AGRICULTURE U3 TUBER CROPS PDF

AGRICULTURE U3 TUBER CROPS prepared for the course team by Dr. Le sl ie Ub aub an d F a la nik o Amo sa  Tuber Crops...

AGRICULTURE U3 TUBER CROPS prepared for the course team by Dr. Le sl ie Ub aub an d F a la nik o Amo sa  Tuber Crops Sweet Cassava Yams Potatoes Importance Morphology Ecology Propagation Development Physiology Husbandry Unit 3 concept map This map represents the core concepts that we’ll be covering in this unit, and the relationships between them. Unit 3: Tuber Crops 3.2 Study organiser Before you begin this unit, please check through your study organiser. It shows the topics that we’ll be covering, the skills you need to acquire (the objectives) and the activities you’ll do to help you acquire these skills. Topic Learning Outcomes Activities 3.1 Cassava Identify and explain the importance of Activity 3.1 the tuber crops to the South Pacific 3.2 Yams people; Activity 3.5 3.3 Sweet Potatoes Demonstrate how to propagate Activity 3.2 different tuber crops in the field; Activity 3.6 Activity 3.12 Grow and manage different tuber Activity 3.3 crops in the field; and Activity 3.4 Activity 3.5 Activity 3.7 Activity 3.8 Activity 3.9 Activity 3.10 Activity 3.13 Identify and name some of the Activity 3.11 diseases and pests of different tuber crops. Activity 3.14 Time for study You can expect to spend 14 hours on this unit. Unit 3: Tuber Crops 3.3  Introduction In this unit you will study the main tuber crops cultivated in the South Pacific region. These crops are cassava (Manihot esculenta), Yams (Discoria spp.) and sweet potato (Ipomea batatas). 3.1 Cassava 3.1.1 Introduction Cassava belongs to the Euphorbiaceae botanical family, of the Manihot genus and esculenta species. Its origins can be traced to Central and South America, Southern Mexico, Guatemala or Northern Brazil. Figure 3.1: Manihot esculenta – Cassava Plant Source: https://www.appropedia.org/ Source: http://bulletin.iita.org/ 3.1.2 M o rp h o l o g y Cassava is a perennial shrub which produces large tuberous roots. The height ranges from 1-4m depending on the cultivar and growing conditions. Stem: The stem is usually slender and glabrous, with leaves near the apex. The lower part of the stem has nodes made up of prominent leaf scars. The stem is woody at the base, but becomes softer towards the tip. Roots: Cassava has fibrous roots. A few (ten) of these develop into storage or tuberous roots. Each tuberous root is attached to the stem by a 'neck.' The tuberous root is generally fattest at the stem end and tapers down to a very thin root at the tip. Unit 3: Tuber Crops 3.4 Leaves: The leaves are deeply and palmately divided with 5-7 lobes, each 4-20 cm long and 1-6 cm wide, obovate-lanceolate, pointed with entire margins. The leaves are arranged spirally on the stem. (See figure 2.1) Figure 3.2: Manihot esculenta - parts of the cassava plant 3.1.3 Ecology of cassava Temperature: Optimum temperature range for cassava cultivation is 25-30C. It tolerates a minimum temp of 18C and yields are reduced above 30 oC. It can not withstand freezing and must have a minimum of 300 frost-free days. Unit 3: Tuber Crops 3.5 Latitudes: Cassava grows in the range of 30N and 30S but grows best in latitudes 15N and 15S. Rainfall: Cassava can grow in areas that receive between 50-250 cm of rainfall annually but the optimum is 100-150 cm. Cassava can withstand long periods of drought except at planting and during its early growth period (about two to three weeks after planting). Soil: Cassava can be grown on a wide range of soils. Light sandy loam of medium fertility gives the best results. Saline and swampy soils are unsuitable. Cassava can tolerate soil of low fertility. Altitude: The crop grows at sea level up to 1000 m. At the highest altitude in this range, growth is slow and yields poor. Day length: Cassava is a short day plant. With long day lengths (10-12 hrs), poor tuberous root growth results, however, some cultivars can tolerate long days. 3.1.4 Propagation Cassava cannot be propagated by tubers like the other crops we have discussed in this course because cassava tubers cannot sprout. Cassava is propagated by hardwood cuttings, cut from the lower parts of the stems. These cuttings are called sticks. Length of sticks: Generally sticks should be 20-30 cm long and have at least three nodes, but 5-7 nodes are better. Cultivars with long internodes require longer sticks. The sticks also root better if they are cut straight across rather than at an angle. Thickness of sticks: Thickness of the sticks should not be less than half the diameter of the thickest part of the stem of the cultivar used. Age of sticks: Sticks should be taken from plants 8-18 months old. Generally, sticks taken from the middle part of the stem give higher yields of tuberous roots because these have enough store of carbohydrates necessary for rapid sprouting. Health of sticks: Sticks should be taken from disease-free plants at all times. Care should be taken when handling planting sticks so that the buds and bark are not broken. Unit 3: Tuber Crops 3.6 Sticks should be prepared just before planting. If planting is delayed, the sticks may be stored as long stems up to one month and then cut before planting. Use fungicide treatment if sticks start rotting before they grow. Soak sticks in solutions of a chemical like Manzate. Activity 3.1 Interview mature people in your community, such as your parents, several farmers or an Agriculture officer to learn the major tuber crops grown in your country and their importance to your country. 