School Science Lessons
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Sweet Potato Project
: Sweet potato

Table of contents

1.0 Sweet potato

1.1 Sweet potato plant (Primary)

1.1a Commercial variety

17 Sweet potato plant (Secondary)

1.2 Planting

1.3 Care & harvest

1.4 Diet

2.0 Leaves

3.0 Stem

4.0 Tubers

5.0 Flowers

6.0 Planting material

7.0 Planting

8.0 Growth

9.0 Fertilizer

10 Pests

18. Sweet potato weevil

11 Climate

12 Harvesting

13 Storing

14 Costs & profits

15 Varieties

16 Food

6.20.0 Records

1.0 Sweet potato, Ipomoea batatas, Convolvulaceae.
See diagram 61.1: Sweet potato plant, leaves, flowers
Sweet potato, Daley's Fruit Trees
1.1 The sweet potato, Ipomoea batatas, comes from South America, but is now grown very widely in many tropical and
temperate countries, e.g. USA, and is now grown more in South West Asia than anywhere else in the world.
It is the chief food crop in Papua New Guinea and is very important in the Solomon Islands and New Hebrides.
This crop is less important on other islands like Tonga and the Cook Islands, but it could be grown more.
In Papua New Guinea it is called kau kau.
In Fiji it is called kumara and in some Polynesian islands its name is kumala.

1.2 Sweet potatoes are very easy to grow.
It is probably easier to grow than any other food plant.
They are dependable, and will always give a crop.
They are very quick in growth.
Some varieties can give a crop in as little as 3 or 4 months.
Sweet potatoes will grow well in many situations from those that are wet to those that are dry.
It grows well at sea level, and some varieties can grow at over 1500 metres.
The plant does not suffer from many diseases.
The crop produces a good yield if it is given some care.

1.3 The emerging shoots, tips, stalks and unformed leaves can be eaten in stir fry dishes or salads.
White varieties can be cooked as chips ( French fries). boiled or roasted.
Grated sweet potato can be used to thicken soup and curries, and also be used as a filler in cakes, breads and fritters.
Dried sweet potatoes can be made into a flour and used is batters and pancakes.
1.4 The tubers and leaves can also be fed to pigs.
1.5 The leaves and excess growth can be added to the compost.
The crop can be harvested a little at a time.
This means that there may be fewer losses of tubers.
1.6 Some people grow sweet potato just a colonizing ground cover on poor soils to keep soil moisture high and add organic matter.

1.1 Sweet potato
Explain the importance of sweet potato in the diet.
It is a creeping plant with trumpet-shaped purple flowers.
It has relatives which are tropical creepers, e.g. the Morning Glory, Convolvulus sp., Convolvulaceae.
It produces starchy underground tubers and young green leaves which can be soaked and eaten.
There are many different varieties producing tubers with different tastes and yields.
Sweet potato can be cut into small pieces and dried in the sun.
This will keep a long time and can be ground into flour.
Sometimes sweet potato is planted for pig food.
It grows best in light sandy soils which contain plenty of rotten plant material.
Show the sweet potato leaves and tubers.
About local names, different varieties, and different ways to use them.
Do some local varieties have special uses, e.g. for babies.
How do people cook them?
Do you need to peel them?
Write on the chalk board a summary of local names and uses.
Sweet potato is an important energy food.
Some varieties are also healthy foods, e.g. red flesh varieties.
Some varieties are also growth foods if you eat the skins.
Draw a leafy shoot and some tubers.
Make sweet potato biscuits
Cut tubers into thin slices and dry them in the sun.
After a few days store the pieces in a tin with a tight lid.
The pieces can be broken up with a hammer to make flour.
Mix flour with margarine, baking powder and some milk.
Roll out dough cut into shapes and bake in hot oven.

1.1a Commercial variety
See diagram: Sweet potato variety
Variety from Polynesia or New Zealand, large tubers, red skin and ivory flesh marbled in violet, with colour intensifies during cooking.
high levels of anthocyanins, small emerald-green leaves, dark purple petioles.

