Potatoes

1.  INTRODUCTION

The potato originally came from South America, near Lake Titicaca on the border of Peru and Bolivia. The crop was first cultivated in the Sierra or mountain zone of the Andes at heights between 3 000 and 4 000 metres. Potatoes were brought to Europe and cultivated there towards the end of the 16^th Century.  The plant is a member of the family Solanaceaer and is closely related to the tomato and the nightshades.

The potato can thrive in places where the day temperatures are warm, but cool nights are needed for proper tuber formation.  The best temperatures for tuber formation are between 18°C and 20°C, and at temperatures above 32°C both tuber formation and yields are poor. The potato plant is very susceptible to damage by frost, so that planting dates and sites for growing the crop must be chosen to avoid frost killing the plants.

The cultivated species of potato, Solanum tuberosum, is a tuber-bearing herbaceous plant which produces stems and leaves above ground, but the so-called root system of the plant is an extension of the stem.  The portion of the stem below ground gives rise to stolon’s which carry the adventitious root system and which terminate with the tuber – see diagramme below.

Figure 1: The Potato Plant

The tuber is a highly adapted part of the stem organised for food storage and vegetative reproduction.  When exposed to the light, the tuber will become green.  The inside or flesh of the potato consists of starch reserves and can be white or yellow.  The skin or corky outside layer can be white, yellow, blue or pink, according to the variety. Each potato has two ends, the heel end, which is attached to the stolon, and the rose end which contains most of the eyes which produce the growing sprouts. Each eye is a collection of sprout buds lying in a depression formed by a scale leaf.  The tubers transpire through small pores or lenticels in the skin, and these can sometimes be seen as white, glistening dots on the surface of the tubers at lifting time.

                Figure 2: The potato Tuber

The three main shapes of potato tubers are round, associated with deep eyes, oval and, kidney.  The diagramme below shows these shapes with each tuber bearing a single apical sprout.

Figure 3: The 3 main shapes of potato tubers

Tubers pass through a period of dormancy immediately after harvest and before they start to sprout.  The rate of sprouting can be increased by treating the tubers with chemicals or by keeping them at high temperatures, and sprouting can be delayed by low temperatures.  Planting unsprouted tubers in cold, wet soil can cause them to rot rather than to grow.  During the growth of the plant, two stages of tuber development are important; ‘tuber initiation’ which is the formation of the very small tubers, and ‘tuber swelling’ which is the time when the tuber enlarges, and this takes place when the plant is flowering.  Dry periods or poor irrigation during these periods will greatly reduce yields.

2.  VARIETIES

There is a fairly wide range of potato varieties, each one having its own characteristics. Potatoes are propagated vegetatively by planting tubers; each tuber will produce the characteristics of its parent plant so that the grower can obtain an even crop.  There are a number of factors which affect the choice of variety, and these are as follows:

Yield – Potatoes are expensive crops to grow because seed is expensive, high levels of fertilizer are required and it is labour intensive.  Due to the high cost, the grower must aim for the highest possible yield from his crop.

Maturity – Some varieties mature earlier than others, and this factor can be important particularly for winter crops.  Late maturing varieties may be damaged by frost.

Resistance to Disease – The most serious disease of potatoes is Late Blight, which attacks the growing crop during warm, wet weather from December to February.  Although sprays are available, the actual spraying can be difficult and ineffective during heavy rainfalls. Certain varieties are more tolerant to disease than others.

Types of Flesh – This can affect the marketing of the crop. White flesh varieties for domestic use are preferred and varieties which keep their shape during cooking.  Certain varieties are better for chips because they do not absorb too much fat during cooking and produce a nice, crisp chip.  These varieties are preferred by the manufacturers of potato crisps.

Depth of Eyes – Potatoes with deep eyes are difficult to peel, particularly with mechanical peelers used in restaurants and chip shops.  This factor can also affect the importance of the shape of the potato.

The following varieties are available, and new varieties are continually being tested and introduced.

