Solanum tuberosum (PROSEA)
- Protologue: Sp. pl.: 185 (1753).
- Family: Solanaceae
- Chromosome number: 2n= 4x= 48
- Solanum esculentum Neck. (1768),
- S. cultum (A. DC.) Berth. (1911),
- S. tuberosum L. subsp. tuberosum [(L.) Hawkes] (1956).
- Irish potato, English potato, white potato (En).
- Pomme de terre (Fr)
- Indonesia: kentang
- Malaysia: ubi kentang
- Papua New Guinea: poteto (Pidgin)
- Philippines: patatas (Filipino), papas (Tagalog)
- Cambodia: dâmlô:ng barang
- Laos: man fàlangx
- Thailand: man-farang (central), man-alu (northern)
- Vietnam: khoai tây.
Origin and geographic distribution
The species of Solanum are considered native to Central and South America, mainly to the highland plains ("puna") and Andes mountains between 40°N to 45°S. The cultivated potato probably originated in the Peru-Bolivia region at least 8000 years ago, and spread rapidly through the Andean highlands during ancient civilization. The potato was introduced into Europe during the latter half of the 16th Century. It was then spread throughout the world, more particularly in the temperate regions of the northern hemisphere, especially continental Europe and the former Soviet Union.
During the 18th and 19th Centuries the potato was introduced into several tropical and subtropical countries, including the South-East Asian region, mainly by colonists from Europe. Most recently, it was introduced into Papua New Guinea in the early 1930s, where it is now a popular cash crop in the highlands at altitudes above 1500 m.
Worldwide, tubers of the potato crop are used for direct human consumption (48%), processing (11% of which 2% is for the production of starch), vegetative propagation (13%), stock feed (20%), and the remaining 8% is waste. Worldwide the use of potatoes for alcohol production is negligible, but can be important in some locations. Consumption per capita in the developing countries of Asia and Oceania is low but rising.
Potato tubers are consumed in many forms. Whole, they may be boiled, roasted or steamed in their skins; they may be peeled and then boiled or steamed and mashed with margarine, butter, or dripping, with or without milk, or they may be baked or roasted. Large quantities are consumed fried as chips (French fries, pommes frites), or as thinly sliced crisps. In many Asian countries, potatoes are part of various curry dishes. The fairly good storing ability of the raw tuber, in addition to processing into many forms of dehydrated, frozen and canned tubers, results in a supply reasonably independent of season. This and the simple cooking methods contribute to potato's importance as a major world crop.
Production and international trade
Total world potato production is nearly 270 million t (1990). Production in Asia is nearly 65 million t, including China with over 33 million t. By contrast, production in Oceania is only 1.4 million t, of which Australia and New Zealand produce over 1.3 million t. Production in South-East Asia is 1.0 million t. Potato production in Papua New Guinea remains negligible. The world total potato export is estimated at nearly 8 million t. Asian exports are nearly 1 million t. Oceania potato exports amount to nearly 33 000 t, virtually all by Australia and New Zealand.
The published analyses of potato's chemical composition and nutritive value are not strictly comparable. Considerable losses of constituents also occur during different preparation and cooking methods. In general, potato is competitive with other foodstuffs as a rich source of energy, high quality proteins and substantial amounts of vitamins, minerals and trace elements.
The freshly harvested, raw tuber consists of 20-25% dry matter, of which 65-80% is starch. Per 100 g the raw, edible portion of the tuber contains: water 77 g, protein 2.1 g, carbohydrates 18.5 g, fibre 2.1 g. It also contains niacin 1.5 mg, thiamine 0.1 mg, riboflavin 0.04 mg, ascorbic acid 20 mg, Ca 9 mg, P 50 mg, K 410 mg, and Fe 0.8 mg. Its energy value averages 335 kJ per 100 g. The tubers also contain a number of phenolic compounds, which are partly responsible for certain types of discolouration in raw or processed products, especially after bruising. In addition, potato contains about 0.01-0.1% (on a dry matter basis) of a steroid alkaloid, solanine, which is toxic. Solanine is mainly concentrated in the skin and exposure to light increases the amount present.
- Erect, juicy herb with numerous fleshy, robust, angular, branched stems up to 1.2 m tall and subterranean tubiform stolons.