1.2 Planting sweet potato
Prepare soil for sweet potato and plant it.
1. On light sandy soil plant into ridges one metre apart.
2. On heavy clay soils plant into mounds.
The soil must be well drained.
Select cuttings about 50 cm long from the growing tips of healthy plants which are producing large tubers.
Some people cut back old vines near the roots and later use the new shoots for cuttings.
Make ridges by digging trenches one metre apart, put in rotten plant material, plant ashes, potash or a low nitrogen fertilizer, e.g.
NPK 5.10.10.
Then hill up over the filled furrow.
The soil should be light and fine with no clods, sticks or stones in it.
Plant the cutting five cm deep, three nodes underground, 50 cm apart.
If you plant two rows on each ridge most of the ground will be covered to control weeds, but tubers will be smaller than if planting
only one row.
Press down around each cutting with the foot.
The soil should be damp when planting.
Mulching will keep the soil damp and protect the ridges from erosion.
The soil should not be very dry just because harvest because the tubers can be attacked by the sweet potato weevil.
Village people have learned many interesting ways to grow sweet potato and you should learn your local method.
However, you should also experiment with new methods, e.g. dry season planting.
Use some clean cuttings.
Draw a diagram to show how to make ridges and where to plant them.
Show how to hill up the soil to make ridges.
Show how to select plant cuttings.
Show how to plant cuttings in the ridges.
You may then use mulch to keep the soil damp.
Make mounds.
Build up heaps of rotten plant material and wood ash.
Cover the heaps with soil to make a round mound about one metre across and half a metre high.
Plant four cuttings around the top of the mound.

1.3 Care and harvest of sweet potato
Care for sweet potato and harvest it.
The sweet potato garden should be weeded every two weeks after planting.
Water the plants if the soil becomes dry.
If heavy rains wash away soil, hill up ridges with a hoe.
Lift the vines to prevent secondary roots growing.
The main pests are the hawk moth and the leaf miner moths.
Their caterpillars eat the leaves.
The sweet potato weevil walks down cracks in dry ground to eat the tubers.
The vine scab disease forms spots on the tips of stems.
The best way to control pests and diseases is to always plant sweet potato in new ground and use clean planting material.
The tubers are ready to harvest when the skin of the tubers is firm and when a cut piece turns creamy white, not greenish black
around the edges, or have a white juice which turns black.
Dig out the tubers with a stick or a fork, not a spade which will cut them.
Dry them in the sun and store in a cool dry place.
Rats will try to eat the tubers.
You can leave the tubers in the ground to harvest only when you need them or cut off the vines at ground level and dig up the lot.
If you burn all the cut vines this will kill all the pests and diseases on them.
Show how to weed properly.
Look for pests and diseases.
If you find anything attacking the sweet potato put the pest or disease and the part of the plant in methylated spirits.
Show students how to tell when sweet potato is ready for digging then show them how to dig up the tubers and store them.
Weigh the tubers.
Let feel the size of some tubers in the ground, then feel the size the following week.

1.4 Sweet potato in the diet
The importance of sweet potato in the diet.
Sweet potato is a creeping plant with trumpet-shaped purple flowers.
It has relatives which are tropical creepers, e.g. the Morning Glory, Convolvulus.
It produces starchy underground tubers and young green leaves which can be soaked and eaten.
There are many different varieties producing tubers with different tastes and yields.
Sweet potato can be cut into small pieces and dried in the sun.
This will keep a long time and can be ground into flour.
Sweet potato may be planted for pig food.
It grows best in light sandy soils which contain plenty of rotten plant material.
You will need sweet potato leaves and tubers.
Show the sweet potato leaves and tubers.
About local names, different varieties, different ways to use them.
Do some local varieties have special uses, e.g. for babies?
How do people cook them?
Do you need to peel them?
Write on the chalk board a summary of local names and uses.
1. Sweet potato is an important energy food.
2. Some varieties are also healthy foods, e.g. red flesh varieties.
3. Some varieties are also growth foods, if you eat the skins.
Draw a leafy shoot and some tubers.
9. Make sweet potato biscuits
Cut tubers into thin slices and dry them in the sun.
After a few days store the pieces in a tin with a tight lid.
The pieces can be broken up with a hammer to make flour.
Mix flour with margarine, baking powder and some milk.
Roll out dough cut into shapes and bake in hot oven.

2.0 Leaves
See diagram 61.2: Leaf shapes
The leaf is the most important part of the plant because it is here that the food is made, and is stored in the tubers after it is made.
The leaves of sweet potatoes have a long thin leaf stalk or petiole.
The shape of the leaf itself is very variable.
Some leaves are entire, i.e. have no lobes but are just one shape.
Other leaves have three, four or five lobes.
Most of the lobes have a pointed shape.
Look at the different leaf shapes in the picture.
There may be different leaf shapes on one plant.
Collect sweet potato leaves of different shapes.
Trace the outline of 3 leaves of different shapes.

3.0 Stem
The stem of the sweet potato plant is thin, bends easily, has little lumps along it called nodes.
The leaves are attached to its younger parts.
Where leaves drop off, a leaf scar is left.
Buds on the stem are just above the position of each leaf scar.
Every fifth leaf is directly in line with another leaf below it.
The stem it carries the food from the leaves down to the tubers.
If a piece of stem is broken off and one end is buried, new leaves will grow from the buds above the ground.
Roots will grow from buds on the part of a stem buried under the soil.
When a stem grows over the surface of the soil, some roots may grow down from some nodes that are touching the soil.

4.0 Tubers
See diagram 9.87: Sweet potato tuber
An outer periderm may produce secondary roots.
The "crown" end may produce shoots.
1. The tubers of sweet potatoes vary greatly in size, shape and colour of the flesh and of the skin.
The commonest shape of a tuber is a long spindle shape as you see in the diagram.
Other shapes are also common, some are nearly round, others are oval.
The outside of the tuber has small pits on it.
There is a bud at the bottom of each pit and it can grow out into a branch.
Tubers with long grooves in them are not good because much peel is lost from this part.
2. The colour of the skin and of the flesh may be white, cream, yellow or purplish, or another colour.
3 . The taste of the tuber varies a lot.
Some tubers are very good to eat and feel good in the mouth.
4. Dark orange varieties contain beta carotene.

5.0 Flowers
See diagram 61.5: Sweet potato flower
1. The flowers are white or purplish in colour.
They are formed singly or in small clusters in the axils of the leaves.
The 5 petals are joined to make a tube.
There are 5 small green sepals under the petals.
Sometimes the flowers make seeds.
These are contained in a small capsule that splits open when it is ripe.
There may be 2 to 4 small brown or black seeds with sharp corners.
The seeds are not usually used for sowing sweet potatoes.
2. Describe the colour and shape of the flowers.
3. Describe the seeds.
Find seeds in the seed capsules.

6.0 Planting material
See diagram 61.6: Methods of planting, Planting material
1. Use pieces of stem and leaves from the middle part of the stem.
The best pieces of stem to use for planting have a new shoot growing in a leaf axil.
The most common planting method is to plant tip cuttings.
2. Leave stem cuttings for a week until the leaves have died, then plant the stem piece.
3. Leave a bundle of stem cuttings in the shade and covered with grass or banana leaves to make roots growing out from the covered
4. Keep tubers in weak light in the shade until they start to sprout. Cut off pieces of the tuber with the shoots attached to them, then
plant the pieces.
5. When the shoots are long enough, 15 -20 cm in length, they can be broken off the tuber and planted.

