older leaves do not get the disease so easily.
13.1 Yam anthracnose disease
Yam anthracnose and dieback disease, dieback disease, lightening disease of yams, is caused by the fungus Colletotrichum gloeosporioides, Glomerella cingulata,), in association with Botryodiplodia theobromae, Lasiodiplodia species and Fusarium species.
It overwinters in plant debris and it occurs worldwide.
Water from heavy rains splashes spores of the disease from the soil up onto the young leaves.
The disease appears as small, round, brown dead spots on the leaves, with yellow colour on the outside of the spots.
The dead spots grow in size until the whole leaf has died.
Then the whole branch with its stem and leaves may die back, spreads rapidly, producing black necrotic lesions on leaves and stems, withered leaves and scorched appearance.
The disease can kill the plant by attacking the terminal bud.
The most effective method of controlling the disease is to practice crop hygiene and to plant yam varieties that are resistant to anthracnose, e.g. TDA 291 or TDA 297.
Spraying at 10 day intervals with zineb or ferbam or mancozeb (maneb + zineb), can be effective.
13.2 Yam leaf spot
Yam leaf spot is caused by various species of Cercospora, Colleotrichum, Phyllosticta, Guignardia dioscoreae in the Pacific Islands especially on Dioscorea esculenta but also on the other common yams. The brown spots, up to 10 mm diameter, with a darker border, hardly affect the yield of tubers.
13.3 Yam mosaic disease, (YMV)
Yam mosaic disease, Yam mosaic polyvirus
The common symptoms are infected leaves show yellow and green patterns (called mosaics) between the veins or may show a narrow green strips bordering the veins, (called vein banding).
If the disease is severe, the leaves become long, thin and strap shape, (called shoe-string symptom) and whole plant become stunted.
Plant may produce few small tubers with less starch content. Some plants may recover from the virus infection soon after first symptom but virus may survive in plant and reduce the vigour. The virus is transmitted by aphids and tubers/setts.
It may cause up to 40% loss in yield.
Yam mosaic virus is always associated with yam mild mosaic virus, yam badnaviruses and cucumber mosaic virus in Africa making this disease more complex.
Disease is reported in West Africa , South America and Caribbean, but not Oceania.
Use healthy and disease free tubers or setts for planting.
Select healthy and large tubers for planting instead of small tubers.
Keep fields free from weeds.
Collect crop debris and destroy them.
Virus diseases, e.g. internal brown spot, of the mosaic type causes leaf mottling, stunting production of numerous basal shoots gives plant a bushy appearance.
Virus infection of leaves is common and although usually tolerated, virus-free planting material is recommended.
Yam mild mosaic virus disease, (YMMV) Bacterial crown gall, caused by Agrobacterium tumefaciens.
Control involves sanitation by removal of crop debris, and fungicide treatment, using maneb, zineb and mancozeb.
13.4 Yam nematodes
The yam lesion nematode, root knot nematode, Pratylenchus coffeae also attacks yams.
The yam tuber dry rot nematode, Scutellonema bradys, causes dry rot of the tubers.
The infected tubers show dry rot of 1 to 2 cm.
Initially this dry rot is of cream and light yellow lesions appear just below the outer skin without any external symptom.
With progress in disease lesion spreads deeper (maximum up to 2 cm).
At later stage the rot become light and dark brown to black in colour and tubers may show external cracks.
Entry of fungus through this wounds causes further decay of tubers in storage.
There is no above ground symptom with yam nematode infestation.
Some time the infected tubers may not show external cracking which make it difficult to diagnose.
In that case, scrap out the external layer of tuber to check the disease incidence.
Use disease free tubers/setts for planting.
Treating tubers with hot water for 40 min at 50-55 C before sowing and after harvest to reduce disease both in field and storage.
In Africa, smearing tubers with wood ash or cow dung shows reduced nematode infection in field.
Follow crop rotation with non host or antagonist crops like ground nut, sorghum, maize, and chill pepper.
Other nematodes
The root knot nematode, Meloidogyne incognita, causes galled tubers with abnormal roots, so the infected plants are stunted with poor growth and the leaves turn yellow
The edible portion of tubers is reduced which reduces the market value. Deep ploughing may expose and kill nematode, otherwise use crop rotation with crops, which are not hosts, e.g. peanut or maize.
13.5 Yam rusts
Yam rusts are seen as white spots on leaves of yams in the Pacific islands region, possibly caused by Goplana species, but not decreasing the yield of tubers.
13.6 Yam scale
The white Scale insects, Aspidiella hartii, cause the leaves and tubers to be covered with small white scales from field to storage.