3.1.5 Developmental physiology of cassava: As we have said for aroids it is important to know the growth cycle of a crop to be able to grow it successfully. Knowing how the crop grows from planting to maturity will enable you to supply the required resources for the crop at its time Unit 3: Tuber Crops 3.7  of most need for those resources. These times of most need are referred to as critical times for the crops’ requirements for these resources which may be particular nutrients, water, sunlight or other management practices. Reading 3.1 Now turn to the Reading by Lebot (2009) at the end of this unit. 3.1.6 Management of cassava Compared to many other crops, cassava will not grow well in high pH soils (pH 8) and yields are better on acid soils (pH 4.6). Where to Cassava can be planted on the 'flat' or on mounds or ridges where soils are poorly grow drained and tend to be water logged. cassava? Cultivars: There are several different cultivars in each country of the region, e.g. Fiji has 30. Spacing: When monocropping, the recommended spacing for both 'flat' and ridge plantings varies from 1 m x 50 cm to 1.2 m x 1.2 m, 1 plant/planting point. For mounds, 1 m between mounds and 2 cuttings/mound gives high yields. Low branching cultivars need to be planted wider than high branching cultivars. In all cases, cassava should be planted close enough so that the canopy will close four months after planting. Planting: Cassava should be planted when the soil is moist, and regular rains are expected to continue for at least three weeks. The best time is at the start of the rainy season. Planting: The sticks can be planted at three different planting orientations:  vertically: recommended for high rainfall conditions  slanted: also recommended for high rainfall conditions  horizontally: recommended for low rainfall conditions; easiest to adapt to mechanical planting. When planting vertically or slanted, one half or two thirds of the stick should be buried. In horizontal planting the whole stick is buried about 10 cm below ground. Unit 3: Tuber Crops 3.8  Activity 3.2 1. How should cassava be planted in wet soils? 2. What stick orientation(s) should be used under such soil conditions? a) b) Fertilisers: Because of its deep root system, cassava tolerates low soil fertility compared to other tuber and root crops. Therefore, unless soils are very infertile, fertiliser is not recommended. When fertiliser is required potassium (K) appears to be the most important. Cassava shows little response to phosphorus (P) because of the mycorrhiza on its roots that enables it to extract P easily from the soil. A general fertiliser recommendation for very poor soils: 60 kg N/ha; 60 kg P/ha; 90 kg K/ha. This can be a split application at 1 and 3-4 months after planting or one application at 2-3 months after planting. Unit 3: Tuber Crops 3.9 Cassava has a high zinc (Zn) requirement; ZnSO4 may be added in the dipping solution if sticks are treated before planting, or it can be sprayed on the plants later. Weed control: Weed control is most important in the first three months after planting. If the right spacing is used, the canopy will have closed at that time, controlling weeds. Weed control methods are:  by hand; and  chemicals. After planting apply pre-emergence herbicides which will give good control for the first month and later use Gramoxone or hand weeding. When using pre-plant or pre-emergence herbicides, dosages should be higher for clay soils than for sandy soils. For example:  In clay soils 1.6-2 kg active ingredient/ha of Karmex, plus lasso at 3 litres/ha.  In sandy soils 1 kg active ingredients/ha of Karmex, plus lasso at 2 litres/ha. Harvesting: Cassava tuberous roots are generally ready for harvest 6-15 months after planting depending on cultivar and growing conditions. Under low rainfall and poor soil fertility conditions, it may take up to 24 months before harvest. In the Pacific all harvesting is done by hand. Average yields in farmers fields in Fiji are 23 t/ha but yields can be as high as 40-50 t/ha. Activity 3.3 1. About what time during the life cycle of cassava is weed control most important? 