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Sweet Potato Cultivation and Post Harvest Handling

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by mohamad on October 29th, 2010 at 03:59 am
helo, thank you
by MARY on May 09th, 2012 at 05:37 am
Very Detailed and Educative.
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INTRODUCTION
The sweet potato (Ipomoea batatas L. (LamK) seemed to have originated in Tropical America, but, the
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exact location is not known. This crop is now grown throughout the tropics for its edible tubers, which
are an important food source in many countries. Guyana is one of the tropical countries in which sweet
potato is cultivated. The Regions of highest production in Guyana, are Regions 3, 4, and 9, but it is also
W grown in Regions 2, 5 and 6.
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Sweet potato is primarily used as food. Generally it is consumed directly- the main type of preparation
being boiling, baking or frying. In processed form, the tubers can be prepared in many ways for human
consumption (canned, frozen, dehydrated or made into flour or starch).
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For industrial uses, the tuber is a source of starch, glucose, syrup, vinegar and alcohol. The tuber and
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plant tops are also a source of animal feed. The tuber is fed directly or in processed form and the leaves
are fed to livestock as fresh fodder or in the form of silage.
VARIETIES
There are several sweet potato cultivars in production in Guyana. Some of these are Black Rock, Strong
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man and Viola.
ENVIRONMENTAL REQUIREMENTS
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Sweet potato is adapted to rainfall ranging from 800 – 1000 millimeters per year but it requires good
drainage. This crop cannot stand water logging and it will not persist in long periods of drought without
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irrigation.
This crop can be grown from sea level to elevations of 2000m. It can also be grown on a wide range of
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soils. In Guyana, sweet potato is mainly grown on soils rich in organic matter, on loamy soils and sandy
soils.
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CULTIVATION
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Land Preparation
Land preparation should be deep enough to accommodate the sweet potato tubers. Since this crop
cannot withstand waterlogged conditions, there must be adequate drainage.
The soil should be ploughed and harrowed and adequate drains made. For lighter soils flat planting can
be done but heavier soils should be ridged to facilitate drainage.
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Planting
Planting material for sweet potato is the slip (stem) of mature plants. The apical portion of the stem
should be used.
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This apical stem cuttings should be about 25 cm long. In order to control pests during the initial stages
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of growth, planting material should be treated by soaking in an insecticidal solution for approximately
ten minutes. The insecticides recommended are Admire at a rate of 1ml / L of water or Vydate at a rate
of 2ml/l of water.
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Ridges should be 0.75 m apart and cuttings should be planted 0.3m apart.
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MAINTANENCE OF CULTIVATION
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General Maintenance
Drains must be properly maintained during rainy periods since sweet potato cannot withstand
waterlogging.
Fertiliser Application
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As a general rule, before applying fertilizers, the soil should be analysed to determine the types and
amounts to be used. If a soil analysis is not done the following fertilizer elements may be applied:
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Fertilizer
Rate of Application
Urea
168 kg/ha
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TSP
150 kg/ha
MOP
140 kg/ha
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Apply the TSP and MOP to the ridges one or two days before planting. Apply all the Urea six weeks after
planting, when the vines are turned to prevent formation of small tubes at the nodes (joints) of the stems.
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Weed Control
Weed control is important during the early stage of this crop before the vines begin to spread. For a
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good crop, the weeds need to be controlled during the first six to eight week after planting.
Turning of Vines
It is customary to turn back the vines from time to time to prevent rooting at the nodes of the plant.
This is to ensure a more even crop and fewer smaller tubers.
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Major Pests and Diseases of Sweet Potato (Ipomoea batatas) in Guyana
Sweet potato is the second most widely grown root tuber in Guyana, after yams and cassava. The crop
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is largely produced under traditional, low input technology and consequently, often suffers poor quality
yields. Much of this is due to inadequacies in crop management practices.
W Economical damage by diseases, pests and weeds of sweet potato is relatively moderate, although sweet
potato weevil can be a menace in some regions, if the problem is not identified early, and remedial action
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not implemented in a timely manner.