Early Maturing – (less than 100 days)

Van der Plank – This cultivar produces good quality, white fleshed tubers with shallow eyes.  They are a medium yielding cultivar and are susceptible to Late Blight.  Tubers keep well in storage and do not break up easily when cooked. It is a slow sprouter and only well-sprouted seed should be planted.

Medium Maturing – (100 – 120 days)

Up-To-Date – Is a high yielding variety with white skinned and white fleshed tubers of very good quality, which keeps fairly well in store. It is mainly used for the frozen chip market. The foliage and tubers are very susceptible to Late Blight, Leaf Roll, Virus X and Mosaic.  This variety should be grown as a winter irrigated crop when Late Blight is less of a risk.

BP 1 – The tubers are oval, white-fleshed and have a hard skin.  This is a very high-yielding variety which is fairly resistant to Late Blight. This variety is mainly used for the production of fresh chip market.

Both the above mentioned varieties make up 77% of the potatoes produced in Southern Africa.

Late Maturing – (120 days +)

Pimpernel – Produces about two thirds of the yield of the Up-to-Date variety. Tubers are red-skinned, yellow-fleshed with high starch content and they are very suitable for the production of chips. Good keeping quality. Has a moderate resistance to Late Blight and is fairly resistant to the virus diseases. It is recommended for the more fertile light soils for both summer and winter planting.

It is most important to use good quality seed from a known source in order to achieve high yields and good quality crops. Growing crops are inspected by Government Inspectors to make sure they are free from Leaf Roll Virus, Mosaic Virus diseases, Bacterial Wilt, Nematodes and other pests.

Commercial growers of potatoes buy new A grade seed periodically. They also plant seed saved from their own crop for some years.  When crops are lifted, they are graded into three grades as follows:

Ware – potatoes large enough for sale.

Seed – Potatoes between 25 mm and 50 mm in length. The ideal size for a seed potato is the size of a hen’s egg.

Chats – potatoes less than 25 mm in size. These can be used for stock feed.

Seed saved from the commercial crop can be bulked up by growing a small area purely for the production of more seed. The following precautions should be followed when producing and keeping potato seed.

• Grow the seed crop on a site at least 200 metres away from other potato and vegetable crops.
• Plant the seed crop early before the normal commercial crop.
• Use systemic pesticides to kill all aphids throughout the life of the crop.
• Keep the headland at least 5 metres around the whole crop, and make sure it is free of weeds at all times.
• Go through the crop at least three times and remove all diseased plants, particularly those with viral and bacterial diseases.
• As soon as the tubers reach seed size, destroy the tops of the plants. This reduces the chance of infection by viruses, and also stops the tubers from becoming too big to be used for seed.
• Discard the potatoes from the first 10 rows from the outside of the plot, and sell these for eating.
• Cultivate the plot as little as possible.

1.  SOIL PREPARATION

Potatoes can be grown in a variety of soils however they do best in sandy loam soils with high organic-matter content. Heavy clay soils should produce good yields although they tend to produce badly shaped tubers. Heavy clay soils can cause difficulties at harvesting if the land is wet. Potatoes are tolerant of soil acidity, and will grow in soils with a pH as low as 4,8 but this generally results in impaired growth. Potatoes prefer a soil pH between 5,0 and 5,5. Lime should not be applied just before the crop is planted because this can cause a scab on the tubers. If lime is required, it should be applied earlier in the rotation.

Potatoes are very susceptible to eelworm (nematodes) infection and the rotation used should allow for potatoes to be grown on the same land once every three or four years. Potatoes should never be grown in the same rotation as tobacco or tomatoes because of the eelworm problem.  Where it is suspected that eelworms are present, soils should be fumigated with a registered nematicide.

Although potatoes are shallow rooted, it is necessary to provide a deep, well cultivated seedbed at least 600 mm deep. This is to allow for ease of making ridges before and after planting, and to allow the tubers to swell during growth. As the seed is large, only a medium tilth is required. Crusting of the soil surface is highly undesirable therefore deep ploughing followed by discing should produce a suitable seedbed.