2. About what time during the life cycle of cassava is fertiliser very important? a) Unit 3: Tuber Crops 3.10  b) Storage of cassava: Cassava does not store well. Physiological deterioration which appears to be connected with enzyme activity in the tubers causes blue/black vascular streaks 1-3 days after harvesting. Therefore, harvest only enough tubers to eat or sell in a few days. To delay vascular streaking, do the following:  grow the resistant cultivars e.g. In Fiji cultivar 'Hawaii';  minimise injury during harvest and transport;  cut off stems 14-21 days before harvesting;  leave harvested roots attached to the base of the stem;  cut the tubers from the stem and leave the neck as long as possible;  store roots in conditions of high humidity, e.g. reburial or put in plastic bags; and  cure for 4-7 days. Cassava can be peeled and frozen for long-term storage or for export. HCN in cassava: Cassava contains glucosides that break down to hydrogen cyanide (HCN). Cultivars differ in their HCN content. Those with high HCN are normally bitter and are called bitter cultivars. Most of the cultivars in the Pacific are low in HCN and are called sweet cultivars. Tubers of sweet cultivars can be boiled and eaten. Tubers of bitter cultivars have to be processed before they can be eaten. The processing involves grating, squeezing out the juice, fermenting, and drying over high heat. Unit 3: Tuber Crops 3.11  Activity 3.4 1. What are the three management practices farmers can use to increase sucker production? 2. What is the best size headsett to use as planting material? 3.1.6 Uses and processing Cassava is eaten as a vegetable, often boiled or roasted. The young leaves are also eaten in some parts of Africa. It is processed into:  starch  dried cassava roots  tapioca  flour  glucose 3.1.7 Diseases and pests In most South Pacific countries there are no economically important diseases or pests of cassava. The diseases and pests of cassava usually do not do enough damage to reduce yields. Diseases: Unit 3: Tuber Crops 3.12   Cassava bacterial blight (Xanthomonas campressris) has been recently introduced into Guam and Pohnpei. The disease can be devastating, and its continued spread is a threat to cassava production in the region.  Leaf spot, (Cercospora hennigsii)  Anthracnose (Glomerella cingulata) Pests:  Spider mite (Tetramychus)  Pigs and rats. 3.2 YAMS 3.2.1 Introduction Botanical family: Dioscoreaceae Genus: All yams belong to the genus Dioscorea. Species found in the Pacific: 1. D. alata 2. D. esculenta 3. D. nummularia 4. D. bulbifera 5. D. pentaphylla 6. D. hispida 7. D. trifida 8. D. rotundata The first six species have been grown in the Pacific for a long time and the last two were recently introduced. Importance: As for the aroids, Yam is a staple food for Tonga, Fiji, Solomon Is, Vanuatu, PNG, Pohnpei. It is important socially in Tonga. Origin: South-East Asia: D. bulbifera, D. pentaphylla, D. hispida, D. alata, D. esculenta, D. numularia South America: D. trifida Unit 3: Tuber Crops 3.13 West Africa : D. rotundata Activity 3.5 1. List the common yams of the South Pacific region by their scientific names. 3.2.2 M o rp h o l o g y Stem: The plant is a vine. The above–ground part (shoot) consists of leaves attached to a stem. A stem usually has branches. Parts of the stem:  leaves;  cataphylls; and  wings, spines or neither. The stem either crawls on the ground or twines (either to the right or to the left depending on the species) up stakes, trees or other supports. After sprouting the stem usually grows some distance before producing any leaves. At this length it has no leaves but there are cataphylls at the nodes. Stems may also have spines, wings or neither.  Leaves: Leaves may have pulvinus and stipules at the base of the petioles. The size of pulvinus and stipules are often used to describe cultivars.  Flowers: Not all yam cultivars flower. Yams are dioecious. Male flowers usually have a sweet smell except D. nummularia which has a distinct smell.  Tubers: The tuber is the part we eat. Tubers are attached to the corm or the primary nodal complex (PNC). Tubers grow directly from the corm, except in D. esculenta in which the tubers are attached to the corm by stolons (up to 35 cm long). Fibrous roots grow from both the corm and the tubers. Parts of a yam tuber:  head Unit 3: Tuber Crops 3.14   middle  tail section (See figures 2.2, 2.3, 2.4) Figure 3.3: Dioscorea spp. - parts of the yam plant Unit 3: Tuber Crops 3.15 Figure 3.4: Dioscorea spp. - parts of the yam plant Unit 3: Tuber Crops 3.16 Figure 3.5: Dioscorea esculenta - parts of the sweet yam plant Dormancy: At harvest yam tubers are dormant (1-6 months) depending on the cultivar. Corms have well preformed buds. Tubers do not. Tubers only form buds if the corm is removed or damaged and the dormancy broken. Tubers also have apical dominance. Sprouts will only come from the head. The other parts will only sprout if the head is removed. Aerial tubers or bulbils grow in axil of leaves like in D. bulbifera. Unit 3: Tuber Crops 3.17 3.2.3 Propagation Types of planting materials:  Small whole tubers are the best planting material, provided they are from healthy plants.  