INTERNATIONAL POTATO CENTER: WORLD SWEETPOTATO ATLAS
UGANDA
HISTORY AND OVERVIEW
The sweetpotato, Ipomoea batatas, preceded the potato, Solanum tuberosum, in its diffusion beyond the American tropics, where it was widely grown in pre-Columbian times. Columbus brought sweetpotato with him on his return to Spain from his first voyage in 1492, eighty years before the potato reached Europe (Purseglove 1968, p. 81). By the 1520s, Portuguese mariners were carrying sweetpotato to ports and territories throughout Africa and Asia, a diffusion later continued by other Europeans (Huntington). The historical record of these various introductions of the sweetpotato to Africa is often unclear, since the term "potato" was derived from "batata" (the Carib term for sweetpotato). Many documents could therefore be referring to either crop, both of which subsequently became important to Uganda. Whatever the circumstances of its introduction(s), sweetpotato has become well established in Uganda due to its high yield relative to both land and labor and its capacity to grow in relatively poor soils where it can be kept until needed, sometimes as a famine reserve crop (Mukasa 2003, p. 329).
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Sweetpotato Production in Uganda
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Sweetpotato was established as an important crop in central and western Uganda prior to 1900, but diffused to the north somewhat later. By the 1950s, sweetpotato occupied about nine percent of Uganda's cropland, ranking behind eleucine millet, bananas, and cassava as the nation's most important food crops. Uganda is now the leading producer of sweetpotato in Africa, and second only to China in the world. Production area has more than doubled since 1980, from 231,000 hectares to nearly 590,000 hectares in 2003 (Hakiza et. al. 2000, p. 47). Reported yields have been fairly consistent over the past decade, at around 4.5 tons per hectare (FAOSTAT 2004). Women generally play the dominant role in the production and utilization of sweetpotato in Uganda, from cultivation activities (planting, weeding, and harvesting) to preparation (peeling, slicing, drying and cooking). Men are typically more involved in the transportation and sales transactions of marketed sweetpotato (Hall et. al. 1998).
Sweetpotato is currently being developed as a means to address one of the most serious health and nutrition problems of sub-Saharan Africa, Vitamin A deficiency. According to a survey undertaken by the World Health Organization in the mid 1990s, approximately fifty percent of the children of Uganda are affected, with symptoms including visual impairment due to any of several specific causes ([e.g] xerophthalmia, corneal scars, and corneal xerosis), as well as greater susceptibility to deadly diseases such as measles, malaria, and diarrhea. Vitamin A deficiency is also a major risk factor for pregnant and lactating women. Green leafy vegetables, the usual source of vitamin A, are not readily and consistently available to many people in Uganda. To address the problem, the Ministry of Health had implemented a national program which included the distribution of vitamin A capsules, but supply and distribution were limited.
In light of these constraints, the Vitamin A for Africa (VITAA) project is addressing this critical need via a widely grown crop, sweetpotato. The International Potato Center (CIP) is a major partner in research directed to develop sweetpotato varieties which are rich in betacarotenes, used by the body to synthesize Vitamin A, and with high dry matter content and other agronomic characteristics sought by Ugandan farmers (Bachou, 2002). The VITAA project is an example of biofortification, intended to develop crop varieties with increased mineral and vitamin content to enhance nutritional standards.
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GEOGRAPHY AND PRODUCTION ZONES
Physical Geography and Climate
Most of Uganda consists of a fertile plateau at an average altitude of approximately 1,000 meters above sea level (masl), surrounded by several Rift Valley lakes and the headwaters of the White Nile. The country's highest peak, Margherita, is over 5,000 masl. Rich soil and regular rainfall permit extensive agriculture in the south, while in the more arid and less fertile north, pastoralism is more common. Approximately 21 percent of the land is cultivated, and 45 percent is woodland and grassland (FAOSTAT; US LOC: Land Use). Uganda's other main food crops include plantains, cassava, millet, sorghum, maize, beans, and groundnuts. A once lucrative cash crop economy (based primarily on cotton, tea, and tobacco) collapsed during the late 1970s and early 1980s (US LOC: Crops).
- Several maps of more specific classifications — for example by soil types, agro-ecological zones, and land use — are available from the European Digital Archive of Soil Maps, Uganda.