- Root system usually 40-50 cm deep, without obstructions up to 1 m. Tubers developing at the tip of the stolons, globose to ellipsoid, very variable in size, weight and colour; tuber skin with scars of scale leaves ("eyebrows"), axillary buds ("eyes", usually several eyebuds per eyebrow), numerous lenticels, almost impermeable to chemicals, gases and liquids, providing good protection against micro-organisms and water loss; number of eyes very variable, normally about 10-15 on a tuber of 50 g.
- Stem up to 1.5 cm in diameter, usually hollow, winged; wings sometimes decurrent and undulate-crenulate.
- Leaves alternate, petiolate, odd-pinnately compound, with or without numerous interstitial leaflets, in outline 10-30 cm × 5-15 cm; lateral leaflets opposite or alternate, usually 3-4 pairs, very unequal in size, largest ones stalked, ovate to ovate-elliptical, 2-10 cm × 1-6 cm, smallest ones subsessile, ovate to suborbicular, 1-15 mm in diameter; terminal leaflet usually largest; all leaflets thinly to densely pubescent, dark green, pinnatinerved.
- Inflorescence a many-flowered cymose panicle, sometimes with small bracteoles; peduncle 5-15 cm long, branching above.
- Pedicel up to 3.5 cm long, articulate at or above the middle; flowers white or white suffused with pink or violet, typically with a greenish-yellow central star; calyx campanulate, 1.5-2 cm in diameter, deeply 5-partite, pubescent outside; corolla subrotate to rotate-stellate, 2-4 cm in diameter, with 5 acuminate lobes, finely veined, pubescent outside; anthers 5, 5-7 mm long, free, erect but slightly curved around the style, yellow, opening by 2 apical pores, each on a short, thick filament; style up to 13 mm long, stigma clavellate to capitate.
- Fruit a subglobose berry, up to 2 cm in diameter, yellow-green, 2-carpellate, many-seeded, poisonous.
- Seed flat, subcircular to ovate, 1-3 mm in diameter, pale yellow-brownish.
Growth and development
The general growth and development pattern of potato plants are characteristic within cultivars, but also vary with the environment and fertilizer treatments.
After harvest, tubers usually enter a period of dormancy, meaning that the buds do not start to grow when exposed to favourable conditions (e.g. darkness, 20°C and high humidity). The duration of dormancy depends on cultivar, maturity of the tuber, soil and climatic conditions during growth, and storage conditions. It often lasts for 2-4(-6) months, but in some cases bud growth has already started before harvest. High temperatures during growth and storage tend to shorten dormancy and low temperatures prolong the dormancy period.
Dormancy can be broken by treating tubers with chemicals such as chlorhydrin, thiourea, or gibberellic acid. However, naturally sprouted tubers without the use of chemicals are preferred, as they give a more uniform germination and better growth. Once the period of natural dormancy has ended, the seed tuber passes through various subsequent physiological stages: apical dominance of sprouts, multiple sprouting, and senility. An apical sprout is dominant over the other buds, so these remain dormant. Only when this apical or top sprout is removed (de-sprouting) will the other buds of the tuber develop sprouts during this phase. The degree of apical dominance depends on the cultivar. The optimum stage for planting is multiple sprouting, not only because of the number of sprouts, but also because of the vigour of individual sprouts. The number of eyes per tuber depends mainly on tuber size and cultivar, the number of sprouts (germinating eyes) per tuber strongly depends on environmental conditions. After prolonged storage seed tubers may reach the stage of senility and have then become unfit for planting.
After planting, sprouts develop into stems. A main stem grows directly from the seed tuber. The lower lateral branches from the main stem are called secondary stems. Apart from secondary stems, a stem may develop branches at higher nodes several times during its growth. Main and secondary stems grow and behave like independent plants, and develop roots, stolons and tubers. Plant population is, therefore, best expressed as number of stems, rather than number of plants.
In plants growing from tubers, adventitious roots arise from the nodes of the underground stems. Plants grown from true seed develop a slender taproot with lateral branches.
The tuber is a modified stem which develops by the swelling of the tip of an underground stem (stolon).
The length of the growing period mainly depends upon cultivar, amount of fertilizer (particularly N) and climatic conditions. In South-East Asia, cultivars generally mature in 3-5 months.
