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PROLOGUE
m e d i a
l t d
Funded by
European Union
Good agronomic practices for
Sweet-potato in Western Kenya
Training manual for Trainers
Good agronomic practices for
Sweet-potato in Western Kenya
Training manual for Trainers
2011
Editor: Daniel Nyambok
Co-editor: John Robins Oyia
Coordination and co-editor: Flavio Braidotti
This work is licensed under a
Creative Commons Attribution Non-Commercial No Derivatives 2.5 Italy License.
To view a copy of this license, visit: http://www.cefaonlus.it/biblioteca/biblioteca.asp
Printed in 2011
Table of Contents
Acknowledgements ................................................................................................................................... 1
1. Introductions ........................................................................................................................................... 2
1.1 Importance of sweet potatoes ............................................................................................... 2
1.2 Botanical characteristics ............................................................................................................. 3
1.3 Cultivars / Variety ........................................................................................................................... 3
2. Ecological requirements ...................................................................................................................... 7
2.1 Temperature ..................................................................................................................................... 7
2.2 Rainfall .............................................................................................................................................. 7
2.3 Soil ...................................................................................................................................................... 7
2.4 Aeration ............................................................................................................................................. 7
2.5 Growing period .............................................................................................................................. 7
3. Field establishment ............................................................................................................................... 8
3.1 Propagation of planting materials .......................................................................................... 8
3.2 Land preparation ........................................................................................................................... 8
3.3 Planting ............................................................................................................................................. 9
3.4 Spacing ............................................................................................................................................ 9
4. Field managementt practices ............................................................................................................ 10
4.1 Weed control ................................................................................................................................... 10
4.2 Fertilizer application ..................................................................................................................... 10
4.3 Crop rotation and sweet potato ............................................................................................... 11
5. Harvesting ................................................................................................................................................. 11
6. Storage ....................................................................................................................................................... 13
7. Sweet potato pests ................................................................................................................................ 13
7.1 Sweet potato weevil: ................................................................................................................... 13
7.2 Sweet potato white fly (Bemisia tabaci) ............................................................................... 14
7.3 Butterfly larvae / caterpillar ....................................................................................................... 15
7.4 Sweet potato vine borer ............................................................................................................. 15
7.5 Mole ................................................................................................................................................... 15
8. Sweet potato diseases .......................................................................................................................... 16
8.1 Potato mosaic disease: ............................................................................................................... 16
8.2 Fusarium surface rot and fusarium root rot ......................................................................... 16
8.3 Black scurf ......................................................................................................................................... 16
8.4 Black rot ............................................................................................................................................. 16
8.5 Leaf rust ............................................................................................................................................. 16
9. General control of sweet potato pests & diseases ......................................................................22
10. Nutrient deficiency symptoms ........................................................................................................22
11. Post harvest handling .........................................................................................................................23
12. Market requirement for sweet potato .......................................................................................... 23
13. Record keeping .....................................................................................................................................23
14. Utilization ............................................................................................................................................... 24
15. Gross margin ......................................................................................................................................... 24
Bibliography ................................................................................................................................................. 25
List of Tables
Table 1 Sweet potato varieties and their characteristics –
South Nyanza Western Kenya.................................................................................................. 5
Table 2 Crops for rotation recommended and not ....................................................................... 11
Table 3 Agents causing diseases and their symptoms ................................................................. 16
Table 4 Important records to be kept ................................................................................................. 23
Table 5 Gross Margin per Acre of Sweet Potatoes (South Nyanza – Western Kenya) ........ 24