2.  SEED

Seed should be sprouted before planting because such seed is seen to be fertile, and will start growing as soon as it is planted.  Unsprouted seed planted into cold, wet soil can rot instead of growing. Seed is sprouted by putting it into small trays or boxes called chitting trays. They should be stacked in tiers so that all the seed has access to daylight. Sprouting in the dark causes long, weak shoots to develop and these are highly undesirable. Newly sprouted seed produces the most vigorous plants and the highest yields. The seed should be planted when the sprouts are between 5 mm and 15 mm long. The tubers selected should be firm, disease free, undamaged and have strong sprouts; green tubers with sprouts are an advantageous to the farmer.

Tubers of the normal seed size/e.g., up to the size of a hen’s egg, are planted whole. The larger tubers can be cut in half and then planted. Large tubers should be cut lengthwise from the rose end to the heel end; there should be two or more eyes on each half. Cutting should be done two weeks before planting, and the cut pieces stored under wet sacks to allow the cut faces to heal and produce a corky layer to prevent the cut potato becoming infected with disease. Cut pieces may be planted without healing, but the cut face should be treated with a fungicide dust to prevent infection; Captam, Zineb or Thiram may be used.

Sprouting can be sped up by using the following methods:

• Placing the seed in chitting trays and keeping it at a constant temperature of 30 – 35°C.  Covering   the seed with a sack or a tarpaulin in moderate sunshine will help to start sprouting.

• Storing the tubers in an air-tight room at a temperature of 21°C – 27°C, and containing 0,1% of acetylene gas in the air. Acetylene gas can be generated by adding the chemical Calcium Carbide to water, and 30 grams of chemical, will produce enough gas for a room 2 m³.

• Immersing the tubers in a solution of acetylene for 4 – 6 hours.  The solution can be prepared by adding 230 grams of Calcium Carbide to 45 litres of water.

• Sprouting can be delayed for up to one year by storing the seed at a low temperature of 4°C, or by     applying a chemical sprout inhibitor to the seed.

3.  PLANTING

The normal method of planting potatoes is to ridge the land using a tractor and ridger. The potatoes are planted in the bottom of the furrows by hand. The ridges are spilt back to cover the seed tubers and the fertilizer.  If decomposed manure or compost is being used, it should be spread along the bottom of the furrows and the seed planted on the manure.

Figure 1: Placement of manure and potato seed

An alternative to planting by hand is to use an automatic planting machine which opens a furrow, plants the seed and ridges the land all in one operation, and it does two rows at a time. The hoppers hold 300 kgs of seed tubers, and the machine can plant 3 hectares in a day.

         Figure 2: A mechanical potato planter

                Source: www.bandbtractors.co.uk

An important point when planting potatoes is to make sure that the sprouts are not damaged or knocked off the tubers.

Depth of Planting: Depending on the soil, moisture, seed size and length of sprouts, the planting depth can vary from 7.5 cm to 15 cm. Seed that is to be irrigated can be planted at a shallow depth, while seed that is for dry land planting should be deeper to prevent the tubers drying out.

Seed Spacing: The distance between rows and that between plants will vary according to seed size, fertility of the soil, and whether the crop is for market or seed production. The standard spacings are 60 cm – 120 cm between rows and 20 cm – 30 cm between plants. Tubers will vary in size from 25 mm to 56 mm so that a pocket may contain a large number of small tubers or a smaller number of large tubers. Planting depth under irrigation is between 7 cm – 10 cm and 15 cm for dryland production.

Where larger, uncut seed is used, the distance between plants should be increased. Each sprout that grows from a tuber is an individual, self-contained plant, so that large seed with many sprouts will produce more plants than small seed with few sprouts.  Large seed planted further apart (up to 60 cm) will produce the same yield as small seed planted close together. The basic factor which determines the yield of the crop is the weight of the seed planted rather than the number of tubers planted per hectare.