7.0 Planting
See diagram 61.7: Sweet potato gardens, Ridges, Mounds
1. Planting on the flat
The garden is prepared by digging up the soil, burying weeds, and making the soil loose.
This method of planting is not used very much because it does not make good drainage.
2. Planting on ridges or moulded beds 30 cm apart
Sometimes ploughs are used to throw earth up into ridges and the cuttings are planted along the top of the ridges.
Sometimes ridges are made by hand.
This method is satisfactory because it makes the soil loose and because it makes good drainage for the soil.
3. Planting on mounds
The best method of planting is to form small mounds and plant the cuttings in the top of the mounds.
The mounds can be made by hand, but sometimes mounds are formed from ridges made by a plough first.
When a mound is made, it may be about 25 to 90 cm wide across the top and it may be heaped until it is 15 to 40 cm high.
When making a mound, bury grass or other plant material in the mound so that the soil is very loose to allow the tubers to grow.
The soil in the mound has good drainage, so it will not become too wet.
Do not dig animal manure into the beds unless it is well composted.
Excess nitrogen fertilizer produces leaves but no tubers.
The buried plant material will make the soil warm and this will make the sweet potatoes grow better.
4. Use cuttings cut about 20 cm from the growing tip of the runner.
Trim all the leaves off except for the two small emerging leaves.
Another method is to make cuttings from pieces of stem that have 5 nodes and to bury the bottom 3 nodes and leave 2 of them above
Plant the cuttings 40-60 cm apart.
5. The depth of planting cuttings also varies.
In some places cuttings are buried 30 cm deep, but in other places they are only buried 6 cm deep.
Planting too deep makes it too hard to harvest the tubers.
In some places the people plant the cuttings so they are straight up in the soil.
In other places they plant the cuttings so they are at an angle of 30o or 60o to level.
In some places the bottom part of the cutting is buried horizontally and the top part is bent upwards.
6. Usually more than one cutting is planted at each place.
Planting 2 to 5 cuttings in one place on a mound is usual.
Plant a new row every week to get a continuous supply.
Dig a sweet potato garden, planting 3 cuttings in each mound.
Use cuttings that have 5 nodes and plant them so the bottom 3 nodes are under the soil and 2 are above the soil.
7. The soil should be pH 5.5-6.5, with good levels of potassium and phosphorus to maximize tuber size.

8.0 Growth studies
See diagram 61.8: Sweet potato diary
Keep records of the growth of the crop.
1. Note how many stems come from each cutting.
2. Note how fast do the stems grow, i.e.
what length of stem is grown each day.
3. Note when the tubers start to form by uncovering the roots of one plant each week.
The normal life of the plant is 4-5 months at sea level but 5-6 months if planted high up in the mountains.
4. Note when the tubers have formed properly and the plant is near the end of its years growth.
At this stage some leaves may turn yellow.

9.0 Fertilizers
Sweet potatoes respond well to fertilizers especially those that contain nitrogen and potassium.
It is a good idea to put some fertilizers on one part of the garden: then students can see the result of using them.
The fertilizers to use are as follows:
1. Ammonium sulfate.
If you cannot get this fertilizer, use urea or IBDU.
2. Chloride or potash or sulfate of potash.
Do not add too much of them or it will burn the plants.
Use about one matchbox full of mixed fertilizer and spread it over one square metre.
Do not put the fertilizer too close to the plants.
Sprinkle it over the ground but keep it 10 cm away from the plants and dig it lightly into the soil.
3. Trim excess growth to increase the size of the tubers.

10. Pests of sweet potato
See diagram 61.10: Sweet potato weevil
Sweet potato weevil, Cyclas formicarius
Eggs are laid in the root and the larvae burrow through the tubers making a sodden rotten mess.
The female weevils chew small holes in the surface of the stems or tubers.
They then lay their eggs in these cavities.
The eggs hatch and a small larva comes out of the egg.
The larvae bore into the stems or tubers.
The larvae have no legs, are white in colour and are nearly a third of an inch or over one centimetre long.
After 3 weeks the larvae turn into a resting stage called a pupa.
Later the adult weevil comes out of the pupa.
The adult weevil is shiny blue and red in colour and is less than 0.5 cm long.
The adult weevils can feed on the leaves, vines, roots and the outside of the tubers.
The worst damage is done by the larvae.
If the larvae have been living in the stem, they often go down into the tubers later.
The weevils may be very bad in dry weather because when it is dry, cracks come in the soil and then going down to the tubers is
easy for the weevils.
Control sweet potato weevil by using crop rotation and crop hygiene.
After harvesting all the tubers, pull out all the remaining roots and stems and burn them.
Termites can infect old growth.
Do not leave any tubers in the soil after harvest.
Do not plant any more sweet potatoes for at least a year in that place so that they cannot go from the parts of the old plants into the
new plants.
Do not use planting material from this place because it may contain weevils.
Tubers with weevils in them can be fed to pigs.
Burn the old parts of the plants where weevils may be living.

11. Climate for growing sweet potato
Sweet potatoes will grow in many warm climates.
The crop likes to have temperatures of 24oC or more with plenty of sunlight and warm nights.
The plants will not grow well when the temperature is below 10oC.
Sweet potatoes can be grown in a cold climate if there is a summer season of 4 to 5 months with no frosts.
Sweet potatoes need 500 mm of rainfall while they are growing, but they need 750, 1000 mm of rain for the whole year.
It is good if there is not much rain when the crop is nearly ready to dig, but it, is bad if the dry time comes 50 days after planting
because then the storage roots are forming.

12. Harvesting sweet potatoes
Test whether the sweet potato crop is ready for harvesting
1. When many leaves turn yellow and begin to fall off the plant it is a sign that the tubers are ready to dig.
2. When a small piece of tuber is cut off with a knife and the juice does not go black quickly, then the tuber is ripe.
It is easy to damage the tubers when you are digging them up.
Dig into the side of the mound carefully with a garden fork and lift.

13. Storing tubers
1. Sweet potato tubers go bad very easily, so storing them for a long time is not usual.
However, there is a way of storing them for short periods.
This is by "curing" them first.
Select only mature tubers with flesh that is white when cut.
Start with air-dried tubers.
Curing is done by giving them a high temperature of 29oC and keeping them in moist air for 4 to 7 days.
Keep them in a room for curing, but there must be air flowing through this place.
After about a week of curing, this hardens the skin of the tubers and this keeps out the fungus that makes them go bad.
However, chilling may cause a hard core condition that remains even after cooking.
2. After curing for about a week, put the tubers where it is cool and the temperature is only about 16oC.
Temperature below 10oC is too cold for the sweet potatoes and they will be damaged by the cold.
In some places the people store the tubers in pits made in the ground in a cool place.
They may keep them for times up to 4 months in the pits, but usually you can only store them for shorter periods, e.g. 4 weeks.
3. Test if the tubers have been properly cured:
4. The skin of the tubers does not slip easily when pushed.
5. The buds may have a purplish colour.
6. The skin looks dry and spongy.

14. Costs and profits
1. Establishment costs are the things you buy, which will probably last for a long time, e.g. spades, hoes, sprays.
Divide the establishment costs by the number of years you think these things will last and you add this amount to the costs, e.g. 3 years.
2. Production costs are things that you must buy each year that you grow the sweet potatoes, e.g. fertilizers and insecticides.
3. ( Profit = returns - establishment costs - production costs)..

15. Sweet potato varieties
1. Where sweet potatoes are grown, there are usually many varieties.
Often these varieties have local names.
Know about these varieties and which are the best to grow.
Try to get some leaves and tubers of as many varieties as you can.
2. Note the features of the different varieties:
2.1 Shape of leaves
2.2 Colour of tuber skin
2.3 Colour of tuber flesh
2.4 Shape of tuber
2.5 How long it takes to grow
2.6 Taste of cooked tubers
The main varieties are:
1. dry and starchy
2. red or purple containing anthocyanin
3. moist and sweet
4. dark orange containing beta carotene ( called "yams" in USA)., and is cured for days to heal damaged skin and develop sugars.
5. The most common sweet potato types in US markets are Beauregard and Red Garnet.
Both types have amylase enzymes which break down more starch during cooking, resulting in a moist, soft texture and sweet flavour.

16. Food value of sweet potatoes
The tubers are full of sugar and starch.
This means they are a good energy food.
However, sweet potatoes only contain a very little protein food.
They are rich in iron and calcium, minerals and in vitamins, e.g. vitamin A.
During cooking, enzymes break down starch to maltose between 60oC and 75oC, so slow baking is better than rapid boiling to
allow longer enzyme action.
Some red varieties become dark during cooking caused by phenolic compounds.

