INTERNATIONAL POTATO CENTER: WORLD SWEETPOTATO ATLAS
PAPUA NEW GUINEA
HISTORY AND OVERVIEW
The sweetpotato, Ipomoea batatas, was widely grown in tropical America when Columbus brought several roots with him on his return to Spain from his first voyage in 1492, eighty years before the potato (Solanum tuberosum), reached Europe (Purseglove 1968). From Europe, sweetpotato was carried by mariners from Portugal, and subsequently other European nations, to ports throughout Africa and Asia in the early sixteenth century.
This dispersal, the "batata line," was not the only means by which the sweetpotato was transferred to the Asia Pacific region. A separate "camote line" (from the Nahuatl word comotli ) was transferred directly from Mexico via Spanish galleons to the Philippines, also in the sixteenth century (Zhang et. al. 2004, p. 116). Following these introductions in historical times, complex interchanges of sweetpotato varieties have occurred in the Asia Pacific region.
Adding to the complexity is evidence of a transfer predating European exploration. Carbonized sweetpotato roots found in northern New Zealand have been dated back approximately 1,000 years (Yen 1982, cited in Zhang et. al. 2004, p. 116). Whether this transfer occurred via natural dispersal or by human voyagers has remained a point of speculation for several decades. Research by Zhang et. al. (2004) compared genetic associations among seventy-five sweetpotato landraces, drawn from Papua New Guinea (PNG) and other countries of the Asia Pacific region, as well as Peru, Ecuador and Mexico. That research demonstrated a relatively weak association between sweetpotatoes from Oceania and those of Peru and Ecuador, suggesting a Mesoamerican origin, and most likely dispersal not by human agency prior to introductions via European exploration.
- An earlier version of this research is available via CIP, The Historic Dispersal of Sweetpotato.
Sweetpotato, whatever the circumstances of its introduction(s), has been cultivated in PNG for approximately 400 years, and has become the most essential crop of the country due to its high yield relative to labor and time, its capacity to grow in relatively poor soils, and its versatility as a source of nutrition for people and for pigs, by far the most significant form of animal protein for PNG and an essential component of cultural exchange.
According to estimates by the United Nations Food and Agriculture Organization (FAO), 520,000 metric tons of sweetpotato were produced in PNG in 2004 (FAOSTAT), but as is often the case, that estimates could be very conservative. A survey of annual household food production undertaken in 1996 estimated that 1,286,000 tons were produced, equivalent to 264 kilograms (kg) annual per capita consumption, more than three times the second ranked food, banana at 85 kg (Gibson 2001, p. 40). For another household survey taken the same year, respondents were asked to identify their most important crop. Over 53 percent indicated sweetpotato alone, and another 8 percent indicated sweetpotato in association with other crops. The second most commonly indicated crop, sago, was indicated by 10 percent (Allen et. al., p. 537). Per capita sweetpotato consumption in PNG could be the highest in the world, but some of that consumption is indirect, as much of PNG's sweetpotato crop is consumed by pigs.
More than 85 percent of the population of PNG depends on agriculture, primarily subsistence production with some cash crops, often tree crops such as coffee and cacao (Gwaiseuk 2001, p. 31). However, longer term food security is a concern, due to the widening gap between population, estimated to be growing at an annual rate of 2.3 percent, and food production, growing at an annual rate of roughly 1 percent (ibid. ). Given the great importance of sweetpotato, even modest gains in productivity can contribute a great deal toward food security. According to the FAO, yields average approximately five tons per hectare (FAOSTAT), suggesting potential for further development.
Sweetpotato consumption rates vary between rural and urban populations, as urban food demand is increasingly met by imports, usually rice and wheat products. There is a modest, but growing, specialization of sweetpotato production intended for urban markets, often using higher levels of mechanical and chemical inputs on a reduced range of varieties.
GEOGRAPHY AND PRODUCTION ZONES
Physical Geography and Climate
The nation of Papua New Guinea (PNG) occupies the eastern half of the island of New Guinea, as well as approximately 600 islands to the east and north. A land area of 464,000 square kilometers extends from the equator to 12º south latitude. PNG is located within the highly unstable zone of the earth's crust surrounding the Pacific Ocean, characterized by relatively young folded and faulted mountain ranges and islands created primarily by volcanic activity (Igua 2001, p. 5).
PNG is mountainous, with over half of the landmass at altitudes over 600 meters above sea level (masl), and several peaks over 4,000 masl. Great topographic complexity can occur over short distances, effectively isolating valleys surrounded by very sharp peaks and ridges. As a result of this physical complexity, landforms can be determined by any of several geomorphic processes: depositional, erosional, or volcanic. The distribution of soil types is therefore very complex and locally variable; eight soil orders have been identified (Igua 2001, p. 6).