The second best planting material is the second harvest from 'milked' yams. Pieces of large tubers are also good for planting material. Head pieces are better planting material than middle and tail pieces. Cut pieces should be dusted with ash, well washed sand, or fungicides and then dried in a shady place for 2-3 days.  Aerial tubers are the best planting materials for D. bulbifera. Size of planting materials: Large planting materials produce large total yields, but they do not necessarily give higher net yields. Net Yield = Total yield - planting material To produce high net yields, sett weighing 150-500 grams are recommended. Farmers normally plant larger setts because under adverse conditions, large setts sprout better than small ones. Health of planting material: Only tubers from healthy plants should be used as planting setts. Therefore you should select planting material in the field so you know from which plant each tuber was harvested. This is why it is very important to mark the diseased plants. The most important diseases are viruses and nematodes. Symptoms of virus:  vein banding;  vein clearing;  mottling and leaf distortion; and  internal brown spotting in the tuber. *for leaf curling and distortion check leaf underside for insects. Symptoms of nematodes:  vertical cracks;  tuber skin flakes off exposing black-coloured dry rot; and  tubers become soft and wrinkled and feel spongy. Unit 3: Tuber Crops 3.18 Pre-sprouting: Setts are cut, treated with ash, sand, fungicide and buried in a well-drained) pit. Layers of setts are alternated with layers of soil. The main advantages of pre-sprouting are:  helps produce a earlier yield; and  provides a homogeneous stand. The main disadvantage is:  the extra time and labour required; and  extra care needed while planting sprouted setts. Supply of planting material: In times of shortages, there are several methods of rapid multiplication. Activity 3.6 1. What is the best planting material for yams? 2. List two advantages of ‘pre-sprouting’. a) b) 3. What would be the best control method for this disease? Why? Unit 3: Tuber Crops 3.19 3.2.4 Ecology Yams grow well in areas with adequate rainfall, and are adapted to areas with a distinct dry period. It is their tuber dormancy that allows them to survive the dry season, Yams will not tolerate water logging; good drainage is necessary. The table below summarises the ecological factors required for yams. Ecological Dioscorea spp. (Yams) Factors Rainfall Major production areas 1100-1500 mm/growing season; very poor yields can be produced from lower and higher rainfall areas Soil Deep well drained is required; good drainage is essential for good yields Temperature optimum 30oC; poor growth below 20oC, most cultivars cannot withstand frost. Altitude Grown successfully at low or medium elevations less than 900m. D. alata can be grown at 2700m. Day length Long days favour vine growth (more than 12hrs); short days (10-11 hrs) favour tuber growth. Activity 3.7 1. Consider the climate in your country and decide whether yams will grow well in there. Give reasons for your answer. Unit 3: Tuber Crops 3.20 2.2.5 Developmental physiology of yams Reading 3.2 Now turn to the Reading 2.2 by Lebot (2009) at the end of this unit. 3.2.6 Management of yams Cropping systems Yams are usually intercropped with other crops. In Africa, yams are intercropped with vegetables and maize. In the South Pacific, yam is grown under coconuts or with other root crops and sometimes grown on its own (monocrop) on small plots. Methods of planting:  planting on the flat (only on deep well drained soil);  planting in holes (trenches); and  planting on mounds (raised beds, beds can be 50-100 cm high and twice this width at the base). Planting on mounds is the best and most widely used method in areas where soils are thin, stony, or not well drained. Harvesting is also much easier when yams are planted on mounds. Time of planting: Yams are planted when they break dormancy and start sprouting. This varies among species and cultivars, but often occurs shortly after the start of the rainy season. This can be as early as July and August in islands south of the equator. To spread the demand for labour at planting, early planting in the dry season while tubers are dormant is practised. Mulch should be applied on planting positions to lower soil temperatures. In Tonga, early yams planted in June and July are first harvested or 'milked' in December. The second harvest is in March. The main growing season is August/September to April and May. Spacing: The same principles for the spacing of aroids (taro) applies to yams. Close spacing results in high yields per area while wide spacing results in high yields per plant. Large setts need to be planted out wider than small setts. For most species of yam- setts (250g) planted on fertile soil as a monocrop, a spacing of 1 m x 0.5 m to1 m x 1 m usually give the highest net yield of medium sized tubers. If soil fertility and rainfall are low then use wider spacing. Unit 3: Tuber Crops 3.21  Activity 3.8 1. Yams prefer to be planted on mounds, under which soil conditions? 