Insect Pests of Sweet Potato (Ipomoea batatas) in Guyana
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1. Sweet Potato Weevil
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(Cylas formicarius)
Among the 300 insect and mite species that feed on sweet potato in the tropics and subtropics, only
sweet potato weevil (Cylas formicarius) is the most destructive insect pest (Figure 1). No resistant
source is available.
A major problem in Guyana.
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Sweet potato
Sweet potato weevil
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weevil
damage
Fig 1. Different stages of the sweet potato weevil and damage caused
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Cultural Control:
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Integrated pest management for this insect is recommended, consisting of the following measures:
• Crop rotation;
• Eradication of Ipomoea weeds;
• Use of clean planting material;
• Deep planting;
• Regular hilling to fill soil cracks around plants; and
• Use of sex pheromone which is effective to trap male weevils;
Chemical Control:
• Pre-plant slip treatment using Triazophos 40%EC; and
• Soil treatment using Basudin 60%EC once monthly.
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2. Vine Borer
Symptoms
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(Omphisa anastomosalis)
O. anastomosalis adults are white with a brownish-yellow pattern on the wing. They lay slightly domed,
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greenish eggs with a flat base, on the upper and lower surfaces of the leaf and on petioles. The eggs are
usually laid singly and egg incubation lasts for about one week.
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Soon after hatching, the larvae bore into the stems and gradually eat
their way down the vines. Full-grown larvae are 30 mm long and
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light purple, although they may also be yellowish-white. The head
capsule is brown, the ventral surface and legs are white, and the
back and lateral sides have yellowish-brown grooves. (Figure 2).
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The larval period usually lasts 30-35 days, but may vary between 21
and 92 days depending upon temperature. Pupation usually takes
place in the vine but larvae may also bore into storage roots and
pupate when roots are close to the soil surface.
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Fig. 2- Larvae of the vine borer
The adults are active at night. Both males and females mate at 1-6 days old.
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Cultural Control:
• Integrated Pest Management; and
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• Crop rotation
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Chemical Control:
• The use of Sevin and/or Decis gives effective control of O. anastomosalis, increasing the yield
of treated plants.
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• Systemic insecticides such as Carbofuran are applied to the soil in the vicinity of the main stem.
The chemical is translocated, killing the insect larvae boring in the stem, but does not come into
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contact with natural enemies. However, treatment with Carbofuran is barely economical because
the price of sweet potato roots in local markets is low.
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3. Cricket
Gryllotalpa spp. (Orthoptera: Gryllotalpidae)
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Acheta spp. (Orthoptera: Gryllidae)
Mole crickets, which have heavily sclerotised front legs that are adapted for digging, are usually common
W in sandy soils.
All crickets are nocturnal, feeding at night and secluded by day, under the soil. They feed at or slightly
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below the soil surface, and can cause considerable damage before being
discovered.
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Crickets spend their entire life cycle below the soil, which may go
through a period of approximately 28 -35 days. They are termed soil
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insects.
Sympton
Crickets eat tubers of sweet potatoes. Fully grown crickets are brown
in colour and are about 2.5 – 3.5 cm long (Figure 3). The various
species of these insects usually live either in the soil, bushes and
Fig. 3 – Adult Cricket
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under decaying crop residues and vegetation.
Cultural control:
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• The areas where sweet potatoes are grown should receive full sunlight, kept clean of weeds
and all crop residues should be removed and burnt.
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• Proper land preparation serves to control weeds, diseases, and soil insects, and also helps in
the destruction of large soil clods, which act as hiding places for cricket.
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Good field sanitation- rid the field of weeds and plants residues from previous crops.
Chemical control:
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• Any approved soil insecticide at the recommended rate may be applied, such as Basudin 60%
E.C (Diazinon) or Vydate L 40%E.C at the rate of 10 mls to 4500 mls water to cultivated
areas.
O 4. White Flies
Bemisia tabaci (Homoptera: Aleyrodidae)
These insects are in fact bugs. The adults are white moth-like insects (Figure 4) that fly upwards from
the plant when disturbed. They are about 2 mm in length and their wings are
covered with a white waxy powder.
The pinhead size nymphs are oval and flattened and are attached to the leaf
surface until maturity. All stages of this pest can be found on the underside of
leaves. Nymphs and adults feed by sucking plant sap, resulting in leaves
becoming mottled, yellow and brown before dying. Feeding whiteflies excrete
honey dew on the leaf surface which encourages the growth of sooty mould,
thus hampering photosynthesis. Ants are also attracted to the honey due.
This pest is also a vector of viral diseases. The life cycle may be completed
Fig. 4 - Whiteflies
in about 28-35 days.
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Cultural Control:
• Do not plant a new crop next to one which is mature: The common practice of having
mature crops adjacent to newly planted ones makes management of the pest very difficult
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since the cycle of the pest is never broken.
• An integrated control strategy is necessary for the effective management of this pest.

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Good farm sanitation, including removal of weeds around the cultivation is necessary
since weeds serve as hosts for the pests.
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Chemical Control:
• Several new generation insecticides are now available for the effective control of white flies.
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Targeting both nymphs and adults with soap based products should be applied very early in the
morning or late in the evening. Other chemicals which may be used include Admire, Pegasus and/
or Basudin and Vydate L at 10 mls to 4500 mls water.
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Diseases of Sweet Potato (Ipomoea batatas) in Guyana
1. Fusarium Wilt
(Fusarium oxysporum)
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Symptom
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The disease is caused by fungi that persist in the soil for long periods of time. Symptoms include a dark
tanned rot that extends into the root and usually has a sharp line separating
the rot from the healthy portion of the root. Stem infections appear as swollen
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areas at the base of the stem that have internal vascular discoloration (Figure
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)
.
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Cultural Control:
• The disease can be avoided by practicing two-year rotations out
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of sweet potatoes and by using clean planting material.
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Chemical control:

Fig 5. Field showing signs
Use Carbendazime, Cuprasan and Rizolex for effective control.
of Fusarium Wilt
2. Soil Rot
(Streptomyces ipomoea)
Symptom
Roots develop black necrotic lesions which traverse its entire cross section
(Figure 6). Usually the necrotic portion of these roots breaks off when the
roots are dug, leaving a small portion of the lesion at the broken end of the
root. Vine growth may be severely stunted with lower leaves bronzed or
yellow and the plants wilted. Lesions on roots are necrotic, black, roughly
circular in outline, and often have cracks radiating from the centre.
Fig. 6 – Symptoms of Soil Rot
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Cultural Control:
• Phytosanitary measures are important to prevent introduction of Streptomyces ipomoea into
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non-infested soils.
• Cultivating equipment should not be moved from infested fields to non-infested fields and /or
should be disinfested before movement into a new area.
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• Storage roots from affected fields should not be used for producing transplants.
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Chemical Control:
• The use of Banrot, Rizolex, Captan or Mankocide is recommended for effective control. These
can also be used as a pre-plant drenches.
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3. Black Rot
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(Ceratocystis fimbriatum)
Symptom
Tubers with this disease show irregular sunken spots which are dark brown to black in colour. Very often
black dots are scattered across the spots (Figure 7). These black dots contain the spores of the fungus
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causing the disease. Pink masses of the spores are sometimes seen oozing from the black dots.
Cultural Control:
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• Use disease free sprouts or tubers for planting;
• Practice crop rotation;
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• Use extra care in storage of planting materials- ‘slips’ and tubers;
and
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• Practice good farm sanitation.
Chemical Control:
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• The use of fungicides such as Captan, Manzeb and Rizolex is
recommended.
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Fig. 7 – Symptoms of
4. Root-knot Nematode
Black Rot
(Meloidogyne spp.)
Symptoms
Underground symptoms
are primarily galls on roots, tubers, abnormal formation and function of the
root system (Figure 8).
Non-specific above-ground symptoms include patchy stunted growth,
discolouration and leaf chlorosis, excessive wilting during dry, hot
conditions, stunting of whole plants, reduced yield and quality, and
sometimes premature death. Root-knot is generally more severe in sandy
soils and under adverse environmental conditions.