List of Figures
Fig 1 - A cow feeding on Sweet Potato vines.................................................................................... 2
Fig 2a - Sweet potato plant and its parts ........................................................................................... 3
Fig 2b - Sweet Potato stems and leaves ............................................................................................. 3
Fig 2c - Sweet Potato Fields .................................................................................................................... 3
Fig 3 - Sweet potato tubers ..................................................................................................................... 4
Fig 4a - KSP 20 (Kuny Kibuonjo)............................................................................................................. 6
Fig 4b - Mugande (Amina) ..................................................................................................................... 6
Fig 4c - Kemb 23 (Nyamigori)................................................................................................................. 6
Fig 4d - Kemb 20 (Nyamisambi)............................................................................................................. 6
Fig 4e - Kemb 10 (United)......................................................................................................................... 6
Fig 5 - Farmers making ridges using oxen ......................................................................................... 9
Fig 6 - A farmer preparing mounts ....................................................................................................... 9
Fig 7 - Some weeds found in Sweet Potato fields:
Amaranthus & Ivy, Black Jack, Lion’s ear ............................................................................... 10
Fig 8 - Sweet potato field ready to be harvested ............................................................................ 11
Fig 9 - A healthy sweet potato at harvest ......................................................................................... 12
Fig 9 - Whole sale harvesting ................................................................................................................ 12
Fig 10 - Farmer building a store for fresh sweet potato roots with dry grass ....................... 13
Fig 11 - Sweet Potato weevil and damage by weevil on tubers ................................................ 13
Fig 12 - Sweet Potato Whitefly ............................................................................................................... 14
Fig 13 - Butterfly larvae causing leaf damage (left)
and effect of Caterpillar on sweet potato leave (right).................................................. 15
Fig 14 - Sweet Potato tubers affected by Mole ............................................................................... 15
Fig 15 - Sweet potato plant affected by viruses
(Note discoloration and wrinkles on the leaves) ............................................................. 17
Fig 16 - Cross section of roots with symptoms of fusarium root rot (left)
and fusarium surface rot (right) ............................................................................................ 19
Fig 17 - Tubers affected by black scurf ............................................................................................... 19
Fig 18 - Internal and external views of roots infected by black rot .......................................... 20
Fig 19 - Roots with black rot lesions (left and right) and uninfected roots (centre) ........... 21
Fig 20 - Sweet Potato leave affected by Leaf Rust fungal ............................................................ 21
Good Agronomic Practices for Sweet-potato in Western Kenya
Acknowledgement
This manual was prepared during the implementation of the EU-funded project
called “Strengthening Product Value Chain of Agro Business Enterprises in
South Nyanza” (2009-2012) implemented and co-funded by CEFA in partnership
with C-MAD.
The generous financial support provided by the European Union to implement
this project and to enable this manual to be produced is highly appreciated and
acknowledged.
Particular thanks to the Field Officers who contributed to the preparation of this
manual: Ms Nohla Achieng Onyuna, Ms Ednah Anyango Okello, Mr Kennedy
Ogoma.
Thanks to Marco Dalla Costa for his tireless work he has done in processing
and editing the pictures of different varieties of Sweet Potato took in the field
during the period that he worked voluntarily as intern in the above mentioned
project.
Acknow ledg em ent
Special thanks to the Agricultural Training Centre (ATC) in Homabay and
particularly to its Principal, Erick Odhiambo Adel, for his feedback and for
significant help in the preparation of the gross margin analysis.
1
Good Agronomic Practices for Sweet-potato in Western Kenya
1. Introduction
Generally sweet potato is a perennial crop but sometimes grown as annual
crop.
It is an important crop in low altitude areas in most parts of Western Kenya
regions. However the crop does well even in high altitude areas.
1.1 Importance of sweet potatoes
• Sweet potato is an important food crop- grown and consumed by majority
of Kenyans. Usually it is consumed by low-income consumers but being
embraced also by high income consumers nowadays;
• Excess is marketed to supplement family income;
• The crop can be produced under marginal conditions such as low soil
fertility and low soil moisture supply where other crops except cassava fail;
• The stems and leaves are used to feed livestock. The tender stems and
leaves are particularly suitable due to their high nutritional value;
• Tender young leaves are consumed as vegetables;
• Orange Flesh Sweet Potato (OFSP) contains vitamin A. Vitamins A is
important in building the body immune system against diseases and
blindness;
Int r odu ction
Source: Kapinga R et all (2009)
• It takes relatively short time to reach maturity. Meaning it can be grown
twice a year;
Fig. 1 – A cow feeding on Sweet Potato vines
Note: The green potato tubers & potato sprouts contains some chemicals
known as glocoside solanine that is poisonous to both animal and humans.
2
Good Agronomic Practices for Sweet-potato in Western Kenya
1.2 Botanical characteristics
Source: CEFA (2010)
(a) Stem & leaves. Sweet potato has long and thin stem that runs along the
soil and put roots in to the soil at the nodes. The stem length may vary
depending on the cultivars and ranges between 1-6 meters. Size and
shape of the leaves change according to the cultivars. See Table 1 for
more details.
Intr oducti on
Fig. 2a – Sweet potato
plant and its parts
Source: Marco Dalla Costa (2010)
Source: Kapinga R et all (2009)
Fig. 2b - Sweet Potato stems
and leaves
Fig. 2c – Sweet Potato Fields
(b) Roots: They normally develop from the stem cutting within (1-2) days
and grow rapidly to form the fibrous root system of the plant. The roots
can go as deep as 2m that is what make sweet potato to be a drought
resistant crop.
As the vines grow along the soils, roots are produced at various nodes.
(c) Tubers: These are produced as a result of secondary growth of the
roots within the top soil height of 20-25 cm which are assimilated and
structured to form tubers.
Tuber formation is normally affected by environmental conditions. High light
initiate tuber formation.
3
Source: Kapinga R et all (2009)
Good Agronomic Practices for Sweet-potato in Western Kenya
Fig. 3 - Sweet potato tubers
Intr oducti on
The composition of the tubers is as follows:
• Water 70- 80%
• Carbohydrates 10-30%
• Protein 1-3 %
• Crude fiber 2-3%
• Fats 0-1%
4
Good Agronomic Practices for Sweet-potato in Western Kenya
There is large number of Sweet Potato varieties. The main characteristics of the
varieties found in South Nyanza (Western Kenya) are provided in the table and
pictures below.
Table 1: Sweet potato varieties and their characteristics – South Nyanza Western Kenya
Variety
(traditional
name in
Dholuo)
Int r odu ction
5
Characteristics
Colour of Colour of Shape of Colour of Yielding Tolerance Tolerance Maturity
tuber skin flesh
leaves
leaves ability
to
to
period
drought diseases
Kemb 10
(United)
Cream
Yellow
Narrow &
divided
Light
green
High
Moderate
Moderate
3-4
months
SPK 004
(Nyathi
Odiewo)
Red/ pink
Orange
Very
narrow &
divided
Green
with
slight
purple
edges
Very high
Very high
Moderate
4-5
months
SPK 013
(Kalamb
Nyerere)
Red
White
Broad &
divided
Purple
High
Moderate
Very high
3-5
months
KSP 20
(Kuny
Kibuonjo)
Red
White
Narrow
and
divided
Green
High
High
Moderate
3-4
months
Kemb 20
(Nyamisambi)
Red
Yellow
Broad and
divided
Green
High
Moderate
High
4-5
months
Kemb 23
(Nya
Migori)
Red
Yellow
Broad and
divided
Purple
High
Moderate
Moderate
3-5
months
Mugande
(Amina)
Red
Yellow
Broad
heart leaf
Green
Very high
Moderate
Moderate
4-6
months
Mwavuli
(Olombo
Japidi)
Red
White
Broad
heart
umbrella
Purple
High
Very high
Moderate
3-5
months
Nyawo
Red
Yellow
Heart leaf
Green
Very high
High
Moderate
3-4
months
Salyboro
(Lodha)
Cream
Orange
Narrow &
divided
leaf
Green
High
Moderate
Moderate
4-5
months
Source: Marco Dalla Costa (2010)
4b) Mugande (Amina)
Source: Marco Dalla Costa (2010)
4a) KSP 20 (Kuny Kibuonjo)
Source: Marco Dalla Costa (2010)
Source: Marco Dalla Costa (2010)
Good Agronomic Practices for Sweet-potato in Western Kenya
4d) Kemb 20 (Nyamisambi)
In trod ucti on
Source: Marco Dalla Costa (2010)
4c) Kemb 23 (Nyamigori)
4e) Kemb 10 (United)
Fig. 4 - Pictorial characteristics of some sweet potato varieties available in South
Nyanza (Western Kenya)
6
Good Agronomic Practices for Sweet-potato in Western Kenya
2. Ecological requirement
Sweet potatoes are adaptable to different agro ecological zones ranging from
0-2100m above sea level and occasionally are found in altitudes of about
2400m.
2.1 Temperature
It requires warm hot weather and does well in high light intensity. Long day
length affects negatively flowering while short day length promote flowering.
Avoid shading of the crop or under canopy. The crop doesn’t tolerate frost.
2.2 Rainfall
It requires rainfall of 750-1000mm per annum.
It is drought resistant crops even during drought vines remain green & healthy.
2.3 Soil
Well drained sandy loam soil is the best. Avoid clay soil and poor water logged
because the crop cannot withstand water logged condition. It requires
moderate soil pH of 6.0 for optimum production.
2.4 Aeration
Clay and water logged soils affect negatively tuber formation. Inadequate water
especially during the stage of tuber development retards its development.
2.5 Growing period
Ecological Req uir eme nt
Sweet potatoes have a growing period of 3-6 months depending on the variety.
However in the short rains farmers prefer those maturing in 3 months and in
the long rains those maturing in 6 months.
7
Good Agronomic Practices for Sweet-potato in Western Kenya
3. Field Establishment
3.1 Propagation of planting materials
Tubers or vines can be used as planting material for propagation.
Characteristics of good planting material are as follows:
•
•
•
•
No sign or symptoms of pest attack;
No sign or symptom of diseases;
The leaves are still green and strong;
2-3 months old.
(a) Tubers – When used for planting they are known as setts obtained from
healthy robust tubers. These are better than using standard runts (small tubers).
The setts should be between 25cm-30cm long. Planting should be done 3 cm
deep. Setts normally give low yields. For this reason setts are mostly used to
propagate vine formation.
(b) Vine cutting
This method is the most recommended because:
• It is free from soil born diseases;
• Gives more yields;
• Tubers produced are of uniform shapes & size.
When using vine cutting the part preferred are those from the apex than the
middle or basal part of the stem.