- 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, Papua New Guinea. Most images are digitized from paper maps, of variable clarity.
The climate is highly variable, both spatially and temporally. Rainfall throughout much of PNG ranges from 1,500 to 5,600 millimeters (mm) annually, but in areas of rain shadow in the central highlands, annual rainfall can drop to approximately 1,000 mm. Where rainfall is heavy, it is generally consistent throughout the year, but areas of lower rainfall generally experience a dry season from May to October (Igua 2001, p. 6). In some areas of PNG, excessive moisture due to heavy rains can be a constraint to sweetpotato production.
Since PNG is equatorial, temperature regimes are not greatly affected by season, but are generally determined by altitude. Uniform high temperatures are characteristic of the humid lowlands, where average temperatures rarely fall below 26º C or exceed 30º C. In the highlands, a drop of 0.5º C in average temperatures is associated with an increase in 100 meters altitude (Igua 2001, p. 6). At higher altitudes, generally above 2,700 masl, frost can be a limiting factor of any form of agriculture (Peters 2001, p.4).
Climate in PNG is also greatly affected by the El Niño Southern Oscillation (ENSO) via changes in sea surface temperature and subsequent air circulation over the Pacific and Indian Oceans. Roughly every ten years, relatively moderate El Niño activity can cause abnormally heavy rainfall followed by reduced rainfall over a period of a year or two, significantly reducing the harvests of staple crops, including sweetpotato. In 1997 a severe El Niño event, the most serious in over seventy years, caused extreme drought and frost with heavy crop losses and local famines in isolated and vulnerable areas (Allen and Bourke 2001, pp. 155-164).
Land use intensity, as measured by the time that land is planted to crops to the time that it is in fallow, is generally very low in PNG, where the predominant agricultural system is forest fallow. Exceptions are most commonly found in highland zones where sweetpotato is the most significant crop and population densities are generally higher (Allen et. al. 2001, pp. 533-534).
Regional Distribution of Sweetpotato Production
Very generally, four main farming systems can be distinguished by their predominant subsistence crops (Gwaiseuk 2001, p. 32):
- High altitudes (sweetpotato and "Irish" potato);
- Highlands (sweetpotato and taro);
- Dry lowlands (yam, banana, and cassava);
- Wet lowlands (sago and taro).
The highlands and high altitude zones (variously defined, but occurring at roughly 1,200 - 1,800 masl, and 1,800 - 3,000 masl, respectively) are characterized by an almost entirely rural population, estimated at 98 percent, of whom 75 to 85 percent are farmers (Benjamin et. al. 2001, p. 94-96). The highlands have long been the area of highest population density in PNG, due largely to the relatively low infestation rates of tropical disease, especially malaria (Ohtsuka, 1995). Sweetpotato is clearly the most important crop above 1,500 masl (Lutulele, p. 684). The upper limit of altitude for sweetpotato cultivation is determined by frost, which can occur even at 2,000 masl in some areas, although in other areas frost-free conditions permit sweetpotato at altitudes as high as 2,850 masl (Ossum et. al. 2001, p. 144). Potato can be an important crop at altitudes higher than the limit for sweetpotato, but the extent of potato cultivation remains unknown. (Reporting by FAO, as of 2007, is almost impossibly low.)
Sweetpotato has not traditionally been a major crop of the lowlands, but there is evidence that it is being more widely adopted following the devastation of taro by leaf blight in the lowlands. It is now likely second in importance only to taro, and/or in some areas banana (Bang and Kanua 2001, p. 670).
Maps of PNG indicating areas of most highly concentrated sweetpotato cultivation are available on Sweet Potato in Papua New Guinea, a presentation by Mike Bourke of the Australian National University. (Images 14 and 15 indicate changes from 1940 to 1990 - 1995.)
PRODUCTION SYSTEMS AND CONSTRAINTS
The Agricultural Calendar
Given PNG's equatorial climate with little seasonal variation, there is no strict cropping calendar for sweetpotato. Cultivation and harvests, under normal weather conditions, are therefore fairly constant throughout the year. However, there is a direct relationship between altitude (thus temperature) and duration from planting to maturity, for example ranging from 17 weeks at 1,400 masl to 49 weeks at 2,600 masl. The ultimate yield is reported to be fairly consistent, regardless of duration (Peters 2001, p. 20).