2. When is the best season to plant yams? Staking: Staked plants yield more tubers than unstaked plants and get more sunlight and are less exposed to diseases. Stakes should be put in before the vines are more than 50 cm high. The need for staking depends on:  Species: For example, D. alata yields quite well unstaked; D. rotundata and D. nummularia often yield poorly unstaked.  Availability of staking material.  The cost of labour.  Wind: High stakes are not recommended for windy areas; unstaked yams get twisted by the wind, therefore in windy conditions low and strong stakes (2m) should be used.  Disease and insects: Some diseases and insects cause less damage to staked yams e.g. rose beetle and anthracnose. Weed control: Weed control is essential during the first four months, especially for unstaked yams. Weeds can be controlled by hand or by chemicals. Chemicals: Gesaprim and Karmex can be applied on the bare soil before the setts sprout. This application can control weeds up to two or three months. Gramaxone can then be used or hand weeding. Fertiliser: In traditional farming, fertiliser is not used on fertile soil. In less fertile soils with close spacing, fertiliser is recommended to obtain high yields. Unit 3: Tuber Crops 3.22 The best rate of fertiliser application will depend on soil analysis and soil type, but a general recommendation can be made as follows: 100 kg N/ha split applied at 10, 14 and 18 weeks after sprouting in the field. If P and K levels are low in the soil, apply at planting 15 kg P/ha and 80 kg K/ha. Inorganic fertiliser should all be applied in the first 18 weeks. Fertiliser applications after 18 months will reduce the eating quality of the tubers. Compost and well-rotted manure should be mixed with the soil before planting. Activity 3.9 1. How does staking improve tuber yields of some yams? 2. What period during the growth of yams is weed control very essential? When is the best time to apply all inorganic fertilisers to yams and what fertiliser would you recommend? Give a reason for your recommendation. Harvesting: Most yams are harvested at 6 to 12 months; D. Nummularia is harvested at two to three years. At the start of the dry season the yam plants normally die back and the tubers are ready for harvest. Harvesting can be spread out over a number of months e.g. Tonga; other places harvest over shorter periods e.g. PNG June - August. For 'milked yams' , the first harvest is done when tubers are large enough to be worth harvesting. The second harvest is done when the vines die. Large yams are dug up by hand. Yam species which produce a number of small tubers are harvested mechanically with a potato spinner. Recorded yields in the Pacific: Farmers field: 5 to 25 tons/ha Research stations: 15 to 40 ton/ha Unit 3: Tuber Crops 3.23 Storage of yams: Good storage begins in the field with good harvesting.  Avoid breaking or scraping skin.  As soon as the tubers are harvested they should be moved into the shade.  If transported, protect them from bruising.  Yams should be stored in dry, shaded (thatched or metal roof), well ventilated places with security against animals.  Tubers should be stored at temperatures from 32 to 40oC with relative humidity of 90% to 95%, for 1-4 days. Chilling damages at temperatures below 12oC. There are several methods of storing yam tubers. In Asia and Africa, yam tubers are stored in covered heaps and yam barns. In the South Pacific storage in thatched houses is common. After dormancy is broken and tubers start to sprout. The tubers should then be planted or marketed as soon as possible. Curing yams: Curing helps to heal wounds in tubers and increase their storage life. There are several ways of curing yams.  Cover heaped tubers with grass and then cover with a tarpaulin.  In Tonga, a single layer of treated setts is buried in a pit covered with banana leaves and more leaves are put on top before soil is put on. After about three days, a hole is made in the top of the pit to let some of the heat off. Later the setts are stored in a cool dry place. Activity 3.10 1. What does ‘milking’ yams mean? 2. List any three good harvesting practices for yams. a) Unit 3: Tuber Crops 3.24  b) c) 3. What is the main purpose of curing yam tubers? 3.2.7 Pests and diseases of yams Fungal disease The major disease of yams in the Pacific is a leafspot caused by the fungus Colletotrichum gloeosporioides,(Glomerella cingulata). This disease is commonly called, dieback, anthracnose, lighting or scorch. Symptoms: Dark brown or black spots on leaves and stem. D. esculenta and D. nummularia are resistant to anthracnose. D. pentaphylla and D. alata are generally susceptible. Cultural Controls for Anthracnose:  Plants should get early morning sun to dry leaves.  Do not work when yam vine is wet.  Stake yams to stop spread of disease by rain splash.  