17. Sweet potato plant (Secondary)
Sweet potato is a climbing herbaceous plant.
It may live for several years, but often it is harvested after 3 months, before it can flower.
Stems may grow to 2 or 3 metres in length; they are thin and climbing or creeping.
They have nodes at varying distances apart.
Varieties with short inter-nodes are said to yield more heavily than varieties with long inter-nodes.
Leaves of different varieties vary greatly in size and shape, and may be entire, heart- shaped or deeply divided with 3, 5 or 7 lobes.
Leaf veins and the leaf- stalks are green or red.
Flowers, usually violet, sometimes white, are clustered in the leaf axils.
Many varieties of sweet potato in cultivation do not have time to produce flowers and fruits before the harvest.
Roots are the most important part of the plant is the roots, because they can develop into tubers.

Tubers are parts of the creeping roots that have built up food reserves.
Tubers are produced at points where the roots cease to spread out near the surface and turn downward into the soil.
Tuber formation is better making mounds that are not too wide.
Varieties of sweet potato differ greatly in the number, shape and size of their tubers and in the colour of the peel and flesh of the tubers.
Sweet potatoes tubers may be round or elongated.
In colour they are white, yellow, red or violet, with soft or firm flesh.
They may weigh between 0.3 and 3 kg.

Sweet potatoes grow well in warm, sunny and humid regions, but can withstand drought quite well, so are suited to dry savanna country.
Sweet potatoes need regular rain to grow well, especially when the leaves are coming into growth, but tubers rot if too much rain at harvest.
The sweet potato will grow in poor soils, the most suitable soil is a light, well-drained, sandy loam.
However, if the soil is too rich in organic matter and nitrogen, it produces much stem and leaves, and much later, very few tubers.

Ridges and mounds
Sweet potatoes are usually grown on ridges or mounds after deep tilling, protect them from too much moisture, so better than growing on the flat
The ridges are made about 75 cm apart, but it is better to plant on round mounds 30 to 40 cm high, 1 m apart.
Mounds should be as narrow as possible to forces the plant to bend its roots downward quickly, build up food reserves and develop tubers.

The farmer must know his varieties well and know how long they take to form tubers, and harvest in the dry season.
If planting several times, at intervals, the whole plantation does not become ready for harvest at the same time.
By doing this, you can lift the sweet potatoes when you need them.
Planting material
Sweet potatoes are propagated from cuttings or tubers, but propagation from cuttings is possible only when sweet potatoes remain
in the field all through the year.
Cuttings should be 20 to 40 centimetres long, with three to five growth buds, and taken from the tips of young stems.
Take the cuttings only when you are ready to plant them, and keep them in the shade until they are inserted in the soil.
Propagation from cuttings is the most economic way of increasing your plants.

Plant cuttings at a slant, leaving 3 or 4 cm above ground, and press the soil down firmly.
If planting on mounds, plant four or five cuttings in a circle on each mound, to give a planting density of 15 000 to 30 000 plants per hectare.
Propagate from tubers, if you do not have any plants of sweet potatoes with enough leafy growth to provide cuttings.
Tubers must be made to sprout in a cool nursery bed.
If the tubers are large, cut them into several pieces.
After about a month, remove from the tubers the young shoots that are 15 to 20 centimetres long and plant them.
This method of propagation from tubers is usually done only on about one third of the area of the sweet potatoes field.
Later, cuttings from the plants thus obtained can be used to enlarge the plantation.

For control of weeds, one or two cultivations in the early stages of growth are enough, and in 4 to 6 weeks after planting, the plant's
own leafy growth will closely cover the soil.
When cultivating, remake the mounds at the same time.

Fungi diseases
Sweet potatoes may be attacked by diseases and insects to yield only a small harvest of poor quality, so you must wait 3 to 5 years before
growing sweet potatoes again on the same field.
Rot and fungi diseases that kill the growing plants are caused chiefly by various fungi that make the leaves turn yellow and wither.
Signs of the disease are yellow leaves and black marks inside the stems and tubers.
Other fungi cause the young plant to rot and it stops growing.
The roots and the tubers already formed turn black.
It is not long before the whole plant withers and dies.
To control rot, choose resistant varieties, so do not use for propagation cuttings or tubers taken from plants attacked by rot.
Do not grow sweet potatoes on the same soil for two years in succession.

Insect pests.
Sweet potatoes may be attacked by certain insects, especially by the sweet potato weevil.
The adult insects eat the leaves, stems and tubers.
The female insects lay their eggs in the stems or roots; the larvae tunnel into the tubers.
Serious damage is caused by weevils.
To control the weevils, use insecticides.
Before planting tubers and cuttings, dip them in an insecticide solution.
In places where harvested sweet potatoes are stored, the soil can be fumigated.
Yields of sweet potatoes and storing.
Depending on the varieties of sweet potato and on the way they are grown, yields vary from 4 to 7 tons per hectare on average.
On a modern and well- cared- for plantation, yields may be much higher, and may even be more than 20 tons per hectare.

The length of time for which sweet potatoes can be kept differs with the varieties and the harvesting season.
If they are harvested in dry weather, the tubers may be stored for 2 or 3 months.
But part of the harvest may be destroyed by rot during storage.
Damaged tubers are most quickly attacked.
Damp conditions encourage rot.
To prevent rot, dry the tubers in the sun for a time after harvesting.
For good keeping, harvest tubers when they are quite ripe, when the stems and leaves have turned yellow.
Take care not to damage the tubers.
Remove all diseased and damaged tubers.
Dry the tubers in the sun.
Store them under cover in a dark, dry, cool, well- aired place.
Put them on dry ground or on boards supported on posts, and do not heap them up too much.

18. Sweet potato weevil
See diagram 61.8: Sweet potato diary
See diagram 61.3: Sweet potato weevil
Sweet potato weevil, Cylas formicarius is the most serious pest of sweet potato.
It causes damage in the field and in storage.
The adult body, legs, and head are long and thin, so it looks like an ant, and measures 5.5 to 8.0 mm in length.
The head is black, with an extremely long rostrum (beak).
The antennae, thorax and legs are orange to red-brown, and the abdomen and elytra are metallic blue.
A complete life cycle occurs in 35 to 40 days and all stages can be found during the year in sweet potato tubers.
. During winter adults may seek shelter and remain inactive until the weather is favourable.
The only vegetable crop they feed on is sweet potato, but they also feed on Convolvulaceae plants, including
railroad vine (Ipomoea pes-caprae), and morning glory (Ipomoea panduratea).
Sweet potato weevil in Australia is found widespread in Queensland, with losses from 5 to 97%.
A symptom of infestation is yellowing of the vines, but a heavy infestation is usually necessary before this is noticed,
so infestation may be overlooked because damage is not apparent until tubers are harvested.
The main form of damage is mining of the tubers by larvae so that the tuber is riddled with dark, spongy cavities.
Also, larvae cause damage indirectly by facilitating entry of soil-borne pathogens.
Even low levels of feeding induce a chemical reaction that gives a bitter taste and terpene odour to the tubers.
Larvae also attack the vine of the plant, causing it to darken, crack, or collapse.
Adults usually have limited access to tubers, so damage by them is less severe than by larvae and adult feeding
on foliage is limited.
However, adults may feed on the tubers, creating numerous small holes on the outside of the tubers.
If you suspect you have seen sweet potato weevil, report your sighting to the Department of Agriculture.
Planting time applications of insecticides are used to prevent injury to the slips or cuttings.
Granular or liquid formulations are used, and systemic insecticides are preferred.
Postplant applications may be made to the foliage for adult control, especially if nearby fields are infected.
If systemic insecticide is applied, some suppression of larvae developing in the vine may also occur.
Due to the long duration of the plant growth period, preplant or planting time applications may be followed by
mid season applications.
Also, insecticides may be applied to tubers being placed into storage to prevent reinfestation.

Before teaching this project, discuss the content 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 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.