Other botanical information
S. tuberosum is a complex species with diploid, triploid and tetraploid representatives. It is mainly known in cultivated state and it is quite questionable whether it has existed or still exists in the wild. The tetraploid plants are most important worldwide; they are classified into two cultivar groups:
- cv. group Andigena (synonym: subsp. andigena Hawkes), mainly occurring in South America from Venezuela to northern Argentina and is supposed to have originated from unknown wild diploid species from the Andean region between Bolivia and Venezuela, by doubling of the chromosome numbers. Its members are tall, often straggling; leaves intensively dissected with numerous leaflets; adapted to short days; they usually produce rather irregularly shaped, deep-eyed and often pigmented tubers that are usually not acceptable in the more sophisticated markets of Europe and North America. To a very small extent they are also grown in Mexico and Guatemala.
- cv. group Tuberosum (synonym: subsp. tuberosum ), mainly occurring in Europe and North America, supposed to have originated from selections during the last 300 years from cv. group Andigena. Its members are smaller, less straggling, leaves less dissected; adapted to long days. This group has become a world crop and is now cosmopolitan in distribution. It was first introduced into Europe where selection began and from where cultivars spread to North America. With growing interest in the crop, supplementary introductions from South America were made to both these regions. Continuing selection has led to the establishment of numerous cultivars, a process still going on wherever the crop is grown. Cultivars vary in characteristics of their tubers, sprouts, foliage, flowers, growth cycle and disease resistance; much of the variability is also influenced by the environment. Propagation by true seed, however, produces variability due to genetic recombination.
Potato requires a well-distributed rainfall of 500-750 mm in a growing period of 3-4.5 months. Most commercial cultivars of potato tuberize best in cool climates with night temperatures below 20°C. Little or no tuberization occurs at night temperatures above 22°C. Optimum day temperatures for dry matter production are within the range of 20-25°C. High light intensities favour dry matter production through their effect on photosynthesis. Short daylengths (12-13 hours) lead to earlier maturity.
In the short daylength conditions of the tropics and subtropics, maximum yields can usually be obtained in cool highland areas and in cooler seasons. In Papua New Guinea, optimum growth of potato takes place at altitudes between 1500 and 2200 m above sea-level, where day temperatures are about 25°C and night temperatures about 20°C.
Potato is tolerant of a rather wide variety of soils, except heavy, waterlogged clays. Good drainage is of great importance. Impermeable layers in the soil limit rooting depth and the amount of available water, and so greatly reduce yields. Deep soils with good water retention and aeration give best growth and yields. The most suitable soil pH is between 4.8 and 7.0. At higher pH, tubers are liable to suffer from scab disease.
Propagation and planting
Potato is normally propagated vegetatively by small (40-100 g) tubers, called "seed tubers" or "seed potatoes". It can also be propagated by pieces of tuber ("seed pieces") or by true seed. The seed rate (tubers) ranges from 1.5-4.0 t/ha. The first problem facing growers in developing countries is that of obtaining supplies of healthy planting material of a suitable cultivar at an acceptable price. In many countries there are no provisions for local propagation of tubers and the import of seed potato is expensive and poorly organized. Some countries undertake traditional propagation of the seed tubers on sites in the highlands with suitable cool but frost-free climates and where the population of insect vectors of diseases can be kept at an adequately low level.
Another line of development is that of plant tissue culture. In recent years a variety of rapid multiplication techniques has been developed based on tissue culture. These techniques start with in-vitro multiplication of disease-free plantlets by nodal cuttings. Subsequently, pathogen-free plants are raised from in-vitro plantlets transplanted to insect-free greenhouses to produce high quality seed potatoes. Alternatively, mini-tubers are produced from in-vitro plants planted at high plant densities. These mini-tubers are then planted in isolated fields or otherwise protected from insects to produce normal size seed potatoes. The next phase is for seed potato farms and private seed growers to conventionally multiply these seed tubers. Optimal planting and crop management techniques ensure high multiplication rates and high health status. In various countries, including Vietnam and Papua New Guinea, this scheme has resulted in a gradual change from dependence on seed imports for field planting to self-sufficiency.
Other useful methods of propagation are the production of micro-tubers through in-vitro tuberization of plantlets, and rapid multiplication methods by means of sprout cuttings, single-node cuttings, stem cuttings and leaf bud cuttings. These rapid multiplication methods are used to multiply healthy plant material of a certain cultivar for which a limited amount of clean material is available.
