ABSTRACT

This module covers

introduction to Irish Potato
Production, varieties, soil,
climatic requirement seed
production, planting,
irrigation, weed, pest and
disease control, physiological
disorders as well as harvesting
of irish potatoes

IRISH POTATO AUTHOR

Chipo Muswehaurari,
Lecturer Chibero College Of

PRODUCTION
Agriculture, Zimbabwe.
(Solanum tuberosum)

BY MUSWEHAURARI CHIPO
Table of Contents
1.0 INTRODUCTION..................................................................................................................... 3
Objectives........................................................................................................................................... 3
1.1 Botany...................................................................................................................................... 3
1.2 Development stages of potatoes............................................................................................. 5
1.3 Nutritional value........................................................................................................................... 6
1.3 Uses of potatoes........................................................................................................................... 7
1.4 External quality of potatoes.....................................................................................................7
1.5 Varieties.................................................................................................................................. 7
2.0 POTATO GROWING CONDITIONS........................................................................................ 8
2.1 temperature requirements...................................................................................................... 8
2.2 Rainfall requirements..............................................................................................................8
2.3 Soil requirements..................................................................................................................... 8
3.0 LAND PREPARATION..............................................................................................................9
4.0 SEED PREPARATION.............................................................................................................10
4.1 Seed size and spacing.............................................................................................................10
4.2 Conditioning the seed............................................................................................................ 11
4.2.1 Storage............................................................................................................................ 11
4.3 Sprouting................................................................................................................................ 12
4.3.1 Forced sprouting............................................................................................................. 12
4.3.4 De-sprouting................................................................................................................... 13
4.3.5 Dipping of Tubers for control of skin diseases...............................................................13
4.5 Cutting and dressing...............................................................................................................13
5.0 FERTILIZER MANAGEMENT............................................................................................... 14
5.1 Macronutrients:.......................................................................................................................... 14
5.2 Trace elements........................................................................................................................... 15
5.3 Timing and Application of Fertilizers...........................................................................................15
5.4 Standard suggestions for potato fertilization are:..................................................................16
6.0 POTATO PLANTING...............................................................................................................17
6.1 Time of planting..................................................................................................................... 18
6.2 Methods of Planting...............................................................................................................18
7.0 IRRIGATION OF IRISH POTATOES...........................................................................................19
8.0 CULTIVATION.................................................................................................................................. 19
8.2 Ridging....................................................................................................................................19

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9.0 WEED CONTROL.................................................................................................................... 20
10.0 PEST CONTROL......................................................................................................................... 21
10.1.1 Nematodes......................................................................................................................... 21
10.2 Insect Pests......................................................................................................................... 22
11.0 POTATO DISEASES.............................................................................................................26
11.1 Late blight........................................................................................................................... 26
11.2 Early blight..........................................................................................................................26
11.3 Potato Viruses X ( mosaic)..................................................................................................27
11.3 Potato Virus Y (PVY)............................................................................................................28
11.5 Black scurf and stem canker...............................................................................................29
11.6 Common Scab.....................................................................................................................29
11.7 Silver Scurf.......................................................................................................................... 30
11.8 Bacterial Wilt...................................................................................................................... 31
11.0 PHYSIOLOGICAL DISORDERS OF IRISH POTATOES......................................................35
11.1 Black heart..........................................................................................................................35
11.2 Hollow heart....................................................................................................................... 35
11.3 Internal browning............................................................................................................. 36
11.4 greening.................................................................................................................................... 37
11.5 Tuber cracking.................................................................................................................... 38
12.1 Haulm destruction.................................................................................................................... 38
12.2 Harvesting time.................................................................................................................. 38
12.3.2 Mechanical digging............................................................................................................ 39
13.0 Storage....................................................................................................................................... 40

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1.0 INTRODUCTION
Irish potatoes (Solanum tuberosum) are one of Zimbabwe’s most popular vegetables. The edible part
of the plant is an underground stem called a tuber (not a root). Potatoes are an important food crop for
food security as well as an important cash crop that contributes to the GDP. As Zimbabweans, there
is need to grow potatoes profitably so that we uphold the country’s vision 2030 of becoming an upper
middle class economy and contribute to the agricultural growth strategy of improving food and
nutritional security.
The production process include: sprouting, planting, weeding, pests and disease control, harvesting
and storage.

Objectives.
By the end of module readers should be able to:
1. Prepare a potato production programme for various regions of the country.
2. Practically manage potato production plot with special emphasis on
a. land preparation
b. sprouting procedures
c. planting proceedures
d. fertilizer management
e. irrigation management
f. weed management
g. pest and disease identification, prevention and control
h. Identification of potato disorders and how they are controlled.
i. Mastering various potato harvesting techniques and storage.

1.1 Botany
Potatoes are dicotyledonous short-lived perennials that are cultivated as annuals for their edible
enlarged underground tubers.

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Fig 1.1 A potato plant showing one stem. (Source) Alberta, Food and Rural Development)

Haulm- Comprises of the leaves and stems. Leaves have 3 - 5 pair of leaflets. Above ground stems
are erect and are initially smooth and become angular and branched with continued growth.
Root system- Potato is a relatively shallow rooted crop (40 - 50 cm) hence requires good water and
nutrient supply. Asexually propagated plants develop relatively shallow fibrous spreading
adventitious roots. Plants grown from true seed form a slender taproot with many laterals.
Tuber & stolon-Tuber is a shortened, thickened, fleshy stem with leaves reduced to scales or scars
known as 'eyes'. This part of stem is adapted for storage & reproduction. Tubers are formed on the
stolons off the stem above the planted seed. The tubers have buds which may vary in numbers. Each
eye has 2-3 buds. Each bud is capable of producing a stem.. The skin has many lenticels. Contain
dormant buds. Conditions favouring tuberisation also favour flowering
Bud- It Give rise to shoot known as the sprout. Sprout is a shoot which grows into a plant identical to
its mother.

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Flowers- Clustered in a primary cymose inflorescence. Has five-lobed fused corolla with colours
ranging from white, pink, blue or purplish lilac. Flowers are nectarless and therefore cross-pollination
mainly occurs by wind although insects can also pollination. Some varieties may not flower however
development has no major influence on the tuber.
Fruit- Fertilised flowers produce small spherical green or purplish berries which are 1.5 - 2cm in
diameter & can contain 50 - 100seeds. Berries are toxic due to the glycoalkaloids they contain.
1.2 Development stages of potatoes
The development of potatoes can be broken down into four distinct growth stages:

Fig 1.2 Growth stages of potatoes. Source: Google pictures.

Growth Stage I: Sprout Development
This stage begins with sprouts developing from the eyes and ends at emergence from the soil. The
seed piece is the sole energy source for growth during this stage. There is a lot of biological activity
happening in the tuber as it starts to sprouting.
Well sprouted seed will ensure an optimum stem density for high yields and good tuber size
distribution. Not all sprouts planted will develop into main stems,
.
Growth Stage II: Vegetative Growth
This stage, in which all vegetative parts of the plants (leaves, branches, roots and stolons) are formed,
begins at emergence and lasts until tubers start to develop. Growth stages I and II last from 30 to 50
days depending on soil temperature and other environmental factors, the physiological age of the
tubers, and the characteristics of particular cultivars.

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Growth Stage III: Tuber Set/Initiation
Growth Stage III, tuber initiation or tuberization starts when the ends of stolons begin to swell.
Tubers form when the plant produces more carbohydrates than are required for haulm growth.
Varying weather and moisture conditions cause uneven tuber set and growth. Tubers are forming, but
are not yet enlarging. This stage will last approximately 2 weeks. In most varieties it will coincide
with first flowering. The canopy is still forming and foliage is still increasing at a very fast rate.