If the crop is being grown for seed production, the distance between the plants should be reduced because the crop will be harvested before it reaches full maturity. The objective in seed production is to produce a crop of small tubers, and the distance between the plants should be reduced to 15 cm.  Distance between rows can be from 60 cm – 120 cm to allow access to the crop for inspection of the plants for disease; this practice is called rogueing the crop.

• Time of Planting.  Although potatoes may be planted at any time during the year, there are two main planting periods. These are designed to avoid damage to the crop from frost and disease, particularly Late Blight.

• Summer Crop planted in November with the first rains (August – January).  It is not advisable to plant into hot, dry soils prior to the rains, but earlier plantings can be made with supplementary irrigation.  Earlier planting can give very high yields, but lifting the crop can be difficult if the soil is very wet at harvest.  November planted crops will mature towards the end of the rainy season and may be subject to Late Blight.

• First Winter Crop.  Planted in the period February to July. In areas which are liable to frost, the crop should be planted in February, with the later planting date being used in the Lowveld to take advantage of the cooler weather. This crop is liable to attack from Late Blight, and resistant varieties should be used.

4.  IRRIGATION

Flood irrigation is much more suitable for potatoes than overhead irrigation because with flood irrigation the leaves and stems of the plants do not get wet. Wet foliage makes the plants much more susceptible to Blight. Due to the crop being shallow rooting, smaller applications of water should be put on at more frequent intervals than for other crops. Once the tubers have begun to form (tuber initiation), water is most important, and the soil should not be allowed to dry out to below field capacity. However, too much water will leach nutrients out of the soil, while too little water will cause badly formed tubers and low yields. Before planting the seed, the soil should be irrigated to bring the water content up to field capacity to a depth of 60 cm. The following table is a guide to the amount and frequency of water applications to the crop according to the type of soil.

Table 1: Irrigation of Potatoes

Moisture Holding Capacity	Soil Texture	Hot Months	Cool Months	Total Water Required
8%	Light	3 – 4 days	5 – 7 days	30 mm
12%	Medium	5 – 6 days	10 – 12 days	40 – 45 mm
15%	Heavy	6 – 7 days	12 – 14 days	50 – 55 mm

Irrigation should be stopped 2 – 4 weeks before starting to harvest the crop or when the potato plant starts dying.

Cultivation: Potatoes can suffer from competition by weeds because they are shallow rooted plants. Furthermore, the tubers are formed on the sides of the plant, below ground. Too much cultivation (particularly by mechanical cultivators) can damage the roots of the plants and knock off the developing tubers.  Weeds should be controlled by hand, by light cultivating or by chemical herbicides. The main cultivating operation should be earthing up the soil by running the ridgers through the crop. This is necessary to protect the developing tubers from being exposed to daylight and becoming green as well as to protect them from damage from the potato tuber moth and from Late Blight. Earthing up can start when all the shoots have appeared above the soil surface, and should be completed by the time the plants are 25 cm high. The figure below shows plants that have been correctly and incorrectly earthed up.

Figure 3: Plants that have been correctly and incorrectly earthed up

Figure 4: A Ridger used to make ridges and to cover

1.  FERTILIZATION

Potatoes are greedy feeders within a relatively short period, and they require fairly large amounts of the major nutrients. However, because of their short growing period, some of the fertilizer nutrients applied to the potato crop are not used up and remain behind in the soil so that the farmer can reduce the fertilizer application to the following year’s crop.  Too much Nitrogen should not be applied to potatoes because it will produce large, soft tops which will be more liable to disease.

The average fertilizer recommendations are:

• Fertile soils:
  • Fertilize with 600kg/ha of 2:3:4(30) at planting time.
  • Top-dress the potato crop at 6 – 8 weeks with 500kg/ha of LAN.
• Infertile soils:
  • Fertilize with 1200kg/ha of 2:3:4(30) at planting time.
  • Top-dress the crop at 6 – 8 weeks with 350kg/ha of LAN.