Cropping Patterns and Fertility Management
The agricultural system is based on shifting agriculture, with sweetpotato the predominant food crop. Some farmers also cultivate some cash crops, coffee being commercially significant in some areas. Coffee production was on the increase in response to higher prices, until the crop suffered losses from the drought and frost of 1997 (Benjamin et. al. 2001, p. 95-96). As noted above, land use intensity is generally low under the forest fallow system widely practiced in PNG. Fertility is therefore maintained by long fallow periods, roughly thirty years in some areas, especially in the wetter lowlands.
In the highlands, there are often two types of gardens: "house gardens" closer to the periphery of settlements which are smaller and intensively cultivated, and "main gardens" often at a greater distance. The combination of crops found in a garden is often associated with the status of the fallow. Gardens that are more recently established, hence more fertile, are more likely to contain a wider variety of crops. Over time, as fertility declines, gardens become more heavily grown to sweetpotato (Benjamin et. al. 2001, p. 96).
However, where population density is higher (and therefore agricultural intensity greater), soil fertility must be more actively maintained. Techniques include (Allen et. al. 2001, pp. 534-537):
- Mounding and composting: a common practice in the highlands, and closely associated with sweetpotato cultivation, involves the construction of mounds of various sizes (although generally, larger mounds are associated with higher intensity production), into which green material such as weeds, vines from a previous harvest, and grasses from outside the site are incorporated;
- Rotations: often using leguminous crops, previously the winged bean (Psophocarpus tetragonolobus), now more commonly the groundnut (Arachis hypogaea), which is more readily used and marketable;
- Tree planting: a common choice being the native species Casuarina oligodon (more detail below);
- Soil retention: placement of poles and brush perpendicular to slopes to impede water flow and minimize erosion, or in a few places the construction of small terraces, though neither is yet a common practice.
The longstanding capacity on the part of people to creatively respond to environmental challenges is evident from extensive casuarina silviculture. Many areas of PNG provide evidence of forest clearing over thousands of years. However, in at least two sites of considerable distance, Baliem Valley to the west and the Wahgi Valley approximately 500 miles to the east, pollen records of approximately 1,200 years ago indicate a surge of casuarina. This innovation probably occurred independently at these sites, and people still commonly transplant seedlings that have sprouted naturally along stream banks into silviculture sites on higher ground. In addition to being fast-growing with wood well suited for fuel, casuarina root nodules fix nitrogen, and copious leaf litter provides a source of mulching material. Farmers also claim that the trees discourage infestation by the taro beetle, although modern research has yet to determine the mechanism behind that effect (Diamond 2005, pp. 277-288).
Farmers are well aware of the value of mulch, not only for nutrient value, but also to maintain optimal soil temperatures and to suppress weeds. Use is limited by local availability, but some evidence suggests an increasing tendency to intercrop with tree crops, such as coffee, to maintain a balance of food and cash income (Lutulele 2001, p. 687). If sweetpotato and coffee intercropping becomes a more common practice, coffee pulp could be more widely used as a mulch, valuable to sweetpotato as a source of phosphorous, calcium, and magnesium, and as a means to reduce soil acidity (a serious constraint in many areas of PNG) (Ossum et. al. 2003).
Another approach to the challenge of maintaining fertility under higher intensity production (hence reduced fallow) is to grow sweetpotato as an intercrop with hedgerows. A study undertaken in the humid lowlands, on sandy loam soils of recent volcanic ash origin, sought to determine how nine leguminous hedgerow species, grown primarily for their foliar mulch, would compare in terms of relative contribution to soil fertility and subsequent yields of vines and roots, vs. competition with the sweetpotato crop. Positive results were seen with some species chosen for high levels of potassium (often a limiting factor to sweetpotato production in PNG) if vigorously pruned to minimize competition (Brook 2000, pp. 137-144). Hedgerow intercropping is apparently not yet a common practice, either in the lowlands or highlands.
Fallows can also be managed, to the extent of planting or favoring particular species that enhance soil for subsequent sweetpotato production. An experimental treatment using three species (Piper aduncum, Gliricidia sepium, and Imperata cylindrica) found that sweetpotato yields of crops grown after managed fallow can be moderately increased, though more consistently than with the use of inorganic fertilizers. But there can be surprises. A low sweetpotato yield response following gliricidia, selected for its nitrogen-fixing capacity, might be due to allelopathic compounds in the leaves, which reduce the growth of adjacent plants (Hartemink et. al. 2001, p. 708).
Although farmers in PNG have not traditionally utilized chemical inputs such as fertilizers or pesticides on sweetpotato, there is some evidence that modern agricultural practices are being adopted, at least on more commercial farms closer to urban markets. However, results from experimental treatments over several years have been inconsistent or inconclusive. For example, vine yields have responded consistently to nitrogen inputs, but root yields less so. