School Science Lessons
Please send comments to: J.Elfick@uq.edu.au
Table of contents
1.0 Importance and distribution
2.0 Climate and cultivation conditions
4.0 Stems and roots
7.0 Planting and growth period
8.0 Record growth
16.0 Harvesting and handling
17.1 Chemical composition
18.0 Dioscoreaceae, the yam family
19.0 Dioscorea genus
20.0 Yam species
21.0 Dioscorea alata, greater yam
22.0 Dioscorea bulbifera, potato yam
24.4 Dioscorea dumetorum, bitter yam
23.0 Dioscorea esculenta, lesser yam
24.1 Dioscorea japonica, Japanese mountain yam
20.1 Dioscorea rotundata, white yam
24.2 Dioscorea transversa, pencil yam
24.3 Dioscorea trifida, cush-cush yam
24.0 Dioscorea villosa, yellow yam
25.0 Global Initiative underway to preserve yam biodiversity
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.
2.0 Climate and cultivation conditions
Edible yams cannot withstand frost, except Dioscorea japonica and Dioscora 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.
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 another),.
Leaves have strongly marked reticulate veining (unusual for a monocotyledon),
sometimes lobed or palmate.
Leaves 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
2. 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.
Stems grow for several metres before branching.
2. 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.
3. 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 .
4. Late, thin, short roots form from tuber.
Roots may have spines.
See diagram 63.5: Yam tubers
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
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
Teach this lesson in the dry season after the yams have been dug.
Use yam tubers of 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.
Bring different shapes of tubers into the classroom.
Draw some tuber shapes.
The tubers of yams are of different 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.
Each yam plant may make one large tuber, but some varieties make several
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.
Form in the axils of the leaves, are like grey to dark brown condensed
stems, may be large and tuberous, and can be eaten like tubers.
Can sometimes make shoots and roots if a stem cutting is taken with the
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.
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
Male flowers are small, borne in panicles from leaf axils, 3 sepals,
3 petals, 3-6 stamens.
Female flowers, larger, borne in spikes from leaf axils, 3 sepals, 3
petals, 3 stigmas, inferior ovary has 3 locules each containing 2 ovules
Pollination by insects.
Capsules, dry, dehiscent, usually trilocular, with 6 seeds, usually
winged for wind dispersal, many of the cultivated forms sterile.
Seed dormant for months after plant senescence then germinated to produce
small seedling with no economic significance.
7.0 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.
Then 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.
2.0 Planting material may be any of the following:
2.1 Whole tubers.
If the tubers are small, they are not cut, but planted whole.
2.2 Cut pieces of tuber.
The tubers may be cut into three pieces, heads, middles and tails.
2.3 The heads are the pieces that grow into the best plants.
There are many buds in this part of the tuber.
2.4 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.
3. To stop the cut pieces of tuber from rotting put fungicide on to
the cut surface.
4. 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.
Planting is done with small tubers (seed yams),
cuttings off the tubers, setts (all types of vegetative planting material,
tubers, pieces of tuber), or bulbils, but tuber production by vine cuttings
Best planting material is the small whole tuber and aerial bulbils can
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
Most yams have a definite period of dormancy, but this may be broken
by the use of ethylene chlorohydrin,
Yams are usually intercropped with maize and vegetables, such as cucurbits,
pumpkins, peppers and okra.
Three types of planting systems:
1. setts planted on the flat, in areas such as river flood plains, where
the soil is deep and soft
2. setts planted in trenches or holes, just below the soil surface.
3. setts planted on mounds, along the sides of ridges or raised beds,
the most widespread method.
Mounds can 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.
Setts are planted deep to avoid drying out of the young shoots and so
the head of the sett is also placed downwards.
Support is necessary for good plant and tuber development, using stakes,
trellises, strings attached to horizontal ropes, corn stalk
However, Dioscorea alata and Dioscorea esculenta, are adapted to trailing
on the ground without support, and other species may also
give satisfactory yields without support.
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.
Yam foliage should become thick enough to cover the ground and eliminate
weeds by shading, especially when the vines are unstaked.
For most large tuber yams, 10 000-15 000 / ha are used, requiring at
least 2.5 t / ha of setts.
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.
8.0 Record growth
See diagram 63.8.1: Yam growth
Keep good records of the growth of the yams.
Make a table to include the date the yams were planted, the date when
the shoots first came, how high the plant was each week
how many centimetres 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.
1. You will learn more about yams if you keep careful records.
2. Make a table for keeping records.
3. Make the first entry of the date yams were planted.
Use growth records to calculate the average daily growth of the yam
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.
See diagram 63.9: Yam staking
Supports for yams:
1. Push long pieces of branches, stakes, into the soil near the planting
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.
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.
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.
See diagram 63.12: Yam beetle, Collar rot, Leaf
1. Do not keep the tubers from diseased plants to use as planting material.
2. Do not continue planting yams in this soil, but use a rotation, plant
another crop next year in this place.
3. Use varieties of yams that do not get this disease.
4. Plant the yams at a time when there may not be many heavy rain storms.
5. The danger of attack is less once the yams have grown higher, because
the older leaves do not get the disease so easily.
12.1 Leaf spot, caused by various species of Cercospora,
Colleotrichum, and Phyllosticta.
12.2 Yam anthracnose, dieback disease, caused by (Glomerella cingulata
/ Colletotrichum gloeosporioides), Colletrotrichum in
association with Botryodiplodia theobromae
Lasiodiplodia species and Fusarium species
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
The disease can kill the plant by attacking the terminal bud.
This is a bad disease because it cannot be cured, and there are not many
things we can do to stop it.
Spraying at 10 day intervals with zineb or ferbam or mancozeb (maneb + zineb), can be effective.
Bacterial crown gall, caused by Agrobacterium tumefaciens
Control involves sanitation by removal of crop debris, and fungicide
treatment, using maneb, zineb and mancozeb.
12.3 Tuber rots caused by fungi may be rapid, destroying
a tuber within a week.
Soft rots caused by Penicillium species, Fusarium species, Botryodiplodia
Dry rots caused by Rosellinia species, Sphaerostilbe species
These rots may also affect the growing plant when the setts consist of
cut pieces of tuber, but are controlled by simple measures such
as the 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.
Another fungus disease of yams is caused by a fungus, Rhizoctonia
This fungus gets into the roots of tubers of the yams and rots them.
12.4 Collar rot, lesions between stem and root form a "collar" of brown
marks low down on the stems, occurs in seedlings growing in
infected soil and can be controlled only by the use healthy big tubers
as planting material.
There may be signs of the disease low down on the stem near the soil.
Dark coloured spots on this part of the stem are called collar rot".
This fungus is carried by the tuber.
If there are any marks of this disease on the leaves, they are on one
side of the leaf only.
The most important way of stopping this disease is not to use tubers
from diseased plants as planting material.
The big tubers are probably good to use for planting, but eat the small
tubers that may have this disease.
12.5 Virus diseases, e.g. internal brown spot,
of the mosaic type causing leaf mottling, stunting production of numerous
gives plant a bushy appearance.
Virus infection of leaves is common and although usually tolerated, virus-free
planting material is recommended.
See diagram 63.12: Yam beetle, Collar rot, Leaf
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 (again the dusting
treatment recommended for yam beetles should be used), and
if aerial parts of the plant are affected, spraying with malathion or
malathion plus 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.
13.1 Yam beetles start as a curled grub that changes
into a beetle, may spoil the tubers by eating holes into them, then the
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
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.
13.2 Yam scale, Aspidiella hartii, attacks stored
yams and young vines, so use scale-free planting material.
Mealy bugs, Geococcus coffeae, Phenacoccus gossypii, Planococcus citri,
P. dioscoreae, causes shrivelling of stored tubers.
13.3 Yam nematode, Scutellonema bradys, causes 'dry
rot' of the tubers.
Root knot nematodes, Meloidogyne spp, and Pratylenchus coffeae attack
13.4 Butterflies, Yam fly (Loxura atymnus), Yam
hawk moth (Theretra nessus), and Large Snow Flat (Tagiades gana),., feed
leaves, then attack the stems.
16.0 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
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.
See diagram 63.17: Yams sprouting
Yams keep better than any other root crop.
Sometimes yams can be stored for 3 or 4 months.
1. Yams can be stored better than any other root crop.
2. The yams must first be dried by spreading them out under a house.
3. Then they can be stored in any cool dry place.
In some islands the people keep the yams in a special yam house.
4. Do not put damaged yams with the others.
They should be eaten.
5. Yams can be stored until they start to sprout, Then they should be
It is bad if the sprouts get too long and are broken off.
Another sprout will not grow from this place.
Stored tubers may lose up to 40% cent of their weight.
Yams may be stored 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
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.
17.1 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.
18.0 Dioscoreaceae, the yam
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
main nerves of entire leaves conspicuous, digitate, tertiary nervation
Tubers, aerial tubers sometimes occur, arising in leaf axils.