Growth Stage IV: Tuber Bulking

Tuber cells expand and divide with the accumulation of water, nutrients and carbohydrates.
Vegetative growth slows down as most of assimilates are sent to the tubers. Tuber bulking is the
growth stage of longest duration. Depending on cultivar, bulking can last up to three months.
Growing conditions must be maintained close to optimum to get good yields.
Fluctuating temperatures, moisture, and nitrogen, especially in the formative stage, can cause
malformed tubers.
Uneven growth of the potato plant leads uneven growth of tubers, can result in abnormalities in tuber
shape. Uneven growth caused by variable climatic conditions is worsened by poor plant stand and
variability between plants caused by variable seed-piece types, sizes and spacings.

Growth Stage V: Maturation

Vines turn yellow and lose leaves, photosynthesis gradually decreases, tuber growth rate slows down
and the vines die. The crop becomes very susceptible to blights. The tubers must have reached
marketable size and skin starts to hardening.

1.3 Nutritional value

Irish potatoes contains 80% water, 2% protein, 1% fibre, 17% carbohydrates. They have high levels
of vitamin C but Low in Phosphorous, Calcium, Iron, Vitamin B2 ,B1 and B 6

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Fig 1.3 Nutrient content of potatoes per 100g Source: United States Department of Agriculture,
National Nutrient Database

1.3 Uses of potatoes

Potatoes are mainly used for production of chips, flour, they can be eaten as boiled or stewed,
used in salad preparation. By-products like starch and alcohol(vodka)can be obtained from potatoes.
Potato vine and tubers can be fed to livestock in Europe and Russia.

1.4 External quality of potatoes

The size can be determined by planting density. Medium sized potatoes of 4-7,5cm are preferred. The
shape-Can be round or oval. Oval most preferred and this is cultivar related. The depth of eye -
shallow eyed potatoes are the best as the eyes are easily plucked out without loss of flesh. Greening is
not preferred because of solanin a poison.

1.5 Varieties
The most common types of Irish potatoes are table and processing potatoes. In Zimbabwe a number
of varieties are available locally at the market. There are early/medium season varieties maturing 3-
4months and long varieties which take 5months to mature.
Montclare
It is a very high yielding variety, producing medium quality tubers, which tend to be
large and of poor shape with deep eyes. It is late maturing variety which is tolerant to drought and
resistant to Late Blight but is susceptible to virus diseases. Poor storage quality and is an uneven
sprouter. The variety produces medium quality tubers with poor shape with deep eyes.
BPI
It is a medium-early variety, taking 3-4 months in the ground. The variety has white flesh, hard skin,
good oval shape and very high yielding. BP1 is fairly resistant to Late blight. And is an even sprouter

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Pimpernel
It is a late maturing variety, medium yield, and red skinned, yellow fleshed tubers for
‘chip’ production. It has good keeping quality, good field resistance to Late blight and is fairly
tolerant of virus diseases.
Inyanga Amethyst
It is a late maturing variety with white purple flowers. The variety is white fleshed flat oval, shallow
eyed, white skin with slight russet and quick sprouting. Inyanga Amethyst is consistently high
yielder, both in summer and winter with good resistance to Late blight.
Garnet
It is a late maturing commercial variety (17 -19weeks). It produces thin haulms with narrow leaves
and white flowers. It also produces white skinned and yellow fleshed round and medium sized tubers.
Garnet combines high yield and good crisping qualities therefore can be used for processing. It is a
good yielder, +/- 26t/ha. Can be produced both under rain fed and irrigated plantings. It has high
tolerance to late blight.
Jasper
It is a late maturing variety with upright haulms. It has white skinned and white fleshed round to oval
tubers with shallow eyes. The skin has some roughness but the tubers have a good appearance. It has
some tolerance to late blight and frost. The yield is +/- 30t/ha especially as an irrigated winter or
spring crop.
Diamond
Diamond is a medium to early maturing variety with good tuber distribution and high yields. It has a
vigorous growth and an open canopy.
It has moderate tolerance to late blight but very susceptible early blight.
Other varieties that are available on the market are:
Opal, mondial, favorite and Emerald

2.0 POTATO GROWING CONDITIONS

Potato growing requires specific temperatures, rainfall and soils. These are to be discussed below.
2.1 temperature requirements
Irish potato originated in Andes Mountain in South America. Day should be worm or tuber formation
is affected resulting in lower yields.
Minimum temperature for tuber formation is 15-20°C.
At temperatures above 30oC tuber formation becomes very poor. Potatoes are very sensitive to frost.
Frost kills all the aerial parts, therefore at planting aim at avoiding frost incidences.
Earl planting is February or March for winter crops so that frost can come in July when the crop is
mature. Late planted crops are planted in late July to August to avoid frost in early July. For best

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production, potatoes need full sun.
2.2 Rainfall requirements
Irish potato require well-distributed rain especially at tuber initiation, then light frequent showers
inter spaced with good sunlight periods for good yields. Drought spells during growth accompanied
by high temperature affect yields.
2.3 Soil requirements
They do best in a loose, well drained, slightly acid soil. It is a shallow rooted quick growing crop,
which grow best in high porous soils with sufficient well decomposed organic matter. Soils should be
well drained with good water retention capacity. Sand loams to sand clays are best because the roots
will easily penetrate and tubers are well formed with good shape and appearance. Clay soils
sometimes make root penetration difficult and misshape the tubers. Poorly drained soils often cause
poor stands and low crop yields. Heavy soils can cause the tubers to be small and rough. This crop
can be planted in all soil types if root penetration is not affected and adequate fertilizers are applied.
Soil pH of 4.2 -5.8 CalCl2 can give very good yields. The optimum pH is 5.0- 5.5. Low pH Inhibit
availability of nutrients e.g. magnesium and phosphorus. High pH exposes the crop to common scab
there by reducing the quality of tubers.

3.0 LAND PREPARATION

Before ridging, remove the stones, trash, and large sticks from the soil. Plough the soil 20 to 30 cm
deep turning the earth over to cover all plant material. Where the soil is shallow it is not advisable to
plough to 300mm as it turns subsoil. If the pH is low, lime at first discing. Re-plough two months
before planting and then disc harrow to produce a suitable tilth. Ploughing lands in a hurry a short
while before planting leads to crop failure. If green manuring precedes a potato crop. Ploughing
should be done a few months before planting or if trash is incorporated. Ploughing should also be
done to allow decomposition of organic matter. Prepare ridges 25cm to 30 high and 90cm apart.
Ridging is vital for drainage.

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 Fig 3.1 Land prepared for planting potatoes

4.0 SEED PREPARATION

Unlike most other vegetables, Irish potatoes are not grown from seed. Instead, pieces from the potato
itself start new plants. Buy good seed potatoes that are free of disease and chemicals.
Do notbuy potatoes from supermarkets for planting. The seed potato contains buds or “eyes” that
sprout and grow into plants. Large tubers sprout faster than small tubers.
Cutting allows large tubers to be used economically. Some varieties produce round tubers which have
eyes of uniform size and produce similar plants. Other varieties with long tubers have eyes
concentrated at the rose end. In such cases it is not recommended to cut the tuber across but
alongside. Round to medium tubers should be used whole.
Small and medium certified tubers should be used as a whole. Only large tubers should be cut.
Cutting is done on unsprouted tubers to prevent or minimize the damage of sprouts. Cutting with a
knife will transmit blackleg, bacterial wilt, and mosaic virus therefore disinfect the knife in a weak
solution and hands in copper sulphate. After cutting store the tubers for 10-14 days at temperatures of
7-18oC and relative humidity of 100%. The tubers will heal and produce a cocky layer over the cut
surface, then will not shrink or rot anymore.
Practically pieces can be pilled on the ground or layer of reeds or grass and cover with wet sack and
water twice a day for 14 days. Place on which potatoes are hipped should prevent loss of humidity.

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 Very little air is required provided the room is not air tight then there is natural conditioning.
After curing the seed pieces may be stored and treated as whole tubers. The difference between the
cut surfaces are more fragile therefore cut pieces should not be carried in bags but in trays. Cut pieces
can also be planted immediately after cutting if the soil is moist and temperatures at 7-18 oC.
In this case cutting be done on sprouted tubers if the soil is moist they will heal. If the soil is dry and
temperatures are higher the pieces will decay.
4.1 Seed size and spacing
Seed from small tubers produce fewer stems and produce larger tubers. There is a tendency of getting
numerous small tubers when large seed potato tubers are used.
Therefore small seeds can be planted closer in the row than larger seed. Where a seed crop is grown
either use large seed at a closer spacing to get a (proper) proportion of small tubers. Ideal tubers are
60-70g for table potatoes production. Standard spacing is 900x300mm (37000 plants/ha) Seed is
supplied in 30kg packs. Seed can be small, medium, or large.
Small 1200 tubers/pocket
Medium 500 tubers/pocket
Large 250 tubers/pocket

It is possible or every farm to produce own seed if he observes the following:

i. Start with the highest quality seed.
ii. Choose site away from other potato crops 200m or more.
iii. Plant as early as possible before any other potatoes crop.
iv. Apply systematic aphicides like Dimethoate to control aphids.
v. Rogue out grasses three times before the potato crop blooms to maintain a weed free
crop to reduce the host plants for pests and diseases..
vi. Remove all plants with symptoms of viral disease and off type plants.
vii. Discard all the potatoes on the outer ten rows since they are exposed to late generation
of aphids.
viii. Never use uncertified seed of unknown origin.

Certified seed should always be used in the production of potatoes.

a) AA grade- only produced under stringent conditions in Nyanga. Plants showing deviations from
normal are removed. The seed is expensive to produce and available from June -July
b) A grade-produced from AA grade and it is produced by potato Producers Association The seed is
produced anywhere in Zimbabwe where conditions are ideal and is available throughout the year.

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c) Grade XX-Production is the same as AA. The tubers are subjected to limited mechanical damage.

4.2 Conditioning the seed

4.2.1 Storage
The reduction in capacity of the potato seed to sprout is known as physiological aging or
degeneration. Better crops and high yields are obtained from seeds sown soon after sprouting. Seed is
planted when sprouts are 5-15 mm long. If allowed to become longer they will easily break during
planting operations.
Tubers are put in trays in the dark and should not be exposed abruptly to sunlight because they may
be scorched resulting in decay. Seeds are then sprouted. Seed potato is handled as little as possible.
Handling is liable to cause cuts and bruises. Seeds are bought unsprouted and put in one layer of in
trays. Trays can be stacked on tires.
Keep potato tubers undisturbed till planting. The major disadvantage of this system is that there is
high initial cost of trays. However, the trays can be made on the farm.
Storage in trays makes treatment for diseases possible without touching tubers. Trays are dipped in a
fungicide e.g. copper-oxy -chloride.

4.3 Sprouting
On a commercial scale, seed potatoes are usually unsprouted when obtained. These should preferably
be sprouted under daylight conditions with protection only from the strongest sun.

Fig4.1 Potato sprouting

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4.3.1 Forced sprouting
A constant temperature of 30-35 o C in a closed room will initiate sprouting. Merely covering
with a tarpaulin in moderate sunshine will help.
a) The tubers may be stacked in an air-tight room at 21-27 o C containing 0.1 per cent acetylene gas.
30 g calcium carbide will generate sufficient gas for 2 m3.
(b) Immersion in acetylene solution for 4-6 hours. For 45 litres of solution, 230g calcium carbide is
added slowly. Sprouting can be retarded by storing the seed at low temperatures, for one year at 3 o C,
or by the application of a sprout inhibitor.

4.3.2 Retarding Sprouting

To retard sprouting, the seed is kept under low temperatures.
-At a constant temperature of 3oC seed do not sprout for a year and keep in good condition.
By exposing potato tubers to sunlight they become green. Tubers exposed to sunlight grow more
slowly and strong than under dark.
Sprout inhibitor which can be used is Fusarex or Technazene which can keep potato tubers without
sprouting at normal outside air temperatures.
Best results are obtained at 7oC and also act as a preventative way of dry rot caused by fusarium.
lf the chemical is applied soon alter lifting it gives control of dry rot in storage.
Once the chemical is applied potatoes are shut in a room without ventilation. Once ventilated the
chemical will volatilize within a few weeks and sprouting begin normally.

4.3.4 De-sprouting
When planting is delayed and sprouts become advanced and spindly it is preferable to break off the
sprouts and leave the tubers to produce new sprouts. Usually no damage results from this process.
The removal of the first sprouts produces makes the tuber produce a greater number of sprouts. This
then results in a crop with a large number of smaller tubers.
When de-sprouting hands are disinfected frequently to prevent the spread of mosaic disease.

4.3.5 Dipping of Tubers for control of skin diseases

Some diseases attack the skin and the outer cortex causing blemishing and spots that make the
appearance of the tuber undesirable.
4.5 Cutting and dressing
In summer when temperatures are above 18C and potatoes already sprouted can be cut and planted
immediately. Tubers should be reasonably clean or be swashed before dipping into a chemical or
dusted. The advantage of dipping is that the tubers are uniformly covered. Success depends on temp
and moisture conditions. The cut seed potatoes are shown in the pictures below

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Fig 4.2 Cutting potatoes for seed
Other products such as wood ash, slaked lime, gypsum, sulphur and cement have been used to dress
out potatoes with success.

5.0 FERTILIZER MANAGEMENT

A complete soil analysis will give the potato farmer scientifically based potato fertilizer
recommendations (including lime, dolomitic lime, organic matter and phosphate recommendations)
to increase the soil fertility using soil amendments to bring the pH and nutrients levels into line for
better soil structure, higher yields, better quality and disease resistance, longer storing tubers and
better soil health for long term sustainable agriculture.
Potatoes require relatively large quantities of fertilizers because of their shallow roots and
because of their need to bulk up within a short growing period.

5.1 Macronutrients:

Nitrogen (N)
This is an important nutrient; however excessive dressings lead to excessive vegetative growth at the
expense of yield. Excessive dressings or late applications of nitrogen result in tuber initiation and
may result in physiological disorders of the tubers such as hollow heart. Potatoes require about 5kg of
nitrogen per ton of tubers produced.
Phosphate (P25)
Potatoes have reasonably high requirement for this nutrient over a short period. Potatoes require about
4kg of phosphate per ton of tubers produced.
Potash (K2)
The crop requirements for potash are very high. An adequate supply of this
nutrient can increase resistance to drought, frost and disease. Potash aids tuber quality and should be

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applied in the sulphate or nitrate forms, as potatoes are relatively sensitive to chlorine. Positive results
have been obtained using Potassium Nitrate foliar sprays to increase yield and increase frost
resistance. Potatoes require about 9.12kg of potash per ton of tubers produced.