Use recommended fungicides. Nematodes Unit 3: Tuber Crops 3.25 Dry rot nematode: is caused by the nematode Pratylenchus coffeae which affects tubers in storage. The nematode is both carried by infected tubers and infected soil. Symptoms:  Longitudinal cracks on the skin.  A black dry rot under the skin. Control methods:  Hot water treatment (50-55o) of whole tubers.  Plant in soil free of nematodes, for example, new soil (an area that has just been cleared of bush.) Root-rot nematode (Meloidogyne incognita): Also attacks yams. Symptoms:  Knobbly tubers.  Tubers have more hairs than usual. Control methods: Do not plant infected material. Pests There are many pests for yams, including:  Rose beetle  Mealybugs and scale insects  Papuana beetle (lay eggs in stored yams and their larvae feed in the flesh).  Weevil (Elytroteinus)  Giant African snail (Achatina fulica). Uses and processing: Yams are a staple carbohydrate food, eaten as a vegetable, either boiled, baked or fried. Yams can be processed into fufu (a dough made from boiled yams which is pounded), flour and yam flakes. Unit 3: Tuber Crops 3.26  Activity 3.11 1. What are some cultural control methods for the ‘dieback’ or ‘anthracnose’ disease of yams? 2. What are the symptoms of ‘dry rot’ in yam tubers? 3. List some pests of yams. 4. List some processed products of yams Unit 3: Tuber Crops 3.27 3.3 SWEET POTATO 3.3.1 Introduction Botanical family: Convolvulaceae Genus: Ipomea Species: batatas Common names in the Pacific: Kaukau, kumala, umala, kumera, simala, umala. Origin : Central or South America, probably Mexico and has been cultivated in the Pacific since ancient times. It is now an important crop in PNG, Solomons, Vanuatu, Tonga and some islands of Micronesia. Uses: Human food and as an animal feed. 3.3.2 M o rp h o l o g y The plant is a vine. The above ground part is made up of a stem and leaves. Stem: The stem usually crawls on the ground, but some cultivars have climbing stems like yams. Length of the stem varies from 1-6 metres depending on the cultivar and growing conditions. There are also compact cultivars with shorter vines. Leaf: The leaf shape is variable, they maybe shouldered, toothed, entire, deeply cleft or variously lobed and the petioles vary in length, thickness and degree of erectness. Pigmentation also varies in colour from green to deep-purple. Roots: There is a main fibrous root system and many secondary root systems growing from nodes which have touched the soil. In some cultivars roots may also develop from the internode. Tubers: The sweet potato tuber is a root that is modified to store starch. (See figure 5.5) These characteristics of sweet potato depend on the cultivar grown:  Tuber skin colour: Can be white, tan, pink, red or copper.  Tuber flesh colour: Can be white, yellow, purple or a combination of these colours. Unit 3: Tuber Crops 3.28   Tubers have no dormancy period, but have apical dominance which is strongest immediately after harvest but deteriorates with storage. Figure 3.6: Ipomoea batatas Unit 3: Tuber Crops 3.29 3.3.3 Ecology Temperature: Sweet potato requires temperatures of 24oC or more for optimum growth. Growth is restricted by cool weather and plants are damaged by temperatures below 10oC. Latitudes: Sweet potato grows within latitudes 40oN to 32oS. However production system and cultivars vary and differ greatly within this range. Rainfall: An annual rainfall of 75-100 cm is best with low humidity as the crop reaches maturity. Sweet potato can tolerate considerable drought periods. It is more drought tolerant than all the other root crops except cassava. Yields are reduced if there is a water shortage within the first 50-60 days after planting when storage root initiation has begun. It is successfully grown under irrigation conditions; 4-8 irrigations supplying 112-150 cm of water is recommended. Soil: It can be grown on a variety of soils but well drained sandy loams reasonably high in organic matter are ideal. Heavier soils can be used if mounds or ridges are made to provide good drainage. Grows best in soil pH 5.6 - 6.6. Altitude: Sweet potato grows well at both low and high altitudes ranging from sea level up to 2100 metres. Day length: Most cultivars are sensitive to day length. Short days with low light intensity promote root development. Sweet potatoes yield highest in short days. South of the equator, short days are in the dry season: May, June, July and August. Flowering also occurs during this time. 3.3.4 Propagation Sweet potato can be propagated by tuber cuttings or by vine cuttings. In farmers’ fields, it is propagated using vine cuttings. To select good planting materials, the type, length, age and health of the vines must be considered. Type of planting material: Cuttings from the tips of the vines are best. Cuttings from the middle and base of the vine can be used but they will give lower yields. Cuttings from the base may carry weevils. Length of planting material: Generally 30-40 cm cuttings are recommended. Age of planting material: Vine cuttings from young plants (2-3 months) produce higher yields than cuttings from older plants (4-5 months old). At 4-5 months old, the plants are putting most of their energy into tuber production and the vine tips Unit 3: Tuber Crops 3.30 are weak and slow growing. The old vines are also less healthy with more disease and weevil than young vines. Health of the planting material: Vine cuttings should be taken from healthy plants. Some tips to follow to maintain healthy plants follow.  