Recently, the use of true potato seed for propagation has aroused great interest. True seed does not transmit most of the potato diseases, is very light and is easy to transport. Promising methods to grow potatoes from true seed include raising seedlings in a nursery and transplanting them to the field. An alternative method includes the production of small seed tubers from true seed under protected conditions in nurseries. Considerable progress has been made in research efforts to reduce the variability among plants and tubers derived from true seed.
No single planting time is considered optimum for all potato growing regions. In regions with no critical dry season, potato can be planted at any time, provided that temperatures are not too high. Under normal conditions the longer the growing season the larger will be the yield.
In regions with a critical dry season, planting early in the rainy season is best. If the rainy season is long and excessive, time of planting is usually towards the end of the rainy season. The optimum time for planting in some parts of the lowlands of Papua New Guinea is towards the end of April when the wet season is coming to an end. In these areas, potato can be planted successfully until early September. Any potato grown after this date is subjected to heavy rainfall during growth or harvest which favours the development of diseases and reduces the effectiveness of chemical applications.
Tubers planted to produce consumption potatoes should generally be planted in rows 75-100 cm apart with a spacing of 30-40 cm within the row (25 000-44 000 plants/ha). The closer spacing should be used in fertile soils and good rainfall areas to avoid the production of very large tubers. Seed potatoes are planted at a spacing of 15-20 cm within the row (about 80 000 plants/ha).
Potatoes are planted at a depth of 5-15 cm (measured from the top of the tuber). Planting depth is greater under warm, dry conditions than under cool, wet conditions. Shallow plantings should be avoided, because the lower nodes of the stem must remain covered to encourage tuberization and to avoid greening of tubers and tuber moth damage. Earthing up or hilling is carried out to control weeds and to avoid greening of the tubers. Potatoes are normally planted by hand in Papua New Guinea, but mechanical planters are available. Wide ridges or mounds are required for intercropping. Potatoes can be intercropped with a wide range of annual crops such as sweet potato, maize or even pyrethrum.
Adequate control of weeds is required to ensure high yields. In the tropics, manual weeding is generally practised in small-scale production, but herbicides are sometimes used in large-scale production.
Potato responds well to high fertility. Fertilizers are needed if the land has been continuously cropped. Well-decomposed animal manure or compost should be applied at 20-40 t/ha. The South-East Asian Programme for Potato Research and Development (SAPPRAD) recommends the use of 750 kg/ha of 12:12:17 NPK and 250 kg/ha of triple superphosphate.
Rotation is a common practice to avoid a build-up of pathogens affecting potato, and to reduce the level of soil infestation once the soil has been contaminated. Rotations should not include crops that are common hosts for these diseases and pests. Rice, maize and legumes are recommended.
Diseases and pests
Diseases of potatoes are many and common in Asia and Oceania, affecting yield and quality. The most prevalent diseases are:
- bacterial diseases, including bacterial wilt caused by Pseudomonas solanacearum, bacterial soft rot caused mainly by Erwinia carotivora, and common scab caused by Streptomyces scabies;
- fungal diseases including late blight caused by Phytophthora infestans, early blight or target spot caused by Alternaria solani, black scurf caused by Rhizoctonia solani, and pink rot caused by Phytophthora erythroseptica;
- several viral diseases including potato leaf roll virus (PLRV) and the mosaic viruses, i.e. potato virus X (PVX), and potato virus Y (PVY).
Physiological disorders and tuber defects cause additional problems for the production of potato in many regions. The commonest physiological disorders are hollow heart and internal brown spot. Common external tuber defects are greening, growth cracking and tuber deformation.
Insect pests of potato are particularly destructive in all regions. The most common pests are aphids, tuber moth, mites, ants and ladybirds. As aphids are the main vectors of potato viruses, their population should be controlled in potato planted for seed production. Cyst nematodes (Globodera spp.) are very prevalent in many traditional potato-growing areas (e.g. the highlands of the Philippines).
Insect attack is usually of primary importance in hot, dry climates, whereas in hot, wet climates, fungal and bacterial attacks are extra virulent. Viral diseases are common in all developing countries or where it is difficult to obtain virus-free seed tubers. In general the use of disease-free planting material and crop rotation are the most common control measures in various parts of the world.