Most images are digitized from paper maps, of variable clarity.
Uganda's equatorial climate is moderated by altitude and proximity to water. Maximum temperatures range from approximately 16° C in the southwestern highlands to 25° C in the northwest; but in the northeast, temperatures exceed 30° C most of the year. The southern region experiences two rainy seasons, usually beginning in early April and again in October, with dryer conditions in June and December. In the north, rains occur more sporadically between April and October, while the period from November to March is often very dry. Mean annual rainfall near Lake Victoria can exceed 2,100 millimeters, while the mountainous regions of the southeast and southwest can receive more than 1,500 millimeters of rainfall annually. The lowest mean annual rainfall, in the northeast, is typically about 500 millimeters (US LOC: Climate).
Regional Distribution of Sweetpotato Production
Of all Uganda's major food crops, sweetpotato is the most widely and evenly distributed, but production is generally concentrated in densely populated regions of mid altitudes, from 1,000 to 2,000 meters above sea level (masl), mostly in the southern region closer to Lake Victoria. Sweetpotato is often a major crop in areas where relatively low precipitation favors it over matooke, a variety of cooking banana which has long been popular in Uganda. In some areas where cassava production has declined due to cassava mosaic virus disease (Hakiza et. al. 2000) or where banana cultivation is threatened by black sigatoka disease, sweetpotato cultivation is expanding to fill the gap (PRAPACE).
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Sweetpotato Distribution in Uganda
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Sweetpotato cultivation does not always conform to its most suitable agro-climatic conditions. In areas of central and southern Uganda where conditions are ideal for sweetpotato, matooke is the primary staple. Here, sweetpotato is considered secondary, especially in terms of social status by the Buganda people of the area, and so is not cultivated to the fullest extent possible (Hall et. al. 1998).
PRODUCTION SYSTEMS AND CONSTRAINTS
Cultivation
Sweetpotato is usually grown by subsistence farmers in small plots of less than one-half hectare. Although plots may contain several varieties, the normal practice is to separate each variety in its own section (Hakiza et. al. 2000, p. 47). Planting material usually consists of apical vines, typically 25 to 30 centimeters long from a previous crop, or obtained from a neighbor (Mukasa 2003, p. 333).
The Agricultural Calendar
In southern Uganda, where there is little seasonal variation and generally adequate and well distributed rainfall, the seasonal pattern of agriculture is not sharply defined. In the north, where rainfall is less reliable, seasonal production and consumption patterns are more defined. Crops planted in May are harvested and available as fresh sweetpotato roughly from July through October, effectively the limit of fresh root storage capacity. For several months thereafter, sliced and dried sweetpotato is a staple until cassava becomes available later, roughly March through June. However, with the decline of cassava in many northern districts due to cassava mosaic disease (CMD), that pattern has been jeopardized. A further extension of time depending on dried sweetpotato products is limited by insect infestation, but this problem might be addressed via improved storage techniques (Hall and Devereau 2000, p. 277). (Please see the "Storage" section for more information on this topic.)
Southern and Central Regional Variations
Sweetpotato cultivation, and crop production systems in general, are locally variable, with five examples briefly described below from the southern and central regions (P. Zorogastúa Cruz, personal communication).
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Please click on this thumbnail to view locations of the areas described below.
- Kumi-Soroti (central eastern, consisting of Kumi and Soroti districts): Sweetpotato often planted in early August is available for harvest around late October, generally following previous harvests of other crops such as groundnut (also called peanut), maize, millet and sorghum. Soil preparation is usually undertaken by hand, often by women. Two to four cuttings are planted in small mounds typically 0.60 to 1.2 meters apart, and often in association with beans planted one bean seed to two sweetpotato cuttings per mound. Other crops of importance during this season include cassava and onions, although cassava is being displaced due to its long growing season (roughly 18 to 24 months) and losses to viral disease, as noted above. Popular sweetpotato varieties include Tanzania, Ejumula, Esapat, Araka, Kala, and Kakamega. Sweetpotato is often harvested "piecemeal," or gradually as needed, so that foliage continues to reseed and roots remain viable until lost to pest infections or rot.