Cuttings can be up to 30cm of length. Cuttings shorter than 30 cm result in
poor yield.
There may be shortage of vine during planting time. This problem can be
solved by establishing a small nursery plot & maintaining it to the next season.
In order to encourage sprouting plant sprouts regulators are used e.g Eitapan or
Ethapam but this mainly apply in large scale production.
3.2 Land preparation
• Seed bed should be prepared before the beginning of the rains.
• Sweet potato can be grown on ridges or mounts or on flat land.
8
Fi el d Est abl ishme nt
Selected tuber can also be used in order to establish vines by growing on moist
beds. Once sprouted, you can pull them & plant them. Sprouting can start within
2 weeks & pulled after 3-4 weeks and used as a planting materials.
Good Agronomic Practices for Sweet-potato in Western Kenya
• Planting on flat land leads to low yield. Ridges are encouraged because they
give high yields.
• Ridges can be up to 30cm high. The higher the ridge the higher the yield.
3.3 Planting
• Planting should be on
ridges and is mostly
recommended because it
gives higher yields.
• Sometimes planting is
done on small mounts. It
is best recommended for
swampy areas or where
water table is high to avoid
risk of water logging in root
zone of the crop.
Source: Kapinga R et all (2009)
• This is done after land
preparation by making
ridges or mounts.
Fig. 5 - Farmers making ridges using oxen
Source: Kapinga R et all (2009)
When planting, it is good to
take precaution against the
pest by dipping the planting
material in Diaznon pesticide
before planting.
The vines should be inserted
at an angle and ½ or 2/3 of
the vine should be buried
in the soil. Note that the
vine should not be inserted
vertically but at an angle.
Field Establ ishme nt
Fig. 6 - A farmer preparing mounts
After planting continue with field management.
3.4 Spacing
Spacing can be 150cm or 90cm apart between the ridges and 30-60 cm within the
ridge depending on the environmental condition and cultivars. Trellising stems
(stems that grow long) are planted wider apart than those that do not trellis
much. Spacing affects yields which can also depend on cultivars, environmental
condition and earthing.
9
Good Agronomic Practices for Sweet-potato in Western Kenya
4. Field management practices
4.1 Weed control
Source: Marco Dalla Costa (2010)
Source: Marco Dalla Costa (2010)
Source: Marco Dalla Costa (2010)
it is very necessary during the first 2 months after planting because during this
time the plant growth is slow. After the 2nd month, the crop is vigorous and
there is usually an effective coverage of the surface that suppresses the weeds.
However, some common weeds such as lion’s ear, black jack and amaranthus
may continue to grow even after first weeding and these should be hand rouged
periodically until the crop is harvested.
Fig. 7 –Some weeds found in Sweet Potato fields: Amaranthus & Ivy, Black Jack, Lion’s ear
4.2 Fertilizer application
Sweet potato does well in fertilized soil and respond well to fertilizer. 500Kg/
ha of DAP could be used but only for deficient soils after soil analysis has been
carried out. Fertilizer should be applied at the time of planting.
Note: Excess nitrogen normally delays tuberisation.
10
Fi el d m anag em ent prac tice s
Manure can also be used (farm-yard manure). Make sure that manure is mixed
thoroughly and deeply in the soil. This is because when tubers get into contact
with manure, the black scab disease will develop.
Good Agronomic Practices for Sweet-potato in Western Kenya
4.3 Crop rotation and sweet potato
Sweet potato can be grown alone or intercropped with other crops.
Table 2 – Crops for rotation recommended and not
Crops for rotation
Recommended
Not recommended
Maize
sorghum
Finger millet
Beans
Cow peas
Soyabeans
Simsim
Cassava
Irish potato
5. Harvesting
Source: Kapinga R et all (2009)
Field m anage men t practi ces/
Har vest in g
Sweet potato harvesting may be ready within 3-5 months depending on
cultivar and environmental conditions although some varieties are ready within
2-4 months (see table 1 for detail for each cultivar).
11
Fig. 8 - Sweet potato field ready to be harvested
Good Agronomic Practices for Sweet-potato in Western Kenya
Sweet potato can be harvested in two ways:
a) Piece-meal harvesting: it starts 3-4 months after planting using
sharp sticks to remove big tubers. This type of harvesting is done
when there is need (for consumption) and the tubers cannot be
stored for a long time.
b) Whole sale harvesting (part of the whole garden): This system is
mainly used for commercial purposes when sweet potatoes have to be
sold in the market. Make sure therefore to minimize damage, cuts to
tubers. Damaged roots easily rot or fetch low price in the market. Signs
of weevil damage will also affect the price.
Whole sale harvesting is traditionally done using a hoe. However when the ridges
system is adopted, it is strongly advisable to use oxen plough along the ridges to
open the soil and collect the tubers afterwards. This method of harvesting results
in less damage to the tuber and it is cheaper since it is less labour intensive.