Avoid plants affected with leaf scab and little leaf disease.  Farmers should make sure that vine cuttings are free of weevil eggs and larvae.  If diseased material must be used as planting material, they must be treated with the right chemicals.  Planting material free of the leaf scab disease can be produced by sprouting the tubers.  Vine cuttings can be hardened by storing in a damp shady place for 1-3 days, so that roots will begin to grow on the cuttings allowing them to resist the 'shock' of planting. Activity 3.12 1. Which part of the sweet potato vine is best for taking cuttings? 2. What is the recommended length for a vine cutting? 3. At what age of the sweet potato plant should vine cuttings be taken? Unit 3: Tuber Crops 3.31 4. How are vine cuttings hardened before planting? 3.3.5 Developmental physiology of sweet potato Reading 3.3 Now turn to the Reading 2.3 by Lebot (2009) at the end of this unit. 3.3.6 Management of sweet potato Planting: Sweet potato can be planted on the flat or on mounds or ridges. Here in the Pacific, it is usually planted on mounds or ridges. Mounds or ridges are recommended when the soil is shallow or poorly drained. Growing sweet potato on the flat is recommended in a few locations where rats are a very serious problem. Time of planting: Sweet potato can be planted year round if rain and diseases are not limiting. In most areas of the Pacific, two crops can be produced in a year. However the dry season is usually higher yielding. In high rainfall islands, e.g. Samoa, reasonable yields are only obtained in the dry season. Sweet potato yields low in rainy season because:  Dry matter is partitioned for the development of the vines.  Leaf scab is severe in the wet season.  Long days during the rainy season and low oxygen content of the soil which is unfavourable for tuber formation. Spacing Ridges: Plant 1m between ridges and 40-50 cm between planting positions in the wet season and 30 cm in the dry season. At these spacings, one cutting is usually planted at one planting point. Wider spacing with more cutting per planting point Unit 3: Tuber Crops 3.32 may be used, e.g. in Tonga, 1m between ridges and 90 cm between planting points on the ridges with 3 to 5 cuttings per planting point. Mounds: Three to five cuttings are planted on each 1 m x 90 cm mound. These are general recommendations for spacing. Adjustments may be required according to the vine type. Spacing may also be closer in the dry season and further apart in the wet season. When growing sweet potato as a monocrop, the fully grown vine should cover the soil completely. If not, your spacing is too wide. Some recommended methods of planting:  Take a long cutting curled into a coil and bury the whole coil with just the tip of the vine above the soil.  Plant the cutting horizontally with both ends out of the soil and cover the rest of the vine.  Plant the cutting at a slant and cover half or two thirds of the cutting with a shallow layer of soil. Fertiliser: Fertiliser is only required if soils have been heavily cropped during the last few years and soil fertility is low. Some general recommendations for fertilisers are: Organic fertilisers: compost up to 40 t/ha chicken manure up to 10 t/ha other animal manure up to 20 t/ha coffee pulp (fresh or rotted) 15 - 30 t/ha Inorganic fertilisers: 40-50 kg/ha of Nitrogen (N) 0-15 kg/ha Phosphorus (P) 60-90 kg/ha Potassium (K) These fertilisers can be mixed into the soil at planting or half the fertiliser can be applied at planting and the other half applied at six weeks after planting. Unit 3: Tuber Crops 3.33 Hilling up: Hilling up is done by pushing or pulling up soil around the plant to form a mound over the main root system. It is done 2-2.5 months after planting. Hilling is done to:  reduce weevil damage by hiding the tubers from weevils;  reduce rat damage;  prevent roots from being exposed to the sunlight, (light prevents tuber formation); and  prevent tubers from being exposed to light, (sunlight discolours and affects the quality of the tuber. Weed control: Important in the first 2-3 months. Weeds are usually controlled by hand or chemicals. Gramoxone can be used between rows. Fusilade is used to control grass weeds. Harvesting: In the Pacific, most cultivars mature in 3-5 months. Harvesting is done by hand. However, if the field is level and stone-free, harvesting can be partially mechanised. Time to maturity can be affected by growing conditions, temperatures and altitudes. Avoid harvesting in wet weather. Yields: Average yields in the Pacific range from 4-30 t/ha. Curing and storage: As with yams, sweet potato should be harvested with care to avoid any damage to the tubers. The tubers should also be cured in the same way as with the yams before storage. After curing (4-7 days), tubers should be moved to a cool place. The same conditions required for the storage and transportation of yams apply to sweet potato tubers. Curing also gives the tubers a sweeter taste. Activity 2.13 1. Is sweet potato planted on the flat or on mounds in your area? Unit 3: Tuber Crops 3.34 2. Give reason(s) for your answer to question one. 3. Why are sweet potato yields lower in the rainy season than in the dry season? 