The placing of the fertilizer is important for potatoes. The phosphate and potash is used early in the plant’s life, and should be placed below the seed so that the young roots can easily reach these nutrients. The different methods of placing fertilizer in the seedbed are shown below.

Table 1: methods of placing fertilizer in the seedbed

Fertilizer placed in the furrow by hand	Efficient method but the fertilizer may burn the seed
Fertilizer placed in the furrow and partly covered by soil	Efficient method with no risk of burn
Fertilizer placed in bands on either side of the seed	Efficient method
Fertilizer broadcast before ridging	Not efficient. Roots can reach some of the fertilizer
Fertilizer broadcast after ridging	Fairly efficient. Roots can reach some of the fertilizer

HERBICIDES

• These are pre-emergence herbicides and should be applied to the land before the crop is planted. This ensures that the land is clean when the young potatoes sprouts emerge from the soil.

• 2-4D: can be used with the potato crop.

• Linuon: is a pre-emergence weed killer

• Metazachlor: kills grasses and broad leaf weeds, pre-emergence.

• EPTC: kills water grass on all except very heavy soils.

When applying herbicides to the land, it is most important to read the directions on the leaflet very carefully. Make sure that the herbicide is mixed at the correct strength and applied in the right quantity. In many parts of the world, potato crops can be grown without using any mechanical cultivation at all, other than ridging up, and this is possible by using both pre-emergence and post emergence herbicides.

2.  PESTS AND DISEASES

 

PESTS

Definition Leys: a piece of land put down to grass, clover, etc., for a single season or a limited number of years, in contrast to permanent pasture.

Nematodes:  These are eelworms, and are the main pest of potatoes. The three main types of nematode are the Root-knot Nematode, the Root-lesion Nematode and the Burrowing Nematode.  Control of these pests is by prevention rather than cure, and the following precautions should be taken:

• Use proper rotations, and do not grow potatoes on the same land without a gap of 3 – 4 years between crops. Include eelworm resistant grass leys in the rotation, such as Yatambora Rhodes grass or Sabi Panicum grass.

• Do not grow potatoes in the same rotation as tobacco, tomatoes and other related crops.

• If it is suspected that eelworms are present in the soil, fumigation may be necessary using a registered nematicide, which is also used for tobacco crops. The fumigant should be applied to the whole land, when the soil is moist, and the land should be rolled immediately after treatment to seal the soil surface. At least 3 – 4 weeks should be allowed before the crop is planted.

Potato Tuber Moth: This pest can cause serious damage to the potato crop because the larvae tunnel into the leaves, damage the stems thereby cutting off the supply of nutrients, and tunnel into the tubers.  These pests can be controlled by adopting the following measures:

• Keep the land free of other hosts of the moth. The moth can live on such a host when no potatoes are being grown in the area.
• Clean the land well and try to remove any potatoes left lying in the soil as these can carry the pest.
• Ridge up the tubers to keep them covered with soil so that they cannot be attacked by the moth.
• Where the moth is a serious problem, a number of chemical insecticides can be used to spray onto the foliage of the growing crop.
• A moth called Copidosoma has been introduced into the potato growing areas. The larvae of the wasp feed on the caterpillars of the Tuber Moth and destroy them.  This method of biological control has proved very successful.
•  

Figure 1 and 2: Potato tuber moth (left) and larvae (right)

Source: mrgoutham.blogspot.com            Source: agritech.tnau.ac.in

Aphids: Control of Aphids on a potato crop is important because they can transmit viral diseases such as Leaf Roll and Mosaic. Control is particularly important where crops are being grown for seed.  A number of systemic insecticides are available for spraying crops against aphid attack.