Fig. 8 – Gall formation due to
Root-knot nematode
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Cultural Control:
• Practice good farm sanitation;
• Practice crop rotation; and
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• Flood fallow
Biological Control:
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• Plant marigold flowers, tulsi, neem in and around cultivated areas
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Chemical Control:
• Use any approved nematicide
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5. Viral Diseases
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Symptom
Viral disease on sweet potato is caused by a synergistic combination of sweet potato feathery mottle
virus (SPFMV) and sweet potato chlorotic stunt virus (SPCSV).
Chlorotic dwarf virus is caused by the synergistic interaction of three viruses: two potyviruses, SPFMV
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and sweet potato mild speckling virus (SPMSV), and a crinivirus (SPCSV).
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SPVD is transmitted by grafting. The individual viruses are transmitted
by their insect vectors.
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Symptoms vary with plant genotype but typically include severe stunting
of plants, the production of small leaves (Figure 9) which are often
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distorted, narrow and crinkled and with a chlorotic mosaic and/or vein-
clearing, giving affected plants an overall pale appearance.
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Whole plant: dwarfing.
Fig. 9 – Symptoms of
Leaves: abnormal colours; abnormal patterns; abnormal forms.
Viral Disease
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Major vectors:
Aphid and whitefly
Cultural Control:
• Use of disease-free planting material and crop rotation are the most reliable means of controlling
these diseases and its insect pest’s vectors.
• Use of resistant varieties.
Chemical Control:
• Spraying with insecticides such as Fastac, Decis or Karate on the undersides of the leaves to kill
aphids.
• Spraying insecticides such as Vydate L, Newmectin and Admire is an effective control measure
for whiteflies.
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Harvest Maturity Indices
Sweet potatoes do not mature as fruits do, but continue to grow as long as conditions allow. Harvest
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time is determined by market price, expected total yield, and root size. Sweet potatoes should be harvested
when the majority of roots have reached the desired size for the intended market. This typically requires
between 3 to 3.5 months from the time of transplanting. Average root size in a field can be estimated by
W removing the soil
around several randomly selected plants. Sweet potatoes will continue to enlarge if left in the ground,
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but root diseases and insect damage typically increase with the amount of time the roots remain in the
soil.
Harvest Method
The sweet potato vines should be cut off at the soil level prior to the intended harvest date (Figure 10).
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During the dry season, the vines should be removed three to seven
days before digging. During the rainy season, the
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vines should be left intact until just prior to harvest. Vine removal
helps to toughen the skin of the root and facilitates harvesting.
The vines can be removed manually with a scythe or machete, or
mechanically with a rotary mower.
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After vine removal, the sweet potato roots can be dug by hand or
by machine. Manual harvesting of sweet potatoes typically involves
the use of a metal spade, pick, or fork which is used to loosen the
Fig 10. Vines should be removed before
O soil and undercut the roots (Fig 11). Care must be taken to avoid digging to allow the skin to toughen.
cutting or
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injury to the roots. The roots are then lifted out of the ground,
separated from the main stem, and temporarily left on top of the
soil or put directly into a field container.
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The roots should be handled gently to avoid skinning and bruising.
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Freshly dug sweet potatoes have a very thin and delicate skin that
is easily removed (Fig 12). Skinned areas of the root surface are
open wounds and become entry sites for bacterial and fungal
O pathogens. Skinning also lowers the appearance and attractiveness Fig 11. Manual harvest of sweet potatoes
of the root. Workers should be advised not to throw or step on
with a hand-made fork.
the roots. A range of mechanical harvest devices also exists for sweet
potatoes.
These include mouldboard plows, middle buster plows, and single or
multiple row diggers. Mechanical harvesters require the vines to be
removed prior to digging. Mouldboard plows turn the soil and roots
over on top of the ground and produce the least amount of physical
damage to the roots. However, they leave many roots covered by soil
that makes them difficult to recover.
Middle buster plows (usually 30-35 cm size) with broad wings, may be
used, although they tend to damage the roots and scatter them on both
sides of the row. It is essential to operate the plow accurately on the row,
at the proper depth, and at the correct speed.
Fig 12. Skinned roots are
unattractive and susceptible to
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