Calcium
Potatoes respond well to calcium as it plays a role in tuber development. Most
compound fertilizers contain 10-12% calcium but if the Calcium status of the soil
is very poor, additional nutrient can be applied as Gypsum before planting or as foliar sprays of
Calcium Nitrate. Potatoes require about 3.25kg of calcium per ton of tubers produced.
Magnesium
This nutrient is required for a potato crop but not in large quantities. Magnesium
deficiencies are unlikely to occur where a liming material has been applied. A magnesium
deficiency can be corrected by a foliar spray of 2kg Magnesium Sulphate per 100L water + 30ml
Sanawett (wetter).
Sulphur
Adequate quantities of sulphur are applied when using a compound fertilizer such as
compound C or compound S as a basal fertilizer. Sulphate of Potash used as a top dressing also
contains adequate sulphur.
5.2 Trace elements
These are required in very small quantities by the crop. Most soils contain adequate levels of
micronutrients but in the past specific instances of zinc and boron deficiencies have occurred. These
deficiencies can be recognized and rectified as follows:
Zinc
Leaves develop irregular grayish brown or bronze spots. Affected leaves are usually half way up the
plant, but in severe cases all leaves show symptoms. In extreme cases internodes remain short and
leaves small and thick. Spots develop on petioles and stems; top leaves assume a slightly vertical
position and margins of leaves may curl upward so that growth resembles that of a fern. To rectify
this deficiency mix 200g Zinc Oxide in 100L water and apply 300 - 400L of solution per Ha as a
foliar spray while crop is young. This treatment is unnecessary where regular sprays of Dithane M45
are being used.
Boron
Deficiency symptoms are indicated by deaths of growing points and lateral buds
develop. The internodes remain short, giving the plant a bushy appearance. The upper leaves tend to
be dark green, tough, with a shiny surface and with an upward curling of the leaf margins. Tuber
symptoms show the vascular rings turning partially or completely brown and rapid discoloration of
the cat surface. As the fertilizer compounds recommended for use on potatoes contain boron this

15
 should be enough for the plants requirements.
The symptoms of excess boron are poor stands due to indifferent sprouting. Sprouts die off after
some growth has taken place. Roots fail to develop satisfactorily. Leaves become bleached or
develop marginal yellowing. Do not exceed application of 2kg elemental boron per Ha.
5.3 Timing and Application of Fertilizers
To ensure that the soil can supply adequate nutrients a carefully planned fertilizer program is needed.
The required rates of fertilization vary considerably according to land history, soil type, variety,
climate and yield expectations. To establish the correct rate of fertilization it is advisable to have a
soil sample analyzed by a laboratory and to discuss the result with your local agronomist, whose
knowledge of local conditions will be of great assistance. All the phosphate and most of the nitrogen
and potash should be applied at or before planting. This is usually done as a basal dressing of a
compound fertilizer. The remaining nutrients are applied as a top dressing. The traditional
recommendations have been to use Compound S (7:21:7). This fertilizer is high in phosphate and low
in boron (0.04%). Compound S should be used on sandy soils and on soils that are low in phosphate,
however the potash content is inadequate, and the crop will require further top dressings of potash if
S is used.
The other alternative is to use Compound C (6:17:15). This is probably the better compound to use
for potatoes as the potash content is relatively high and phosphate is adequate.

5.4Standard suggestions for potato fertilization are:

Method 1
Apply 1300 - 1500kg/Ha Compound S at planting, rate depending on the sandiness and phosphate
status of the soil. At three weeks after emergence apply 150 - 200kg/Ha AN as a top-dressing. Top
dress with 400 - 500kg/Ha Sulphate of potash split into two equal applications, one at flowering and
the other two weeks after flowering. (Potassium Nitrate at 450kg/Ha can be used in place of AN and
Sulphate of potash).
Method 2
Apply 1300 - 1500kg/Ha Compound C at planting. Apply 200kg/Ha AN 3 weeks after emergence as
a top dressing. Top dress with 200 - 300kg/Ha Sulphate of potash split into two equal applications,
one at flowering and the other two weeks after flowering. (Potassium Nitrate at 450kg/Ha can be used
in place of AN and Sulphate of potash).
When applying the basal Compound S or C applications, it is preferable to band the fertilizer to the
side of the seed. If suitable banding equipment is not available, it may be placed in the furrow with
the seed. A degree of fertilizer burn can result from this practice. To reduce this possible effect a
single line may be drawn along the bottom of the furrow to mix it with the soil, or a fertilizer covered
with a thin layer of soil before planting the seed. The fertilizer can be broadcasted after ridging, set

16
the seed in the furrow then split the ridge back over the seed. When tubers are in direct contact with
the fertilizer a good initial irrigation will reduce the chances of burn occurring.

Table 5.1: Amount of fertilizers kg / ha

Nutrient status of the soil
Nutrient Good Medium Poor
Nitrogen Up to 70 70-100 110-160
Phosphate Up to 325 350-400 400-450
Potash Up to 70 70-110 100-135

All phosphate and potash requirements should be applied at/before planting, by banding to the side of
the seed. To reduce fertiliser burn a single line may be drawn along the bottom of the furrow to mix it
with the soil, or the fertiliser covered with a thin layer of soil before planting the seed.
The other method of application is to broadcast the fertiliser after ridging, set the seed in the furrow,
then split the ridge back over the seed. When the tubers are in direct contact with the fertiliser, a good
initial irrigation will reduce the chances of burn occurring.
6.0 POTATO PLANTING
Tubers are planted 70-100 mm deep under irrigation. Dry-land planting may be up to 150 mm deep.
Inter-row spacing is 900 mm and the in-row spacing is 300 mm. Large seed tends to produce more
stems and a higher proportion of smaller tubers, small seed. Planting is deeper in light soils and
shallower in heavy soils. Under irrigation planting should be shallower in all types of soils. In lands
which are cold and less drained plant shallower in winter and deep in the dry season. Harvesting
deeply planted potatoes require a lot of power during the harvesting process.
A seed rate of 1050kg / hectare is used. This is shown in the figure below.

Fig 6.1 Planting potatoes.

Seed producers will use a spacing of 600-1200 mm between rows and from 150-160 mm in the row.
Seed is supplied in 30 kg pockets containing sizes from 25 to 56 mm and an average of 400 tubers

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 per pocket.

Table 6.1Irish Potato Planting Distance Table

SEEDING RATE

60 pkt/Ha 70 pkt/Ha 85 pkt/Ha 100 pkt/Ha 110 pkt/Ha

DISTANC 90 120 90 120 90 120 90 120 90 120
BETWEEN
ROWS cm cm cm cm cm cm cm cm cm cm
DISTANCE IN THE ROWS (cm]
SEED SIZE
Small 16 12 14 10.5 11.5 8.5 10 7.5 9 7
Medium 39 29 33 25 27.5 21 23 17.5 21 16
Large 74 55 63 48 52 39 44 33 40 30

NB: Spacing is related to hollow heart - large tubers tend to be more prone to this physiological
disorder, therefore varieties which tend to produce larger tubers should be planted at closer spacing to
reduce tuber size. A pocket of seed weighs 30kg
6.1 Time of planting
Potatoes can be grown all year round in the middleveld and highveld. However because of
considerations of frost, wet months and utilisation of irrigation equipment, three main planting times
are recognised.
a) Summer crop
Normally planted in November- December to mature towards the end of the rainy season.
Earlier planting is possible and can lead to very high yields but lifting can be difficult.
Use late blight resistant varieties since complete chemical control during rains is extremely difficult.
The crop is planted under summer rainfall conditions. It is the most unpredictable crop because of
uncertainty of rainfall. Summer plantings can be attacked by late blight in hot summer period. Plant
varieties which are tolerant to late blight. Controlling the disease by spraying is difficult during
periods of heavy rainfall. Planting takes place in November with the first rains. Do not plant in hot
soil, as stands may suffer. Where irrigation is available planting of late blight susceptible varieties in
early August reduces the risk. Harvesting can be difficult in January unless there is a dry spell. Where
rainfall is unreliable supplement with irrigation is applied.

b) First Winter Crop

Planted from February to April to mature before frost according to area, but later in the lowveld to
take advantage of cooler weather. Varieties resistant to Late blight should be grown. Supplementary
irrigation is essential.
c) Second Winter Crop
Planted in late July to early August after the risk of frost has passed. Irrigation is required and when

18
 flood irrigation is used Late blight is not a problem. With the use of overhead irrigation the risk of
late blight. Susceptible varieties to late blight are not planted early. Control is of late blight is
effective when rain is over and where irrigation stop to allow foliage to dry before night fall.
6.2 Methods of Planting
A uniform tilth is required and where flood irrigation is used ridging is essential. Where overhead
irrigation is used planting is done on flat land and ridging is done later.
a) Hand planting
This is done behind a tractor-drawn ridger, which opens the furrows. It is necessary that the land be
level before planting The tubers are placed in the open furrow with sprouts facing upwards then cover
and irrigate if necessary.
Furrows are spaced 90-120 cm and tubers are placed 30-60 cm inrows.
b) Machine planting
Potato can be planted using potato planters which have fertilizer hoppers so that fertilizer can be
applied at the same time seed is dropped. Potato planter opens the furrow, apply fertilizer and drop
the tubers at required spacing and cover. Setting of the planter is very important for the flow or
fertilizers. Rate is checked during planting. Fertilizer chutes require regular inspection during
planting in wet weather to avoid clogging.