- Mbale (eastern): With a very high population density (over 520 people per square kilometer), agricultural land holdings are usually very small, typically 20 to 100 square meters. Sweetpotato can be cultivated to some extent throughout the year, but the main season is March through August, often following a groundnut harvest. Other crops of local importance include soybean, yam, and rice cultivated in swampy areas. At higher altitudes in the district, main crops include bananas, coffee, maize, beans, and to a lesser extent vegetables such as tomatoes, onions, or cucerbitaceous crops (various melons). Most popular sweetpotato varieties are those of white or yellow flesh.
- Tororo (southeastern): This district is generally similar to Mbale, although population density is not as high (around 300 persons per square kilometer) and the area is more reliant on pastoralism, primarily cattle. Sweetpotato is established from late March through April, for harvests in June through August. Varieties reported to be popular include: Rubongi, Panyangasi, Kidera, Kisoko, Naigongera, Kirewa, and Butalenja.
- Luweero (central): Compared to the eastern areas noted above, Luweero is characterized by higher temperatures and more intense rainfall, as well as heavier textured soils of silt and clay. Population density is lower, though still relatively high at around 83 persons per square kilometer. A wide variety of crops are grown, but pastoralism is also important. Unlike areas noted above, sweetpotato is often cultivated in furrows, set at distances roughly two meters apart interspersed with maize planted around April. Sweetpotato is usually established in July, sometimes as a companion crop with okra (Hibiscus esculentus).
- Kabale (southwestern): This region is characterized by some areas of higher altitude and cooler temperatures than others noted above. Population density is also high, at around 273 persons per square kilometer. Predominant crops are grains, mainly maize, sorghum, and millet established in late February into March. Following the main harvests in July, land is prepared for sweetpotato cultivation, usually in August.
Viral Disease
Sweetpotato production can be constrained by viral diseases that cause yield reductions, potentially from about fifty percent to even as much as seventy percent (Mukasa 2003, p. 329; personal communication W. Grüneberg and J. Espinoza). However, the relatively low overall incidence of viral disease (less than twenty percent of all plants surveyed across the south where most of the crop is grown) suggests that farmers are fairly successful in selecting and planting virus-free vines from previous harvests (Mukasa 2003, p. 333).
Suspected viral disease in East Africa was first reported in Uganda in 1944, following similar reports several years earlier from Ituri province in the current Democratic Republic of Congo. Symptoms of a virus transmitted by aphids, then called virus A, suggest that it was sweetpotato feathery mottle virus (SPFMV). Another, transmitted by whiteflies and initially labeled virus B, resembles sweetpotato mild mottle virus (SPMMV), which has yet to be reported outside the East Africa region (ibid).
Several viruses currently recognized as significant constraints to sweetpotato (SP) in Uganda, listed roughly by their frequency of occurrence, include:
- SP chlorotic stunt virus (SPCSV): genus Crinivirus, family Closteroviridae, transmitted by whiteflies, appears to be very common, occurring as a single infection and combined with others which can cause severe symptoms;
- SP feathery mottle virus (SPFMV): genus Potyvirus, family Potyviridae, transmitted by aphids;
- SP mild mottle virus (SPMMV): genus Ipomovirus, family Potyiridae, transmitted by whiteflies;
- SP chlorotic fleck virus (SPCFV): a Carlavirus transmitted by vector still unknown;
- SP virus disease (SPVD): Caused by the co-infection of SPCSV and SPFMV, this is the most serious viral disease infecting sweetpotato in Uganda. (Mukasa 2003). Symptoms include small, distorted leaves which are often narrow and crinkled, and general stunting of plants (Kreuze et. al. 2000).
In his survey, Mukasa (p. 329) also noted that several plants (eleven percent of the total sample) with apparent virus-like symptoms did not react with the antisera used to identify the four well known strains listed above, suggesting that other, still unknown viruses or virus-like agents are infecting sweetpotatoes. Viral strains, like sweetpotato varieties, also seem to demonstrate strong local variability. SPFMV isolates from Uganda, and perhaps elsewhere in East Africa, have been shown to be distinctive from SPFMV strains from West Africa and elsewhere in the world (Kreuze et. al. 2000).
By region, the lowest incidence of viral infection was reported in the short grassland agro-ecological zones on the northeastern fringe of sweetpotato cultivation (Carey et. al. 1996, cited in Abidin et. al. 2002, p. 61). Here, drought is more common, and sweetpotato is less likely to be continuously grown, in contrast to other areas where many gardens are planted according to a staggered schedule in order to obtain a nearly year-round harvest (Mukasa 2003, p. 334).