Even though it won't effect yield much, sometimes the foliage shows poor growth and tubers may show delay in germination or even stopped.
Severe infestation may cause tubers to shrivel.
Use scale-free planting material.
13.7 Tuber rots
Tuber rots caused by fungi may be rapid, destroying a tuber within a week.
Soft rots are caused by Penicillium species, Fusarium species, Botryodiplodia species.
Dry rots are caused by Rosellinia species, Sphaerostilbe species.
These rots may also affect the growing plant when the setts consist of cut pieces of tuber, but may be controlled by painting of the cut surfaces with lime wash or Bordeaux mixture, or coating with wood ash.
Rotting during storage may be minimized by treating cut or bruised surfaces of the harvested tubers in the same manner.
The fungus, Rhizoctonia species gets into the roots of tubers of the yams and rots them.
Yam weevil, Palaeopus costicollis, causes similar damage.
Termite, Amitermes evancifer, attacks yam tubers in Africa.
13.9 Yam beetles
See diagram 63.12: Yam beetle, Collar rot, Leaf spot (dieback).
Yam beetles start as a curled grub that changes into a beetle, may spoil the tubers by eating holes into them, then the tuber rots.
Greater yam beetle (Heteroligus meles), lesser yam beetle (H. appius), Heteronychus licas, Prionoryctes rufopiceus, P. caniculus and Lilioceris spp, attack tuber setts and may prevent sprouting.
Eggs are laid in swampy ground, adult beetles fly to the yams early in the rainy season, burrow downwards from the stems to the tubers, leaving holes in the body of the tuber which are unsightly and may lead to rotting during storage.
Dusting of planting setts with insecticides has not been a successful control.
Prevent yam beetles by washing the planting material in lead arsenate (POISON!), and Bordeaux mixture.
Also prevent yam beetles by rotation, do not plant yams twice in the same soil, but use crops for the next year.
Yam weevil, Palaeopus costicollis, causes similar damage.
Termite, Amitermes evancifer, attacks yam tubers in Africa.
13.10 Mealybugs
The mealybugs Dysmicoccus brevipes, Planococcus citri, Planococcus dioscorea, Geococcus coffeae, Phenacoccus gossypii, feed on yams and cause shrivelling of the stored tubers.
Conserve natural enemies of mealybugs, e.g. parasitic wasps, ladybird beetles, by less use of insecticides.
Avoid heaping of tubers to allow light and air through them.
Place yam farms away from previously infested fields to avoid carrying mealybugs into storage.
Practice crop and land rotation with cereals, e.g. maize, sorghum, millet, to reduce field infestation.
Mealybugs are a problem in prolonged storage of yams, so monitor mealybug populations after 3 months of storage.
Mealybugs prefer warm humid conditions and spread by crawling between tubers, where they suck the sap from the yam tissues.
Mealybugs look like oval-shaped flightless bugs that have cottony white powdery appearance, so look for white powdery mass on the surfaces of tubers.
Spray harvested tubers or yam plants with an high pressure stream of water to knock off the mealybugs.
Spray potassium soap solution or add liquid soap in a sprayer or spray neem seed extract or horticultural oils.
They may cause sooty mould due to fungal colonization of sugary honeydew excreted by the mealybugs.
Ants may protect them for their sugary honeydew secretions.
Prune out heavily infested branches.
The Philippine mango mealybug, Rastrococcus spinosus is a declared pest in Western Australia.
13.11 Butterfly pests of yams
Common yamfly (Loxura atymnus) feeds on Dioscorea pentaphylla, Asia
Yam hawk moth (Theretra nessus) feeds on Dioscorea bulbifera, Asia
Large snow flat, Suffused snow flat (Tagiades gana), feeds on Dioscorea pyrifolia and Dioscorea orbiculata, Asia
Preface
Before teaching this project, discuss the content of the lessons with an agriculture extension officer and get advice on planting material, planting distances, site for planting, approved mulch, composting, and control of pests and diseases.
Use only the procedures, agricultural chemicals and insecticides recommended by an agriculture extension officer.
If you cannot control insects by hand-picking, ask the Ministry of Agriculture to recommend a chemical spray.
All insect sprays are dangerous.
Show the students how to use them safely.
Do not get the spray onto your hands.
Do not breathe in the spray.
Wash your hands well after using spray.
Keep the spray container in a safe place where students cannot get it.
Spray on a day of no wind, but if you must spray when there is a wind, spray down wind.
Make sure the spray does not blow on other people.