7.0 IRRIGATION OF IRISH POTATOES

Adequate water is required from tuber initiation to maturity. Stress during tuber bulking period may
lead to malformation and poor quality. Various irrigation systems can be used to irrigate potatoes.

Fig 7.1 Sprinkler and drip irrigation of potatoes (source-Potatonewstoday.com)

Potatoes have shallow roots and exploit 600 mm which should be wetted during planting to allow the
sets to emerge. After wetting the top 600 mm, no additional moisture during sprouting. Next
irrigation should be applied at the first flower which coincides with tuber initiation. From tuber
initiation to maturity stress is avoided because drought will limit tuber expansion. Avoid excessive
watering as this cause undue leaching of nutrients and reduces the keeping quality of the tubers. A
useful and practical test is to examine the tubers. If the tubers have white flecks on the lenticels then
you have applied too much water.

19
Table 7.1:Irish potato Irrigation guide.
Soil type Hot Months Cold Months Irrigation required
Light soils 3-4 days 5-7 days 25-30mm
Heavy Soils 4-5 days 6-9 days 30-35mm

8.0 CULTIVATION
Potatoes are very sensitive to root damage, for this reason cultivation only be done when necessary.
Potatoes grow successfully on a flat soil without any cultivation from planting to harvesting. Tubers
must be covered to protect them from late blight.
8.2 Ridging
Earthing up by ridging is necessary to protect the tubers from greening, and potato tuber-moth attack
and Late blight. It should be planned so as to be an integral part of weed control.
Ridging is done 4-6 weeks after emergence. A baselier ridger can be used.

Fig 8.1 Potato ridging using a baseliner ridger.(Source-Youtube.com)

Fig 8.1 Poatato ridging using a ridging hiller. (Source google images)
Re-ridging should be completed by the time the plant is 25 cm high. The ridges should be made as
low as is consistent with good coverage for the tubers and for efficient flood irrigation.
High steep ridges are subject to greater erosion and more rapid drying due to the greater surface area
exposed to the sun. Wide flat-topped ridges are satisfactory for making adequate soil available for
tuber development and tuber protection. To avoid greening and potato tuber moth ridging up is
necessary and weeds be removed. Where surface irrigation is used ridging is done at planting. Where

20
overhead irrigation is used they can be planted on the flat and ridged later. Another re-ridging may be
required. Cracks allow access of potato tuber moth to the potato and should be covered as a
preventative measure. No tubers should be exposed up to harvesting.

9.0 WEED CONTROL

Potatoes are sensitive to weed competition. Weeding should start as soon as potatoes emerge from the
soil. First ridging destroy weeds as the crop grows older the cultivation should be shallower to
prevent root damage. Invariably some weed seeds survive cultivation operation. These can be killed
by hand hoeing or by puling. Chemicals which can be used in weed control are as listed in table 9.1.

Table 9.1 Weeds and herbicides to be used

Chemical Weed
Alachlor EC Some grass weeds and broad leaved weeds
Metalachlor EC Yellow nut sedge, broad leaved weeds and grasses
Linuron Annual broad leaved weeds and grasses
MCPA-K salt Annual broadleaved weeds.

10.0 PEST CONTROL

Many insecticides are available at agro-dealers. Sevin is a synthetic insecticide; organic options
include sulphur -based insecticides. Before using a pesticide, read the label and always follow
cautions, warnings, and directions.
10.1.1 Nematodes
Three main types of nematodes occur in Zimbabwe
Root-knot nematodes (Meloidogyne spp)
These give rise to characteristic watery blisters on the tubers at lifting time. These subsequently
collapse and present a rough surface of crinkly scar tissue. This type is most common and
economically dangerous. The demage caused by root knot nematodes is shown in Fig10.1 below

21
 Fig 10.1 Irish potato damaged by root knot nematodes.

Root-lesion nematodes (Pratylenchus spp)

They are characterised by lesions on tubers which are visible as slightly raised, light to dark brown-
purple areas with a groovelike depressed border. Symptoms may develop during storage.

Fig 10.2 Root lesion demage in potatoes (Sourceomfra.gov.on.ca)

Burrowing nematodes (Radopholus)
They cause minute intensely black sunken spots easily visible on freshly lifted tubers. This nematode
is as yet of little economic significance. Control of nematodes is by rotation with nematode resistant
grass leys of Ermelo Love, Katambora Rhodes and Sabi Panicum grasses.
Soil fumigation with EDB can also be done three or four weeks before planting. Due to the ban on
EDB alternatives can be used e.g. Nemacur

10.2 Insect Pests

a) Potato tuber moth (Phthorimea operculella
It affects production, reduces quality and increases the risk of other pathogens entering the crop. It
feeds on a restricted range of plants (potatoes, tomatoes and tobacco). It attacks plants in the field
and in storage. The causal insect is a small brown moth – that is active at dawn and dusk. Moths
cannot fly far Main spread is via infected tubers. The demage is shown in fig 10.3 below.

22
 Fig 10.3 Potato tuber moth demage
The larvae of this pest cause leaf damage by tunnelling between the upper and lower leaf surface thus
hindering photosynthesis, they also tunnel the stems and the tubers.
Control
Can be controlled using the parasitic wasp Copidosoma as biological control.
Clean fields by removing all tubers after harvesting and by weeding out alternative hosts e.g
Nicandra (Apple of Peru). Ridge up tubers to bury them with about 250 mm soil. Do not ridge just
before spraying since covered leaves will receive no insecticide. Monitor numbers with pheromone
traps, weed, and get together with your neighbours to plant your potatoes in at the same time.
Chemical control using chemicals such as Monocrotophos or Carbofuran.
b) Aphids
Aphids transmit several virus diseases such as Leaf roll, Necrotic Virus Y and Virus Y.
They cause wilting, premature senescence and a consequent decline in yield.

Fig 10.4 Potato leaf attacked by aphids

Chemical control can be done using Dimethoate.

23
c) Cutworms
Cutworms are the larvae of certain moths that chew through the stems at surface level. They are
plump, dark, greasy grey caterpillars 25-38 mm long that feed at night

Fig 10.5 Cutworm demage in Potatoes.Source- ephytia.inra.fr

Control:
Apply a band of insecticide along the rows as a spray or bait.
Chemicals registered for use in potatoes are Monocrotophos, Deltamethrin, Chlopyrifos and
Endosulfan.
d) Potato leaf hopper (Empoasca fascialis)
These are small wedge-shaped insects that feed on the underside of the leaves.
They rarely cause economic damage.

Fig 10.6 Potato leaf hoppers

These are controlled by using Malathion.