For piece-meal harvesting, make sure you return the soil as young tubers may
be affected.
For whole sale harvesting, prior pruning is required by removing sweet potato
canopy 14 days before harvesting. In this way the amount of water/milky fluid
in the tuber is reduced (curing) and this allows the tubers to be stored for long
after harvesting.
In general for harvesting follow these indicators:
• Harvest when the soil is moist or wet to avoid bruising the roots;
• Readiness is indicated by yellowing and drying of the lower leaves. Early
harvesting results in low yields. Late harvesting on the other hands results
in tuber becoming fibrous and coloured and attacked by pests or prone to
rotting ;
• Not all tubers reach the maturity at the same time. Presence of tubers is
indicated by cracking of the soil.
Source: Kapinga R et all (2009)
Source: Kapinga R et all (2009)
Har ve sting
Fig. 9a - A healthy sweet potato at harvest
Fig. 9b - Whole sale harvesting
12
Good Agronomic Practices for Sweet-potato in Western Kenya
6. Storage
Source: Kapinga R et all (2009)
Source: Kapinga R et all (2009)
Tubers can be left in mounts and harvest piecemeal or can be harvested and
stored in a pit or clamp stores. Place fresh sweet potato in a bed of dry grass.
Fig. 10 - Farmer building a store for fresh sweet potato roots with dry grass
7. Sweet potato pests
7.1 Sweet potato weevil
Adult weevils feed on leaves, the underground storage roots (tubers) and the
vines of sweet potatoes.
Fig. 11 – Sweet Potato weevil and damage by weevil
on tubers
13
Source: Marco Dalla Costa (2010)
Source: Kapinga R et all (2009)
Storag e
Sweet p otato pests
Symptoms: A symptom of infestation by sweet potato weevils is yellowing,
cracking and wilting of the vines, but a heavy infestation is usually necessary
before this is apparent. Damage by weevils can be recognised by the holes
in the vines or the tunnels in the tuber when you pull them up from the soil.
Attacked tubers become spongy, brownish to blackish in appearance.
Good Agronomic Practices for Sweet-potato in Western Kenya
Host plants:
Sweet potato. Minor host plants: Coffee, maize, cowpea, sesame.
Control:
• Crop rotation: Avoid planting sweet potatoes in the same area for two to
three successive seasons.
• Plant early or plant early maturing varieties. This will allow harvesting before
the end of the growing season e.g. Kemb 10 and SPK 013.
• Carry-over of the weevils from an infested crop to the new planting could be
reduced by carefully selecting fresh cuttings for planting a new crop.
• Planting away from weevil-infested fields, and or use barrier crops such
as cassava, maize, bananas or sorghum planted around the perimeters in
stripes of at least 3 to 5 m in width between fields to restrict movement of
weevils between fields.
• Avoid or minimise cracks in the soil. Soil cracks are the major route of weevil
access to roots. This can be done by:
1. Planting cuttings deep in the soil and using of deep-rooted cultivars
reduce weevil damage.
2. Ridging: it prevents the soil from cracking by hilling the area around the
plant.
3. Mulching: mulches conserve soil moisture and minimise soil cracking.
• Remove and destroy (through burying, burning or feeding to livestock) any
crop residues left in the field after harvest.
• Early harvesting -Harvest the crop as soon as it has developed roots of
acceptable size.
Note: Cultivars resistant to sweet potato weevil are not available.
Sweet pot ato pests
The females lay eggs on the
underside of the leaves where they
spend the majority of their lives.
When leaves on a host plant are
rustled, the sweet potato whiteflies
flutter up, and then settle back
down onto the plant. Damage on
plants includes yellowing of leaves
accompanied by curling and stunted
growth.
Source: arbico-organics.com (http://www.arbicoorganics.com/product/sweet-potato-whitefly/)
7.2 Sweet potato white fly (Bemisia tabaci)
Fig. 12 – Sweet Potato Whitefly
14
Good Agronomic Practices for Sweet-potato in Western Kenya
7.3 Butterfly larvae / caterpillar
Source: Marco Dalla Costa (2010)
Source: Kapinga R et all (2009)
Butterfly damage to sweet potato is through their larvae which eat the leaves.
Fig. 13 - Butterfly larvae causing leaf damage (left) and effect of Caterpillar on sweet potato
leave (right)
7.4 Sweet potato vine borer
The butterfly larvae may penetrate the vines causing damage and eventual
sudden withering of the whole plan.