The constraints in the control of potato diseases and pests are not that methods of control are unknown but that farmers have limited access to adequate information and materials. In order to protect his crop, the farmer needs a reliable service to identify pathogens and pests, advice on control methods and access to pesticides and disease-free planting material. Increasingly, modern cultivars have resistance to or tolerance of some of the major diseases and pests.
Time of harvesting of potato varies with cultivar, cultural practices, climate and price. Tubers harvested while still immature tend to have a low dry matter content and to suffer more skin damage, resulting in easier infection by fungal and bacterial pathogens. However, seed potatoes are often harvested early, to avoid virus infection which may occur during the latter part of the growing season. Late blight attack may also be a reason for early harvesting.
The harvesting operation involves destroying the aboveground parts (haulm), lifting and collecting the tubers. The haulm is destroyed either by manual or mechanical pulling, cutting, chemicals, or chemicals combined with haulm beating. The merits and demerits of each of these methods make the choice a compromise.
In Papua New Guinea, the crop is mature when the haulm is completely dry. This happens in 100-120 days with the cultivar Sequoia (most common), depending on altitude. The haulm is removed by hand after it has been cut with a bush knife, or is killed chemically with gramoxone. The tubers are left in the ground to harden the skin for 5-7 days and then dug up by hand or machine and bagged in the field with minimum handling or grading.
In small-scale farming in the tropics, lifting is done manually using simple implements such as sticks and spades. Mechanical harvesting is carried out only in large-scale farming areas using various types of potato diggers, e.g. ploughs, spinners, or elevator diggers. Semi-automatic diggers lift the tubers from the soil for hand-picking or collection. Harvesting should not be done during or immediately after rain.
In 1990, the average yield of storage potato tubers throughout the world was about 15 t/ha. The average yield in Asia was 13.3 t/ha: 29.6 t/ha in Japan, 15.8 t/ha in India, 12.3 t/ha in Indonesia, 8.9 t/ha in Vietnam, 12.2 t/ha in the Philippines, 9.2 t/ha in Thailand and 11.5 t/ha in China. In Oceania, the average yield was 28.7 t/ha: 30.9 t/ha in New Zealand, 29 t/ha in Australia, 4.5 t/ha in Papua New Guinea. In many tropical and subtropical regions potential yields are much higher than actual yields. However, various constraints (e.g. environmental/seasonal, propagation, crop protection, economic and social) prevent the full expression of this potential.
Handling after harvest
Harvesting or any other handling to which potato tubers are subjected may cause damage ranging from external injury to internal bruising. After harvesting it is advantageous to allow the tubers to dry in heaps for about 1-2 weeks at 10-20°C under high humidity before further handling. During this time the skin hardens, wounds heal, adhering soil dries and disease symptoms become more visible, which facilitates the removal of the infected tubers. Grading should not be started before the curing and hardening have taken place, otherwise further damage occurs.
Before the tubers go into storage, rotten and infected tubers, which may become sources of infection, should be removed. Potato tubers are usually delivered into stores in bags, baskets or crates. To facilitate handling, containers should not be too large; if they are large they should not be filled completely.
Adequate storage methods for seed are essential to ensure that seed tubers of the correct physiological stage are available at the required planting time. After cold storage, seed potatoes should be pre-sprouted in diffuse light to ensure optimal development of sprouts prior to planting. Light is a good alternative to low temperature when storing seed potatoes. By storing seed tubers in diffused light at ambient temperatures, excessive sprouting is avoided and they can be kept in good physiological condition for a long period of time. In South-East Asia, diffused light storage of seed potatoes has been widely adopted by farmers.
Storage of ware potatoes for the market is associated with undesirable quality changes (mainly sprouting, high sugar content, and weight loss due to evaporation and respiration). For prolonged storage, ware potatoes are best stored at about 4°C. However, for processing purposes tubers are better stored at 7-10°C to avoid high sugar levels which cause the browning of fried products. For short-term storage (1-2 months) in the tropics, ware potatoes may be stored at ambient temperatures in the dark, in well-ventilated buildings. In South-East Asia, emphasis is placed upon on-farm storage using inexpensive, well-ventilated constructions.
Processed potato products are of increasing importance in South-East Asian countries, particularly in the Philippines and in Thailand.