Insect Pests
The sweetpotato weevil of the genus Cylas is the most serious insect pest of sweetpotatoes worldwide (Hakiza, et. al., p. 47). C. puncticollis and C. brunneus occur in Uganda, and are uniquely African species (Downham et. al. 2001, p. 163). The larvae feed upon roots, but are not always readily observed until they have caused significant damage. Adults are likewise difficult to detect given their nocturnal habitat (Downham et. al. 2001, p. 163).
Sweetpotato varieties with significant resistance to weevil attack have not yet been developed, although varieties can vary by the depth of their roots, a shallow rooting variety (perhaps even exposed at the soil surface) generally being more susceptible to attack by weevils (Mwanga 2001, p. 385).
The use of pheromones to control weevil populations by disrupting their mating behavior has proven potentially effective, but little evidence yet demonstrates the practical effect of reducing sweetpotato root infestation. One study undertaken using the formulation of C. brunneus on both Cylas species proved effective on C. brunneus and somewhat effective on C. puncticollis, with preliminary evidence of reduced infestation (Downham et. al. 2001).
VARIETIES AND SEED SYSTEMS
Due to lack of records, it is not known which varieties of sweetpotato were the first to be grown in Uganda, but varieties of more than fifty years duration have been identified (Hakiza et. al. 2000, p. 47). Many landraces are currently grown, bearing local names and presumably descendant from stocks introduced by Europeans centuries ago.
Hall et. al. (1998) report that varieties planted by farmers do not necessarily indicate their assessments of which varieties are best suited for a given area, but might reflect what is available, in the form of vines for vegetative propagation, at the beginning of the planting season. This constraint might be more common in semi-arid rangelands.
New varieties can be introduced from other areas, though sometimes new varieties arise via local mutation (Aldrich 1963, cited in Hakiza et. al. 2000). As indications of the varietal richness of Uganda's sweetpotato crop, in 1938, 47 varieties were recorded in Bukeba Village in Buganda, southern Uganda (Hakiza et. al. 2000). In 1965, 27 varieties were identified in the immediate vicinity of Makerere University Farm, also in the south ([MacDonald] 1965, cited in Hakiza et. al. 2000). A collection undertaken in the northwest in 1999 included 206 accessions, in the form of cuttings, which were characterized using standard morphological descriptors, and resulted in the identification of 181 morphologically distinct accessions (Abidin 2001).
Varieties can differ in many respects: leaf shape, color, size, yield, nutritional value ([e.g] carotene content), flavor, and local adaptation and resistance to diseases and pests. The relative popularity of a variety can change quickly, in large part due to deterioration of stocks due to viral infection, or proven superiority of a new variety to local conditions. As often seems to be the case for root and tuber crops around the world, the responsibility for identification, selection, and maintenance of particular varieties is usually borne by women.
In addition to high potential yield and disease resistance, farmers generally prefer varieties of higher dry matter content, especially if they intend to utilize the crop in the form of amukeke, sliced sweetpotato roots which are dried for longer storage life (Abidin et. al. 2002, p. 61).
Although most locally occurring varieties could be characterized as low yielding, narrowly adopted, and susceptible to many diseases and pests, there are exceptions. Of five varieties released by the Ugandan Plant Variety Release Committee in 1995, four (Bwanjule, New Kawogo, Tanzania, and Wagabolige) were selections from Ugandan landrace germplasm, presumably chance seedlings selected and retained by farmers. The fifth, Sowola, was newly bred. All were selected based on criteria of consumer acceptance and consistently high yields and disease resistance across a wide variety of sites in Uganda (Mwanga et. al. 2001, p. 385). Farmer-participatory programs undertaken in the harsh environment of the northeast seem to hold promise for selecting varieties useful elsewhere in Uganda, subject to the uncertainties of local adaptation (Abidin et. al. 2002, p. 65).
CONSUMPTION, STORAGE, AND MARKETING
Consumption
Since sweetpotato is generally consumed close to the area where it was grown, consumption is well estimated by production. In 1988, FAO estimated annual per capita production at 95.4 kilograms. Based on a mid 2004 population estimate of 26,083,000 (PRB) and most recent FAO national production data of 2,600,000 tons, annual per capita consumption now stands at approximately 100 kilograms. Some estimates are even higher, [e.g] 119 kilograms (Mwanga et. al. 2001).