4. Why is ‘hilling up’ important? 3.3.7 Diseases and pests Two most destructive diseases of sweet potato are: 1. Sweet potato scab caused by Elsinoe batatas. Symptoms: small scabby lesions or spots occur on the leaf; and leaves distorted and twisted and look silvery from a distance, and when severe, vine tips are killed. Control:  Plant disease resistant cultivars;  Use only clean planting materials;  Crop rotation;  Isolate plantings from infected fields;  Use chemicals, Benlate and Manzate. Unit 3: Tuber Crops 3.35 2. Little leaf caused by a mycoplasma-like organism. Symptoms:  leaves much smaller than usual, rounded and yellow;  internodes are much shorter;  plant grows more upright;  there are more fibrous roots under ground and on the stem;  yields are reduced by as much as 50%; and  small tubers. Control:  Use clean planting material.  Practise good sanitation - rogue and burn diseased plants. 3.3.8 Uses and processing Sweet potato is used primarily for human consumption. It is eaten as a vegetable, after boiling, baking or frying. It is also canned in the USA and used in a variety of products like pie fillings, purees, candied pieces, souffles and baby foods. It is also processed into:  animal feed  starch  flour Activity 3.14 1. What are the two most destructive diseases of sweet potato? Unit 3: Tuber Crops 3.36 2. How are these diseases controlled? You have now finished Unit 3. Congratulations you are now about half way through the course! Unit 3: Tuber Crops 3.37 Reading 3.1 Cassava: Developmental Physiology in V. Lebot 2009. Tropical Root and Tuber Crops (Lebot). Wallingford: Oxfordshire. Pp 39-48 Unit 3: Tuber Crops 3.38 Unit 3: Tuber Crops 3.39 Unit 3: Tuber Crops 3.40 Unit 3: Tuber Crops 3.41 Unit 3: Tuber Crops 3.42 Unit 3: Tuber Crops 3.43 Unit 3: Tuber Crops 3.44 Unit 3: Tuber Crops 3.45 Unit 3: Tuber Crops 3.46 Unit 3: Tuber Crops 3.47 Unit 3: Tuber Crops 3.48 Reading 3.2: Yams: Developmental Physiology in V. Lebot 2009. Tropical Root and Tuber Crops (Lebot). Wallingford: Oxfordshire. Pp 229-238 Unit 3: Tuber Crops 3.49 Unit 3: Tuber Crops 3.50 Unit 3: Tuber Crops 3.51 Unit 3: Tuber Crops 3.52 Unit 3: Tuber Crops 3.53 Unit 3: Tuber Crops 3.54 Unit 3: Tuber Crops 3.55 Unit 3: Tuber Crops 3.56 Unit 3: Tuber Crops 3.57 Unit 3: Tuber Crops 3.58 Reading 3.3: Sweet Potatoes: Developmental Physiology in V. Lebot 2009. Tropical Root and Tuber Crops (Lebot). Wallingford: Oxfordshire. Pp 127-136 Unit 3: Tuber Crops 3.59 Unit 3: Tuber Crops 3.60 Unit 3: Tuber Crops 3.61 Unit 3: Tuber Crops 3.62 Unit 3: Tuber Crops 3.63 Unit 3: Tuber Crops 3.64 Unit 3: Tuber Crops 3.65 Unit 3: Tuber Crops 3.66 Unit 3: Tuber Crops 3.67 Unit 3: Tuber Crops 3.68 Unit 3: Tuber Crops 3.69 References 1. Lebot, V. 2009.Tropical root and tuber crops: cassava, sweet potato, yams and aroids. CABI: Wallingford. Call No. SB 210.T76 L43 2009. 1 copy in Pac & 1 copy in Main. 2. Onwueme, I.C., & Sinha, T. D. 1991. Field Crop Production in Tropical Africa: Principles and Practice. CTA. Call. No.: SB 123.C485 O65. 1 copy in Main. 3. Gerlach, W.P. 1988. Plant diseases of Western Samoa. Samoan German Crop Protection Project: Apia. Call No.: SB 723.S3 G4. 1 copy in Pac. 4. Purseglove, J.W. 1972. Tropical crops: monocotyledons. Longman: Harlow, Essex. Call No.: SB 111.P872 1972. 1 copy in Reserve. 5. Purseglove, J.W. 1968. Tropical crops: dicotyledons. Longman Scientific and Technical: New York. Call. No.: SB 111.P8721 1968. 1 copy in Reserve and 1 copy in Main. 6. Bokanga, M. 1999. Casava: Post-harvest operations. FAO & International Institute of Tropical Agriculture (IITA): Nigeria. Electronic Document: http://www.fao.org/3/a-au998e.pdf 7. Nelson, S.C. & Elevitch, C. 2011. Farm and forestry production and marketing profile for Sweet Potato (Ipomoea batatas). Permanent Agriculture Resources (PAR): Hawaii. Electronic Document: http://www.agroforestry.org/images/pdfs/Sweetpotato_specialty_crop.pdf 8. Opara, L.U. 2003. Yams: Post-Harvest operation. FAO & Massey University: New Zealand. Electronic Document: http://www.fao.org/publications/c ard/en/c/848b3a59-3a1e-47d9-b1f3- 18a294e0ab95 9. Onwueme, I. 1999. Storage and processing of roots and tubers in the tropics. FAO, Thailand. Electronic Document: http://www.fao.org/docrep/x5415e/x5415e00.htm Unit 3: Tuber Crops 3.70