7.5 Mole
Fig. 14 – Sweet Potato tubers affected by Mole
15
Source: Marco Dalla Costa (2010)
Source: Marco Dalla Costa (2010)
Sweet pot ato pe sts
These are rodents which live in the underground but come on the soil surface
to search for food. Moles eat and cause heavy damage to sweet potato tubers
resulting into heavy loses to the farmer. During harvest affected plants should
be separated as they are further sources to other predisposing diseases like the
surface rot thereby causing damage to the entire harvest.
Good Agronomic Practices for Sweet-potato in Western Kenya
Control: there are rodenticides that have been developed to kill moles which
can be bought from local vet stores. However, some farmers have succeeded
controlling moles by laying traps.
8. Sweet potato diseases
Below are reported different agents causing various diseases and their
symptoms on the sweet potatoes.
Table 3: Agents causing diseases and their symptoms
Disease causing agents
Symptoms
Swellings or cracks on the storage root
Nematodes
Swellings on the fibrous roots
Death of large portions of the root system
Black areas, often roughly circular in the roots
Fungi
Spots or powdery areas in the roots
Large groups of filaments in the roots
Wounds-at the surface
Bacteria
Root rotting
Dead plant parts
Stunted growth
Viruses
Pale green leaves with clear veins
Distorted and curled leaves
Pigmented leaves (mainly purple colour)
Reduced production of storage roots
Source: Kapinga R et all (2009)
Sw ee t potat o di sease s
Fig. 15 - Sweet potato plant affected by viruses (Note discoloration and
wrinkles on the leaves)
16
Good Agronomic Practices for Sweet-potato in Western Kenya
8.1 Potato mosaic disease
Potato mosaic disease is caused by several species of viruses. It is the most
common sweet potato disease.
Symptoms: Affected plants show a variety of pigmentation on leaves ranging
from pale green yellow leaves to purple coloration. This leads to poor food
development in the leaves resulting in reduced production of storage tubers
and stunted growth of whole plant. Affected plant leaves are also distorted and
curled.
Control: It is controlled by using clean planting materials, resistant cultivars
e.g. SPK 013, removal and burning of infected plants in the field. In addition
chemicals such as 0.1% Carbaryl, 0.1% Tenthion and 1% Hebtachlor can be used
for disease control.
Swee t pot ato dise ases
(http://www.aces.edu/pubs/docs/A/ANR-0917/)
Source: aces.edu
8.2 Fusarium surface rot and fusarium root rot
Fig. 16 - Cross section of roots with symptoms of fusarium root rot (left)
and fusarium surface rot (right)
Fusarium surface rot and Fusarium root rot are caused by species of the fungus.
Symptoms: Surface rot occasionally occurs prior to harvest on roots that have
been mechanically injured, split by growth cracks, or damaged by nematodes,
insects, or other soil pests. Lesions on fleshy roots are circular, light to dark
brown, firm, and dry. Decay remains shallow, usually not extending beyond
the root’s vascular ring. Externally, lesions appear solid brown and are often
cantered on a broken rootlet. When infected roots are stored for an extended
17
Good Agronomic Practices for Sweet-potato in Western Kenya
period, the tissue around lesions dries and becomes shrunken, and the roots
eventually become hard and mummified.
Fusarium root rot may be difficult to distinguish from Fusarium surface rot by
external symptoms. In some cases, surface rot may be an early stage of the
more aggressive root rot. Root rot lesions are circular and commonly exhibit
light and dark brown round rings. Internal rotting extends may eventually affect
the entire root. This pattern distinguishes root rot from surface rot.
Species of Fusarium that cause surface rot and root rot can persist in soil for
many years. Infection in the field may occur through rootlets or growth cracks.
More commonly, however, roots contaminated with the fungus are invaded
through wounds that occur during harvest. Surface rot or root rot that develops
during storage spreads to other stored roots only when new wounds are made.
Surface rot is prevalent when sweet potatoes are mechanically harvested,
when soil is wet and cold at harvest or excessively dry prior to harvest (causing
increased skinning of sweet potatoes), when sweet potatoes are exposed
to high or low temperatures for extended periods after digging and prior to
curing, or when conditions are favourable for desiccation of wounded tissue.
8.3 Black scurf
Also known as soil-stain, scurf is also caused by the fungus. Damage from the
disease is primarily superficial but still detracts from market value.
Symptoms of scurf begin during the growing season as small, dark brown to
black spots develop on roots and later merge to form irregular lesions. Lesions
enlarge until the entire sweet potatoes usually have brown lesions, and redskinned sweet potatoes have black lesions.
18
Sw ee t potat o di sease s
Control:
• To reduce Fusarium root rot, control surface rots. If surface of tuber is rotten,
discard the tuber.
• Use sanitary practices and properly handle harvested roots. These are the
most effective control procedures for both pathogens.
• Minimize injury during harvesting and handling, especially if the crop is
lifted from wet soil.
• Cure roots immediately after harvest. (Cure roots at temperatures between
29.4°C to 35°C and at 29.4°C to 32°C percent relative humidity for 5 to 10
days).