Sweetpotato is generally consumed in fresh form, but where weevil attacks limit the length of time that roots can be kept underground prior to harvesting, they can be harvested, chipped, and sun-dried to make products such as inginyo and amukeke (PRAPACE).
Storage
Where adequate and well-distributed rainfall allows annual production and continuous harvesting, there is little need for storage. However, in the drier northern areas of Uganda, especially where cassava has been threatened by mosaic disease, storage of fresh sweetpotato roots for longer periods could help alleviate a potentially serious food shortage, especially in the months of May through June (Hall and Devereau 2000).
Piecemeal harvesting, where only enough is taken for one or two meals, is a common practice for home consumption and small-scale marketing (Hall et. al. 1998). A first flush of roots typically matures three to four months after planting, depending on the variety and conditions. Mature roots, harvested from the mound, make room for additional roots to develop. The process of piecemeal harvesting can continue for about three months, again depending on the variety and conditions, but after that time any roots remaining in the soil will succumb to sweetpotato weevil attacks or other pests, or otherwise deteriorate. The harvesting of roots close to or protruding from the ground might, however, help deter weevil attacks (ibid).
Storage of dried sweetpotato has become a practice of increasing importance to year-round food security, especially where cassava has declined. Storage is in the form of either dried slices known as amokeke or kaseede, or in dried crushed form known as inginyo. Sweetpotato slices can generally be stored for as long as four to six months, and in crushed form for somewhat longer, though both are at risk from infestation by grain borers. Re-drying slices can extend storage life, but in larger quantities (necessary where sweetpotato has become an important staple), the process becomes very time-consuming (Hall et. al. 1998).
Longer-term storage of fresh roots has been attempted on an experimental basis in Soroti, in the mid northeastern region, with the participation of local farmers using below-ground pits, above-ground clamps with conical thatched roofs, and wooden boxes kept indoors. Ash and other materials have also been used to extend storage life. The storage capacity of particular varieties has proven to be a significant variable, thus providing a new element to varietal selection factors. Although this effort was still very much in a pilot stage as of 2000, an optimal mix of techniques and varieties seemed to offer an opportunity to store fresh sweetpotatoes for up to three months, long enough to make a difference when local food security is most uncertain (Hall and Devereau 2000).
Marketing
Sweetpotato is now a major source of cash income almost everywhere in Uganda. Specialist commercial production takes place in areas with market access, served by roads, primarily for the lower income market of the capital city, Kampala. Local markets in the north are annually variable, but prices generally begin to increase when cassava would normally be available, roughly March through June (Hall and Devereau 2000). Studies cited by Hall et. al. (1998) refute the notion that traders and various middlemen in the marketing system make exorbitant profits, especially when considering risk factors.
There is potential for new uses of sweetpotato flour and/or mashed sweetpotato meal as an ingredient in the production of products such as chapati, mandazi, and kabalagala, which are usually made from imported wheat (PRAPACE).
The site for Uganda's Ministry of Agriculture, Animal Industry and Fisheries includes a link to the National Agricultural Research Organization (NARO,) as well as statistical data.
Uganda is a member of the Regional Potato and Sweetpotato Improvement Network in Eastern and Central Africa (PRAPACE), which developed out of PRAPAC, a network established in 1982 by the national research institutions of Burundi, Rwanda and the Democratic Republic of Congo to coordinate their potato programs. Currently PRAPACE collaborates with potato and sweetpotato programs of ten countries, the other nine being: Burundi, Democratic Republic of Congo, Eritrea, Ethiopia, Kenya, Madagascar, Rwanda, Sudan, and Tanzania. All are members of the Association for Strengthening Agricultural Research in Eastern and Central Africa (ASARECA.) The International Potato Center (CIP) provides support in the forms of improved germplasm, scientific information, training, and administrative assistance. The United States Agency for International Development (USAID) funds the network.
The Famine Early Warning System (FEWS) sponsors FEWSNET, which features reports of food security status in several countries, including Uganda (in English).
CONTRIBUTORS
Kelly Theisen is the principal contributor to the initial (2006) Uganda chapter of the Sweetpotato Atlas.