DISEASES

Late Blight: (Phytophthora infestans). This is a very serious disease of potatoes in many parts of the world, and was the cause of the famous potato famine in Ireland. It is a fungal disease, which attacks both the foliage and the tubers. In warm, damp weather it spreads very quickly and can destroy all the foliage of a crop within five days. It starts as small dark green to brown spots on the leaves, and these run together to turn the leaves and stems of the plant black.  Infected tubers can carry the disease over from one crop to the next.  Plant breeders have produced varieties that are fairly resistant to the disease and this has helped to contain it, but Summer potato crops should be sprayed as a precaution. Captab and Chlorothalonil based chemicals are two that are used to prevent the disease, but they must be sprayed onto the crop before any signs of the disease have appeared.

Early Blight: (Alternaria solani). Also causes dark brown spots on the leaves, and can be prevented by early spraying with fungicide.

Figure 3 and 4: Late blight on potatoes

Source: usablight.org                                                 Source: www.longislandhort.cornell.edu

Common Scab: This causes deep rough or raised circular brown marks on the tubers, and is a fairly common disease. It can be caused by high soil temperatures, liming just before planting the crop, or by ploughing in large amounts of un-decomposed organic matter before planting which causes large amounts of air to be trapped in the soil.  No fungicide is available for treating scab.

Figure 5: Common scab

                                  Source: en.wikipedia.org

Virus Diseases: If seed is saved from successive crops and planted on the farm for a number of years, the crops become poorer and poorer.  The reason for this is disease caused by a number of viruses which are spread by aphids.  The main virus diseases are:

Leaf Roll: causes the leaves to roll up and reduces yield.

Figure 6: Potato leaf roll virus                            Source: www.quirkyscience.com        

Leaf Drop Streak caused by Potato Virus Y which produces fine black streaks along the veins of the leaves.

Mosaic: Caused by Potato Virus X.  This produces mottling on the leaves, but some varieties may show no symptoms apart from a drop in yield.  The effects on the yields of the plants are shown in the diagramme below.

Figure 7: Potato mosaic virus

Source: commons.wikimedia.org

3.  HARVESTING

Normally the haulms, or tops, of the potato crop are allowed to die off naturally and the crop is then harvested.  However, for crops that have been grown for seed production, or if an attack of Late Blight is feared, the tops may be destroyed.  This can be done by hand, by cutting with a rotary cutter, or by applying a chemical.

Harvesting can start once the tops have died off and the skin on the tubers has hardened enough to avoid mechanical damage during lifting.  The best yields are obtained if the tubers are left in the soil for about 2 weeks after 95% of the leaves have died off.  If the crop is being lifted during hot, dry weather, the tubers should be stored in a cool place out of the sun to avoid shrinking and a reduction in keeping quality. 

The following table, taken from F.B. Barnes of CONEX shows the average mass of tubers harvested per plant at various stages.

Table 1: Average Mass of Tubers Harvested per Plant

Condition of Plant	Average Mass of Tubers per Plant
Leaf tips beginning to die off	584 grams
About 80% of the leaves dead	740 grams
All leaves dead, stalks partly dead	794 grams
Stalks brown and dry	760 grams

Potatoes can be lifted by hand or by mechanical diggers.  As the crop is shallow-rooted and is grown along ridges, lifting by mechanical digger is very effective. Both these machines leave the potatoes on the surface of the soil, and they are then collected and bagged by hand.

Yields: Should average 17 – 20 tons per hectare for Summer crops, and 24 – 27 tons per hectare for Winter-irrigated crops and, under conditions of good management, they can go as high as 40 tons per hectare.

Storage: Potatoes are grown all the year round and are not stored for long periods.  In the U.E. the main crop is lifted in the autumn, stored and sorted and sold during the winter and spring. Tubers that are to be stored should be sound, clean, and stored in a cool, dark place to prevent greening and sprouting. If stored at temperatures between 2°C and 4°C, they will only start to sprout after 8 to 12 months. Ware potatoes to be stored and should be dressed with a 1% Malathion dressing.