24
e) White grubs
These are soft bodied, white larva with a brown head that feed on the tubers making shallow holes.

Fig 10.6 White grabs in potatoes.

They are controlled by chlorpyrifos at planting.

f) False wireworms (Trachynotus and Psammodes spp)

Tough skinned cylindrical, yellow-brown larvae which eat into the plant stem just below soil level.

Fig 10.7 Wireworms

Control is by use of Chlorpyrifos.

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11.0 POTATO DISEASES.
Potatoes are affected by several diseases.
11.1 Late blight
It is the most serious disease which cause appreciable loses in potatoes. This disease can occur in
summer and first winter crops. To avoid the disease, plant non-susceptible varieties. The disease first
appears on the leaves as small brown patches which in damp humid weather enlarge to form water
socked lesions, characterized by formation of a zone of white fungal mycelia at the edge of the lesion
on the underside of the leaf. Severe infection leads to defoliation. This is largely responsible for the
yield loss. Tubers underground may also be infected through spores finding their way to the tuber.
Produce dusty brown markings underneath the skin of the tuber and these are associated with
depressions on the lesion as shown below.

Fig 11. 1Potato leaf and tuber infected by late blight

The disease requires prolonged wet weather conditions for epidemic to development. There is
severe outbreak of the disease in summer crops in December to January.
Ensure that the crop is ridged up to prevent tuber infection. Once the tuber is infected no chemical
control can be used. Adjust the planting date to avoid hot humid weather which speed up the
development of the disease.
Application of fungicides e.g. Dithane M45.

11.2 Early blight

Is caused by a fungal pathogen Alternaria solani The disease occasionally causes severe yield losses.
Where the crop is well fertilized the disease occurs late to do any material damage to the crop. On
poorly growing crops the disease may appear earlier and reduce yields. The primary infection is

26
caused by inoculum from host plants (weeds / tomatoes / volunteer plants) or infected plant debris
from the previous crop. High humidity and free moisture are required for the spores to germinate and
penetrate the plant as shown in the figure 11.2 below.

Fig 11.2 Potato crop and tuber infected by early blight

Spores infect the lower mature leaves of the potato plants causing brown dots that enlarge, become
angular and then are easily identified because they contain light and dark concentric bands. Under
high disease pressure the lesions join up and the leaves become chlorotic and die. Remaining on the
plant. Severe foliar infection will cause reduced yields and low dry matter content in the tubers.
Once in a crop the potato disease can spread fast as the spores are both wind and water borne and
move around in dust, air currents, and water splashes. Alternating wet and dry periods are most
conducive to the spread of the disease. The fungus can get infect the tuber at harvest reduce affect
quality and storage
The disease can be controlled by spraying fungicides like Dethane M45

11.3 Potato Viruses X ( mosaic)

Potato virus X is the most common and is carried in many varieties with slight or no obvious
symptoms. Infected plants may show a mild interveinal mottle with no distortion of foliage and no
apparent reduction in yield. The severity of the mottling may be considerably affected by
environmental conditions and complete masking often occurs.
Symptoms of mild mosaic are best seen on a dull overcast day. Bright sun makes detection difficult.
There are a number of strains of the virus which differ in severity of symptoms. Virus X is sap-
transmitted. Transmission normally occurs by contact of plants aided by mechanical operations.
Cutting of seed can also spread the virus.

27
Fig 11.3 Crop infected by potato virus X

11.3 Potato Virus Y (PVY)

PVY is an aphid-borne virus that causes mild to severe mottling and eventually leaf drop in the
leaves. It greatly reduces the yield, tuber size and marketability of the potatoes. Infected seed
potatoes are the main cause of spread.

28
Fig 11.4 A potato crop infected by potato virus Y. Source ephyytia.inra.fr

Once in the field the PVY has a wide host range (Solanaceae, Amaranthaceae, Chenopodiacaeae –
ie tomatoes, eggplant, cape gooseberries, many common local greens and weeds). Aphids pick up the
virus within a few seconds of feeding and proceed to transfer to one or two healthy plants before until
it feeds off another infected plant. Use clean seed potatoes, control aphids.

11.5 Black scurf and stem canker

This disease is caused by fungi Rhizoctonia solani
Occur frequently in potatoes and tobacco characterized by black superficial wart growth on the skin.
Growth resembles soil particles but when tubers are washed they still remain on the tuber. This
disease characterized by dark brown lesions just below or above ground level. Stems may be
completely girdled. Leaves curl upwards from margins and turn yellow. In severe cases young stems
are completely killed before or after emergency resulting in poor stem.

Fig 11.5 Black scurf in potatoes. Sourcecropscience.bayer.uk

Control of black scurf

Reduce damage to young growing plants through operations like ridging. This should not be done
when the soil is hot as it predisposes plants to Rhizoctonia.

11.6 Common Scab

The disease is caused by fungus Streptomyces scabies.
Affects mainly tubers and is common. The disease have very little effect on the yield but

29
disfigurement caused to the tubers results in low market value Disease appear as roughly circular
brown scabby lesions. Lesion enlarges and coalesces to cover extensive areas of the tuber. Sometimes
relatively deeper pits or cracks may occur on scabby areas as shown in fig 11.6 below:

Fig 11.6 Common scab in potatoes

In severe attacks the causative organisms come from the soil. Favourable conditions are not known
but aeration of the soil seems to be an important factor in reducing the disease. Ploughing in of large
amounts of un-decomposed organic matter may increase disease attack. Poor land preparation
enhance the disease. High soil temperature also favour the outbreak of the disease. Other factors are
neutral or alkaline pH and deficiency of soil humus. To reduce the incidences, proper land
preparation is done to obtain a good tilth. Should the land require liming the lime should not applied
immediately prior to potatoes but within the rotation. Infected seed should not be planted.
Control: all cultural control measures apply and regular sprays of fungicides reduce the disease
incidences.

11.7 Silver Scurf

Is a fungal disease caused by Spondylpcladium atroverns
More prevalent in the Eastern Highlands. The disease only infects tubers producing silvery gray
roughly circular patches on the skin of the tuber. If closely examined, tiny black Sclerotia are found
on the patches. Fungus only attacks the skin. Severe attack causes losses in storage due to loss of
water through damaged skin resulting in shriveling of all affected tubers

30
Fig 11.7 Potato silver scurf on tubers
.No control measures other than culling affected seed crop.

11.8 Bacterial Wilt

It is caused by bacteria Pseudomonus solanecearum
Is a soil borne disease in warm weather conditions Symptoms include wilting, stunting and yellowing
of plants, which finally die. Early symptoms include wilting in the day and recovery overnight. The
bacteria gets into the tubers, which may show no external symptoms, but they rot inside, and a
bacterial ooze comes out of the eyes and stem end attachment, which soil sticks to. Infected plants
should be removed from the field and destroyed, and ash spread on the area. Ralstonia is spread with
infected seed potatoes, soil and irrigation water. These can be tested at our laboratory before you
plant. Surface water is a great risk for Ralstonia in Zimbabwe. There are many weeds that harbor and
spread the bacteria but show no symptoms. It causes wilting of the foliage and discolorations of
vascular tissue.

11.9 Blackleg
Blackleg and bacterial soft rot are diseases that are caused by different, but closely related bacteria.
These pathogens are found worldwide. These bacteria reside in the lenticels of the potato tubers.
They only require the proper environmental conditions to cause disease. Warm, moist conditions
favor disease development in the field. In storage, a film of water will induce anaerobic conditions of
the tuber, which encourages soft rot.