Wolfgang Gruneberg and Percy Zorogastua Cruz provided information and advice via personal communication.
REFERENCES
Abidin, Putri. 2001. Sweetpotato Germplasm Collected in Northeastern Uganda in 1999: Passport Data and Morphological Description. Hortscience 36(3): 487.
Abidin, Putri E.; Fred A van Eeuwijk; Piet Stam; Paul Struik; [D.P] Zhang; Michael Herman; Edward E. Carey. 2002. Evaluation of Sweetpotato (Ipomoea batatas (L.) Lam.) Germplasm from Northeastern Uganda through a Farmer-Participatory Approach. Proceedings of the First International Conference on Sweetpotato Food and Health for the Future. Acta Horticulturae Number 583. ISHS. Brugge, Belgium.
Aldrich, [D.T.A] 1963. The Sweetpotato Crop in Uganda. East African Agriculture and Forestry Journal (29):42-49.
Bachou, H., MD. 2002. The Nutrition Situation in Uganda. Nutrition 18(4): 356-358.
Carey, [E.E]; [R.O.M] Mwanga; S. Fuentes; S. Kasule; C. Macharia; [S.T] Gichuki; R. Gibson. 1996. Sweetpotato Viruses in Uganda and Kenya: Results of a Survey. Paper presented at the sixth triennial symposium of the ISTRC, Africa Branch, Lilongwe, Malawi, 22-28 October 1995.
Downham, [M.C.A]; [N.E.J.M] Smit; [P.O] Laboke; [D.R] Hall; B. Odongo. 2001. Reduction of Pre-harvest Infestations of African Sweetpotato Weevils Cylas brunneus and C. puncticollis (Coleoptera: Apionidae) Using a Pheromone Mating-Disruption Technique. Crop Protection 20: 163-166.
FAOSTAT. (Agriculture: Agricultural Production, Crops Primary, Sweet Potato; Population: Annual Time Series; Land: Land Use).
Hakiza, [J.J]; G. Turyamureeba; [R.M] Kakuhenzire; B. Odongo; [R.M] Mwanga. 2000. Potato and Sweetpotato Improvement in Uganda: A Historical Perspective. African Potato Conference Proceedings, Vol. 5. pp. 47-58. (Link to abstract of subsequent publication)
Hall, [A.J], Bockett, [G.N], Nahdy, S. 1998. Sweet Potato Postharvest Systems in Uganda: Strategies Constraints and Potentials. International Potato Centre (CIP) Social Science Department Working Paper Series No. 1998-7 CIP, Lima, Peru.
Hall, Andy J. and Andrew Devereau. 2000. Low-cost Storage of Fresh Sweetpotatoes in Uganda: Lessons from Participatory and On-station Approaches to Technology Choice and Adaptive Testing. Outlook on Agriculture 29(4): 275-282.
Kreuze, [J.F], [R.F] Karyeija, [R.F] Gibson, [J.P.T] Valkonen. 2000. Comparisons of coat protein gene sequences show that East African isolates of Sweet Potato Feathery Mottle Virus form a genetically distinct group. Archives of Virology 145:567-574.
Mukasa, Settumba. 2003. Incidence of Viruses and Virus-like Diseases of Sweetpotato in Uganda. Plant Diseases 87(4): 329-335.
Mwanga, [R.O.M]; B. Odongo: C. Ocitti p'Obwoya. 2001. Release of Five Sweetpotato Cultivars in Uganda. [HortScience] 36(2): 385-386.
Ocitti p'Obwoya, [C.N] and [N.E.J.M] Smit. 1998. In-ground Post-maturity Storability of Five Local Sweetpotato Varieties. Uganda Journal of Agricultural Sciences (3):1-6.
PRAPACE. Socio-Economic Contribution of the Two Commodities (Potato and Sweetpotato).
PRB (Population Reference Bureau). The Uganda Country Profile cited here is not accessible as of May, 2007, but other information about Uganda is available via the PRB DataFinder.
Purseglove, [J.W] 1968. Tropical Crops: Dicotyledons. Longman Scientific and Technical. New York.
US LOC (United States Library of Congress) Country Studies: Uganda
Zorogastua Cruz, Percy. 2005. Trip Report to Uganda and Western Kenya. CIP. Lima.