• Reduce surface rot in the field by controlling root-knot nematodes and
insects that can rupture the skin of sweet potatoes.
• Reduce spread of Fusarium root rot to sprouts in transplant beds by planting
disease-free roots treated with fungicides.
• To avoid transporting the stem canker phase of Fusarium to the field, cut
transplants above the soil line.
(http://www.aces.edu/pubs/docs/A/ANR-0917/)
Source: aces.edu
Good Agronomic Practices for Sweet-potato in Western Kenya
Fig. 17 - tubers affected by black scurf
Swee t pot ato dise ases
Symptoms: are restricted to the skin of storage roots and do not directly affect
the underlying tissue.
Affected tissue can be easily scraped off.
Cracks may develop on severely affected sweet potatoes and result in shrinkage
because of water loss. Scurf-infected sweet potatoes are more susceptible
to invasion by other fungi. Scurf lesions continue to enlarge when sweet
potatoes are put into storage, and new lesions appear if high relative humidity
is maintained. The optimum temperature for disease development is 24°C,
but scurf can develop to a lesser extent over a wide range of temperatures.
Disease development is greatest when soil moisture is optimal for plant growth.
Most scurf infections result from the use of infected propagating material. The
fungus can also survive in crop debris in the soil for 1 to 2 years.
Disease severity is greater and persistence of the pathogen longer in finetextured, highly organic soils. The disease has a narrow host range that consists
only of species in the genus Ipomoea.
Control:
• Use only scurf-free, fungicide-treated sweet potatoes as seed roots. Bed
these in soil free of the disease.
• Cut transplants at least 1 inch above the soil line, and dip them in a
fungicide.
• Grow sweet potatoes after a 3- to 4-year rotation with other crops.
8.4 Black rot
Black rot is caused by the fungus. The disease can cause significant losses in
storage, in the transplant bed, and in the field. The pathogen not only reduces
yield and quality but also gives sweet potatoes a bitter taste.
19
Good Agronomic Practices for Sweet-potato in Western Kenya
Black rot may develop on sweet potatoes during transit or in the marketplace.
Entire lots may become infected as the fungus spreads quickly to roots
surrounding a rotting sweet potato. As a result, entire crates of roots may be
quickly destroyed in storage. Insects, such as the sweet potato weevil, may also
transmit the disease in storage. Development and spread of the disease is rapid
at temperatures greater than storage temperatures (12.7 -15.5 0c).
(http://www.aces.edu/pubs/docs/A/ANR-0917/)
Source: aces.edu
The black rot fungus can produce tremendous numbers of spores during
storage. These can contaminate washing machines, crates, and structures as
well as the hands of workers. Contaminated items or dip tanks can serve as
sources of fungal inoculum for new infections. Using contaminated equipment
or washing and packing roots infected with black rot before curing may spread
the disease.
Fig. 18 - Internal and external views of roots infected by black rot
Symptoms: Small, circular, slightly sunken, dark brown spots are the initial
symptoms of black rot. Spots enlarge and appear greenish black when wet and
greyish black when dry.
20
Sw ee t potat o di sease s
Within the spots there are small black fungal structures (perithecia) with long
necks which appear to the naked eye as dark bristles. The rot usually remains
firm and shallow. If secondary fungi or bacteria invade the tissue however, the
flesh beneath the spot turns black, and this blackened area may extend to the
centre of the root. Tissue near the discoloured area may have a bitter taste.
Eventually, the entire root may rot. Roots may appear healthy at harvest but
rot in storage, during transit, or in the market. The fungus survives in the soil in
crop debris. Infected storage roots escape detection at harvest or bedding. The
fungus either colonizes the young shoots or infects the stem. Transplants and,
subsequently, the main stem and daughter roots are thereby infected. When
slips are pulled for transplanting, the stem carries the pathogen along with the
plant.
(http://www.aces.edu/pubs/docs/A/ANR-0917/)
Source: aces.edu
Good Agronomic Practices for Sweet-potato in Western Kenya
Fig. 19 - Roots with black rot lesions (left and right) and uninfected
roots (centre)
Control:
• Control black rot with crop rotation, since most crops are unaffected by the
disease.
• Disinfect seedbeds if a clean site is unavailable.
• Propagate plants from healthy stem cuttings.
• Apply a postharvest fungicide.
• Do not wash and package roots showing symptoms of black rot.
• Decontaminate equipment that comes into contact with an infected crop.
• Fumigate storage structures.
8.5 Leaf rust
Symptoms: White pustules containing
disease spores develop on the undersides
of leaves causing yellow or brown
spotting on the plant leaves. While the
rust causes cosmetic damage, the sweet
potatoes aren’t affected.
Control: Use clean planting materials.