31
Fig 11.9 Potato black leg
Once proper environmental conditions occur, disease can result. Decay can actually occur between
the temperatures of 100C and 350C, but rots increases dramatically when temperatures are between
220C and 300C. These bacteria are spread in and on potato seed pieces, in old potato stems and stalks,
water, soil on potatoes, insects and any mechanical methods, such as soil movement or human
activity. The pathogens can cause losses in the field as well as losses in potato storages.
Blackleg is caused by the bacterium Erwinia carotovora subsp. atroseptica. The name of the disease
comes from the black lesions produced on infected stems. The disease affects stems and tubers.
Stems of infected plants typically have inky black symptoms, which usually begin at the decaying
seed piece and may extend up the entire length of the stem. Stem pith can be decayed above the black
discoloration, and vascular tissues can be discolored. Leaves turn yellow and leaflets tend to roll
upwards at the margins. Leaflets, and later entire plants, may wilt and eventually decline. In wet
weather, decay is wet and slimy and may spread rapidly form plant to plant by wind or rain.
Tubers of infected plants may show symptoms ranging from slight vascular discoloration at the
stolon end to wet breakdown of the entire pith, extending inwards from the stem end.

11.10 Bacterial soft rot

Bacterial soft rot is caused by the bacterium Erwinia carotovora subsp. carotovora, a common soil
resident. Bacterial soft rot occurs on a wide range of crops and is one of the most severe postharvest
diseases of potatoes worldwide. Loss may occur during storage, transit or marketing. All potatoes
varieties are susceptible.
Contamination of potato tubers occurs anytime they come into contact with the bacterium, most
commonly during harvest, handling or washing. The bacterium invades the potato tuber chiefly
through wounds. Most of the soft rot infections are in tissues that have been weakened, invaded or

32
killed by pathogens or by mechanical means. Soft rot in tubers is favored by immaturity, wounding,
invasion by other pathogens, warm tuber and storage temperatures, free water and low oxygen
conditions. Decay can be retarded by temperatures less that 10 0C, the lower the temperature, the
better. Immature tubers are susceptible to harvester-related injury and bacterial infection.
Soft rot on tubers first appears as small, tannish, water-soaked spots on the surface. These spots
rapidly enlarge and the tissue decomposes in a soft, blister-like area on the surface of the tuber.
Often, a slimy or watery substance oozes from breaks in the blister. The blister margin is darker than
the tuber skin. Soft rot often follows bruising As secondary rot occurs, the rot becomes very foul
smelling. The rot typically progresses to the point of a chalky-white, foul-smelling mass.

Fig 11. 10 Bacterial soft rot in potato tubers

To control the disease, ensure good drainage by avoiding water logged conditions to occur in the
field. Delaying harvest until the skin has set reduces tuber injuries. This will reduce the entry points
for the pathogen. At harvest, watch tuber handling practices, and ensure good sanitary procedures to
reduce spread of bacteria. Harvesting during wet, muddy conditions generally leads to an increase in
bacterial soft rot in storage. Properly suberize (heal) potatoes to insure wound healing and reduce the
infection sites for the pathogen. Leave potatoes a minimum of seven days after the vines are totally
dead to encourage skin set and reduce bruising. Eliminate as much soil as possible before the tubers
are stored, as soil can restrict air movement. If harvesting wet potatoes, ventilate continually until the
potatoes are dry. Isolate problem potato lots in a separate bin. Check the pile temperature at regular
intervals. Early detection aids in control, thereby reducing loss. Avoid washing seed or potatoes into
storage unless absolutely necessary. Potatoes that have soft rot should be removed before storing and
during packing. After washing, potatoes should be dry before shipping. Tubers need to be protected

33
 from bruises, excessive heat or cold during harvest and transport.

Good cultural practices to control potato Pests and diseases

i. Field sanitation – remove all early disease inoculum in field before planting – potato plant residues,
volunteer plants, culled tuber piles, residual tubers in soil, weeds.
ii. Source good clean certified seed. If this is not available, check the quality of the tubers you buy, cut
open and discard any with symptoms. Wash tubers away from field to remove soil before planting.
If you use own seed, mark the healthiest plants before harvest. Check quality of tubers after harvest,
cutting some, if in doubt store in a warm place for 15-20 days and discard rotten ones, wash off field
before planting.
iii. Spray preventative fungicides for late blight – these also work on early blight, rotate active
ingredients to prevent fungicide resistance build up.
iv. 2-3 weeks before digging up – cut stems and leave tubers in soil to allow tuber to mature and skins to
thicken – this reduces post-harvest infection. Harvest carefully to reduce damages. When harvesting
– remove infected potatoes before storage.
v. Proper plant spacing to allow aeration
vi. Mulching soils can reduce water splashes and transmission of late and early blight
vii. Good soil health and fertiliser use (have your soil tested and use a proper fertiliser program), healthy
plants are more resistant to diseases and insects..
viii. Irrigation can greatly increase yields. Use drip to reduce splashes and disease spread and check the
water source – underground water has less risk of disease transmission. Irrigate midday to allow
leaves to dry before the evening.
ix. Check for aphids / leaf minors / cutworm and spray insecticides if necessary.
x. Equipment hygiene – clean all tools / tractors / shoes when going from one field to the next. In the
morning start clean – in the newer fields and access older fields last – to avoid moving the inoculum
from the older crop to the younger crop.
xi. Understand the disease pressures in your area and plant resistant varieties.
xii. Plant potatoes deep (15 cm) and hill up after germination to reduce disease / moth infestation of
tubers.
xiii. Soil solarization in field, by placing weighted down clear or black plastic sheeting over damp soil for
several weeks can pasteurize the soil and reduce inoculum loads.
xiv. Weeding, many local weeds and greens eg nightshade can harbor these viruses and diseases.
xv. Rotation, Rotation, Rotation – rotate with none Solanaceae crops eg cabbage.

34
11.0 PHYSIOLOGICAL DISORDERS OF IRISH POTATOES
Physiological disorders are not caused by pathogenic organisms; they are caused by adverse
environmental conditions.
11.1 Black heart
Black heart is the black centre of tuber caused by storage at high temperatures

This condition is a result tubers stored at high temperatures with inadequate ventilation. This leads to
the flesh at the centre breaking down and turning black. There are no external symptoms. The
condition can only be seen when tubers are cut into halves. It develops on tubers left in the ground
when temperatures are high. If tubers are to be left in the ground for any period they should be well
ridged to insulate them from the sun’s heat.
11.2 Hollow heart
Hollow heart is caused by rapid or irregular growth of the tuber. This is more common in large
tubers. Irregular cavities are found in the Centre of the tubers when it is cut. No discoloration of flesh
or cavities.

35
Fig 11.2 Hollow heart in potatoes

Occurs when the crop resumes growth after a check or when a heavily fertilized crop is growing
under favorable conditions. This problem is severe when there is moisture stress followed by long
conditions favouring rapid growth.

11.3 Internal browning

Brown flecks characterized by spots of brown dead tissue are seen if the tuber is cut. They vary in
size from a mare fleck to 13mm in diameter. The areas are scattered throughout the flesh of tubers
which may exhibit a radial arrangement. Brown flecks when cooking turns black.

36
Fig 11.3 internal browning

The condition occurs in fertile soils and is attributed to potassium deficiency in acid soils. Dry
conditions may increase likelihood of brown flecks in these soils. Lands severely affected should not
be cropped to potatoes. Liming is done within the rotation 2-3 years before potato planting. Drought
Spots occur as a result of water shortage. This is common when dry periods occur in January and
February. Symptoms differ from those of brown flecks in that discolouration occurs in strands
associated with vascular tissue and tissues outside. To avoid this situation irrigate during drought
spells.

11.4 greening
Potato tuber turn green due to accumulation of chlorophyll in the tuber. This makes the potatoes un-
markatable due to solanin (poison). The green tissue may extend 2cm into tuber, and is often
accompanied by purple pigmentation.
Some greening maybe cultivar related. Immaturity of tubers when harvested leads to solanin
accumulation of solanin accumulation. Entry of light reaching potatoes in storage is another
cause .High storage temperature also leads to tuber greening.