Perianth suberect or spreading, with an oblong 3-locule ovary immediately
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.
19.0 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.
Food yams are grouped as follows:
1. Enantiophyllum group.
Dioscorea alata greater yam
Dioscorea rotundata / Dioscorea cayenensis complex.
Dioscorea japonica Japanese mountain yam.
The most economically important group.
Vines twine to the right, i.e. clockwise viewed from below.
In the following groups, the vines twine to the left
2. Lasiophyton group.
3. Combilium group.
Dioscorea esculenta lesser yam.
4. Macrogynodium group.
Dioscorea trifida cush-cush yam.
5. Opsophyton group.
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.
20.0 Yam species
Dioscorea astericus, South-Central Africa
Dioscorea batatas (D. polystachya), Chinese yam, cinnamon vine, China
Dioscorea bulbifera, potato yam
Dioscorea cayenensis, yellow guinea yam, twelve-months yam, Lagos yam, vegetable, Africa
Dioscorea composita (D. tepinapensis), yam, barbasco, Mexico
Dioscorea dregeana, wild yam, similar chemical properties to D. villosa
Dioscorea dumetorum, bitter yam
Dioscorea esculenta, lesser yam
Dioscorea floridana, Dioscorea villosa subspecies, Florida yam
Dioscorea hispida, intoxicating yam, grows wild in India, Philippines, Papua New Guinea, need to detoxify before eating
Dioscorea japonica, Japanese mountain yam
Dioscoreaa lata, water 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
Dioscorea rotundata, white yam
Dioscorea transversa, pencil yam
Dioscorea trifida, cush-cush yam
Dioscorea villosa, yellow yam
Dioscorea wallichiii, kruo
See diagram: Dioscorea wallichii leaves.
20.1 Dioscorea rotundata, white yam, white Guinea yam
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
21.0 Dioscorea alata (D. rubella), greater yam, winged yam, water yam, purple yam, white
yam, Guyana arrowroot, ten
months yam, name-de-Agua, greater Asiatic yam, Lisbon yam, white Manila
yam, common yam, "yam" (Enantiophyllum group),
perennial, deciduous, hardy plant, climber, herbal medicine, culinary
tuber in ground and bulblets that form on vine, eaten cooked, most grown
cultivated yam, Southeast Asia, Dioscoreaceae.
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
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
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.
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.
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
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.
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 ricer
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.
22.0 Dioscorea bulbifera,
potato yam, air yam, round yam, aerial yam, aerial potato, air potato, bulbil yam,
"wild yam", perennial, deciduous
potato and root peeled and diced, soaked well, then cooked or roasted,
Southeast Asia, Dioscoreaceae.
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
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
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.
23.0 Dioscorea esculenta,
lesser yam, pan is a small climber producing more than 1 small tuber.
The lesser yam 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.
Lesser yam, Dioscorea esculenta (Combilium group), Dioscoreaceae, Asiatic
yam, Lesser Asiatic yam, Chinese yam, Karen potato
Pana, Potato yam
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
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%).
24.0 Dioscorea villosa
See diagram: Dioscorea villosa
yellow yam, "wild yam", perennial, deciduous, climber, herbal medicine, pre- and post- menopausal women, skin cream, contains
| Diosgenine | steroid sapogenin disgenin C27H42O3,
herbal medicine, act on the contractile force of cardiac muscle, cardiotonics,
root saponins used for different disorders but no evidence for anti-cancer
function, North America
sold as root and root powder.
Dioscorea villosa, wild yam,
24.1 Dioscorea japonica,
Japanese mountain yam, "wild yam, shan yao, paeonol, Japan, China, Korea.
24.2 Dioscorea transversa, pencil yam, long yam midiny, native yam, herbal eastern and northern Australia
heart-shaped shiny leaves with 5-7 prominent veins, rounded seed pods are green or pink before drying,
slender long edible tubers, .eastern rainforest form has no bulbils, but northern open forests form has small
bulbils and large tubers, Australian native food, Dioscoreaceae.
24.3 Dioscorea trifida, cush-cush yam, Indian yam, aja, yampi,
mapuey, equatorial growth, thin skin tubers, good taste, South America. Dioscoreaceae
24.4 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
25.0 Global Initiative underway
to preserve yam biodiversity, 16 September 2010
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
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.
Before teaching this project, discuss the content of the lessons with
a field officer of the Ministry of Agriculture 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 the local field officer of the Ministry of Agriculture.
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.