Practice crop rotation.
21
Source: Marco Dalla Costa (2010)
Swee t pot ato dise ases
Sweet potato leaf rust is caused by
a fungus. The disease affects sweet
potato leaves and vines during rainy wet
seasons.
Fig. 20 – Sweet Potato leave
affected by Leaf Rust fungal
Good Agronomic Practices for Sweet-potato in Western Kenya
9. General control of sweet potato pest and diseases
Use integrated method of pest and disease control e.g. clean disease free vines,
early planting, crop rotation, and rouging infested plants.
The following on sweet potato cultivation practices should be avoided because
they favour the spread of diseases:
1.
Planting sweet potato fields close to each other will speed the
transmission of diseases between crops.
2.
Planting the sweet potato fields at intervals that are not regular, so
that new crops find old ones not harvested. The old field can be the
source of diseases.
3.
Rouging should be carried out i.e. uprooting of infected plants
4.
Piece-meal harvesting allows the sweet potato crop long duration in
the field, which in turn allows more time for the disease to progress in
big quantities.
5.
Leaving the leaves and storage roots in the old field after harvesting
because they can grow again into sick plants and start spreading the
diseases.
In the case of sweet potato it is advised to use:
•
•
•
•
Only approved pesticides.
Farmers must have the knowledge to apply pesticides.
Farmers must take all records regarding pesticides used.
Farmers must dispose off surplus spray mix in an orderly manner.
10. Nutrient Deficiency Symptoms
Phosphorus: Young leaves tend to be dark colour; older leaves appear yellow
on the surface and purplish under surface.
Potassium: Tubers are thin, yellowing of leaves, followed by browning and
necrosis along the leaf margin .
Calcium: Leaf appear to be light green.
Iron and magnesium: Result in interval chlorosis.
22
General control of potato pest and
diseases/ Nutrient deficiency symptoms
Nitrogen: The plant is stunted; the leaves tend to turn light green and turn
yellow finally.
Good Agronomic Practices for Sweet-potato in Western Kenya
Boron: Restriction of plant terminal growth of the shoot and production of very
short inter nodes, older leaves turns yellow on the margin.
Note: Excess nitrogen delays tuber formation.
11. Post harvest handling
After harvesting, dip the roots in water and wash them thoroughly but carefully
to avoid bruising the skin. Sorting should be done according to colour, size and
shape. The tubers should be dried under the shade.
12. Market requirement for sweet potato
•
•
•
•
•
•
•
•
•
The skin of the tubers should be intact and retains characteristics of the
variety.
No pest or disease damage.
Should be relatively uniform in size.
Must not be bruised.
Tubers must not be deformed.
Should be packed dry.
Must be devoid of dust, mud or any other foreign material.
Harvested tubers should be kept under shade to avoid turning of skin
colour.
The produce should reach the market at most three days after the
harvest.
13. Record keeping
Keep all record right from planting to harvest e.g. date of planting, number
of mound/ridges per acre, variety planted, source of planting material, area
planted, pests and diseases identified, yield per acre.
Table 4 – Important records to be kept
Post ha r vest han dl i ng /
Recor d keepin g
variety
planted
23
date of
planting
number of
mound/ridges
per acre
source of
planting
material
area
planted
pests and
diseases
identified
yield
per acre
Good Agronomic Practices for Sweet-potato in Western Kenya
14. Utilization
• The fresh tubers can be boiled or steamed and then smashed.
• Dried and ground to produce flour which is used as supplements porridge.
• Can be processed to produce -bread, crisps, biscuits, cakes.
15. Gross margin
Note: Different production level (level 1, 2 and 3) means farmers adopting all best
good agronomic practices (level 3), some essential good agronomic practices
(level 2) and poor adoption of GAP (level 1).
Bags : Assumption were made of bags used with capacity of 50 Kgs and packed in
1 and 1/2 bags. The total weight of a 1 ½ packed bag of sweet potato is assumed
to be 50 Kg.
Table 5: Gross Margin per Acre of Sweet Potatoes (South Nyanza – Western Kenya)
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Good Agronomic Practices for Sweet-potato in Western Kenya
Bibliography
Elske van de Fliert and Ann R. Braun (1999). Field Guides and Technical Manual. Farmer
Field School for Integrated Crop Management of Sweetpotato. International potato centre.
Bogor, Indonesia.
Kapinga, R., Byaruhanga, P., Zschocke, T., & Tumwegamire, S. (2009). Growing orangefleshed sweetpotato for a healthy diet. A supplementary learners’ resource book for upper
primary schools. International Potato Center (CIP), Kampala, Uganda. 142 pp.
Bibl iograp hy
PRAPACE. (2004), Sweet Potato varieties for good security, health, local and export markets
- Training manual, Sweet potato coalition project, Kampala, Uganda .
25