Fig 11.4 Greening in potatoes.

37
Control is through excluding light and control atmosphere in the storage. Wax or oil the tubers.
Avoid damaging the tubers.

11.5 Tuber cracking

This is caused by pressure imposed on tubers during growth. Such as machinery are harvesting. Viral
infections and excess fertilizer application.

Fig 11.5 Tuber Cracking in potatoes

12.0 HARVESTING
12.1 Haulm destruction
Haulm destruction may be necessary if the crop is attacked by late blight. Tops can be chopped off by
hand taking care not to disturb the tubers. Step on both sides of the crop and pull up the haulms. A
tractor mounted haulm pulveriser is quicker though a lot of power is required. Some farmers can use
grass mowers to cut off the haulms. Chemicals can be used alone or combined with haulm pulveriser
for greater efficiency. A number of products are available e.g. Gramoxone, Sulphiric acid, Tar oil.
After using chemicals balm killers there may be intense attack by black scurf on the tubers therefore
lift the crop within 10 days of haulm destruction.

12.2 Harvesting time

Potatoes should be harvested when the skin harden when the tubers are least likely to be damaged
mechanically. Lifting should not be done when the soil is too dry or too wet. Tubers reach maximum
weight and size when the leaves die but vines are still green. Normally harvesting is done when 95%
of the leaves have died off. When chemical haulm destruction has occurred lifting should be
completed inside 10 days to avoid attack by Black scurf (Rhizoctonia)
Harvesting begins when the potato skin has hardened sufficiently to reduce physical damage during
lifting For a highest yield of mature table potato crop to be achieved the crop should be left in the soil
for 12-15 days when leaves have died when vines are destroyed in advance.
For seed production the same period is required to allow the skin to harden. Harvesting time depends
on weather conditions, where temperatures are too high or threat of bacterial blight, no time should

38
be lost in lifting potatoes and disposing them even if plants are still green. Potatoes require cool
conditions and should not be allowed to dry faster soon after harvesting. Cool overcast conditions are
preferable.
12.3 Harvesting Methods

12.3.1 Hand lifting

This is done using a hoe or garden fork. A modified fork with long and bend tines to operate as a hoe
is used and is easier to handle.

Fig 12.1 Hand harvesting of potatoes

A moldboard plough or a ridger can be used to break the ridges and partly lift the potatoes.

12.3.2 Mechanical digging.

Done by a number of implements of which the spinner digger, link belt digger. Swing sieve digger
are the main types. The swing sieve or oscillating sieve follows the rows of tile potatoes.
Soil is then taken into two oscillating grids which are at right angles to the direction of travel. The
sieve vibrates and quickly sieve off the sol and deposit the potatoes on one side. Grids are so efficient
that the soil tubers and haulms are completely separated. This machine with appropriate alteration is
very useful for lifting beetroot and carrots. In hot dry conditions the tubers should be moved to a
sheltered place immediately after harvesting to avoid a reduction in the keeping quality. Only very
muddy potatoes should be washed.

39
FIG 12.2 Potato harvesting
A yield of up to 40t/ha can be obtained but on average the summer crop produces a yield of 17-20
t/ha and the winter crop 24-27 t/ha.

13.0 Storage
The crop is marketed soon after harvesting because it is a perishable product.
The crop may be left in the soil or harvested and stored. This is done on some crops where harvesting
can be delayed until August. If left in the ground when temperatures are high then potatoes will
sprout faster and shrinkage and deterioration of quality. Leaving potatoes in the ground when mature
results in greater damage by fungi, insects, mice and wild pigs etc. In hot dry conditions the tubers
should be moved to a sheltered place immediately after harvesting to avoid a reduction in the keeping
quality. Only very muddy potatoes should be washed.

40
Restriction of ventilation in storage induces faster sprouting. Temperature below 3 oC induces
sprouting because respiration is stopped. At low temperatures tuber moth attack and rotting are
reduced to minimum or stopped. Humidity is recommended to storage. Potatoes should not be kept
too wet especially freshly cut or bruised.
Cleanliness and freedom from pests are other possible causes of deterioration. Tubers with slightest
attack by tuber moth are discarded before storage.
Old tubers and dirty tubers are removed before storing a new crop. Floors, walls and ceiling should
be sprayed with malathion or carbaryl to control tuber moth. During loading potatoes are be dusted
with malathion. Where there was a heavy rotting in the previous year store, boxes are sprayed with
2% formalin or copper sulphate. Keeping quality depends on correct fertilization of growing crop.
Excessive nitrogen or late application of nitrogen reduces the keeping quality when applied in the
sulphate form. Over watering is detrimental if accompanied by unbalanced and untimely fertilizer
application.

Activity
Describe the sprouting process of potatoes.
Give a brief account of fertilizer management of Irish potato.
Identify the physiological disorders and diseases of potatoes.

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Further reading
This publication was revised from earlier versions such as:
1. Floyd .M. Ashton, Thomas, J. Manaco (1991) Weed Science Principles and Practices, John wiley
and sons Inc.
2. Joseph Masabni, Assistant Professor and Extension Horticulturist, Easy gardening Irish potato
Production The Texas A&M University System
3. Julia Sibiya (1999) Crop Physiology and Crop Protection, Plant pathology, Zimbabwe Open
University, Harare, Zimbabwe Easy gardening Irish potato Production by Joseph Masabni, Assistant
Professor and Extension Horticulturist, The Texas A&M University System
4. Ministry of Agriculture , Mechanization and Irrigation Development (2010) Farm Management
Handbook, Horticulture Crops volume 2. Ministry of Agriculture Mechanization and Irrigation
Development, Harare.
NB: More Extension publications on Irish Potato can be found at www.ZIMAGRIHUB.co.zw

42
 Appendices
Pest problem agrochemic rate/ ha grams or Spray EU MRL Day
al ml/100 Lts interv mg/kg s to
water @ al Har
500 Lts days vest
water/ha (phi

Actara 25 400 gm 4 g/1,5Lts n/a 1 )n/a

Nematodes
soil pests and aphids WG drench 25 200 gm
Actara water/100m
40 7-14 1-2 7
Ampligo
WG foliar 200-400 40-80 14 2-4 7
leafminor- lyrichmyza Amistar
150 ZC Top 500
ml ml 100 7-14 2-4
Bravo
325 SC 720 1.5 Lts 300 7-10 4-6 3
SC
Dynamec 560 ml 115 14 3-4 3
redspider mites Folio Gold 5 2Lts 400 10 2-3 14
potato tuber moth 5Karate37Zeon
SC 100 ml in 70 n/a 1 3
Match
5CS 050 150
600 ml Lts 120 10-14 2-4 7
aphids EC
Maxim XL 500 ml / n/a 1 n/a
disease problems 035 FS
Oritiva ton tubers
600 - 800 60 - 120 10 2-1 7
Polo 500 SC ml
500 ml 100 7 2 21
Black and silver scurfs, Proclaim 05 250 g 50 7-14 2 1
soft rot, common scab SG
Revus 250 500 ml 100 7-14 2-4 3
Early blight/ alternaria SC
Ridomil 2.5 kg 100 7-14 1-2 14
Gold MZ 68 3 Lts
Solvigo 50 n/a 1 n/a
late blight Trigard 75 150 g 30 7-14 3-4 14
virus complex Dual 1 Its 200 n/a 1 n/a
WP
Magnum 960 1.25 Lts
Fusilade Forte 250 n/a 1 28
Gramoxone 9 Lts 400 n/a n/a
Touchdown 2 lts 400 n/a 28

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