According to the International Plant Genetic Resources Institute (IPGRI), there are about 60 000 accessions of germplasm maintained in 40 countries worldwide. However, many cultivars, wild species and hybrids are duplicated. The largest collections are in the former Soviet Union, United Kingdom, United States and Peru. The collections in India, Japan, and China are the largest in Asia. In Oceania, many accessions are also maintained by individual national programmes, though, often in the face of serious management problems. The International Potato Centre (CIP) in Peru has accepted responsibility for maintaining a global collection of potato germplasm for long-term conservation, as a base collection within the IPGRI network of designated genebanks. During the last decade, SAPPRAD has provided special and significant support to countries in the region to enhance, conserve and use global potato genetic resources in production and breeding.
Potato selection and improvement had in fact been conducted by farmers and a few interested scientists in the 19th Century. Since the 1930s it has been concentrated in public agencies as well as private companies. Despite prolonged efforts, progress has been limited. The cultivar "Russett Burbank" was released in 1876 but still accounts for 32% of the potato area in the United States and Canada. The cultivar "Bintje" was released in 1910 and still accounts for 20-30% of the potato area in Western Europe and 40% in the Netherlands, and the parents of "Early Rose" are still a major crop in the former Soviet Union. These cultivars are, of course, high yielding and of good appearance, but they are far from ideal in their agronomic and disease and pest resistance qualities.
Varietal genetic improvement of potato in South-East Asia has received special support from CIP and SAPPRAD.
The Philippines is the leading country in the CIP regional research network for South-East Asia. The collaborative breeding programme in the Philippines has made progress in developing cultivars with resistance to bacterial wilt, heat tolerance for the lowlands and thrips resistance.
Thanks to the leading role of CIP, great progress has been made over the last 30 years in taxonomic and genetic studies of numerous tuber-yielding potato species in tropical and subtropical regions. In 1987 CIP reported that 70 potato programmes in developing countries had received germplasm for evaluation and that 36 superior clones had been multiplied by 22 national programmes and released to farmers. In addition, a large number of clones are in advanced selection and cultivar trials in developing countries, to test their resistance to or tolerance of frost, heat, nematodes, viruses, late blight, or bacterial wilt.
Certainly, potato alone will not solve the impending tropical and subtropical food crisis, but millions of the rice-loving people of South-East Asia eat potato. The quantities of potato marketed are at present relatively small, but there is widespread interest in the crop. Recent biotechnological developments may be useful for producing improved cultivars, including cultivars with improved resistance to the most damaging diseases and pests, and cultivars adapted to the South-East Asian environment.
- Beukema, H.P. & van der Zaag, D.E., 1990. Introduction to potato production. Centre for Agricultural Publishing and Documentation (Pudoc), Wageningen, the Netherlands. 208 pp.
- Burton, W.G., 1966. The potato. 2nd edition. Veenman, H. & Zonen, Wageningen, the Netherlands. 382 pp.
- Dodds, K.S., 1962. Classification of cultivated potatoes. In: Correll, D.S. (Editor): The potato and its wild relatives. Texas Research Foundation, Renner, Texas, United States. pp. 499-507, 517-539.
- FAO, 1991. Potato production and consumption in developing countries. Plant production and protection paper 110. Food and Agriculture Organization (FAO), Rome, Italy. 47 pp.
- Harris, P.M. (Editor), 1992. The potato crop: the scientific basis for improvement. 2nd edition. Chapman & Hall, London, United Kingdom. 909 pp.
- Hughes, M.J., Pitt, A.J., Gorogo, G.D. & Waiange, J.K., 1989. Papua New Guinea seed potato scheme technoguide. Department of Agriculture and Livestock, Food Management Branch, Port Moresby, Papua New Guinea. 45 pp.
- Twiss, P.T.G., 1963. Quality as influenced by harvesting and storage. In: Ivins, J.D. & Milthorpe, F.L. (Editors): The growth of potato. Proceedings of the 10th Easter school in agricultural science. University of Nottingham, London, United Kingdom. pp. 281-291.
- Woolfe, J.A., 1987. The potato in the human diet. Cambridge University Press, Cambridge, United Kingdom. 231 pp.
- van der Zaag, D.E. & Horton, D., 1983. Potato production and utilization in world perspective with special reference to the tropics and subtropics. Potato Research 26: 323-362.
M.E. Wagih & S.G. Wiersema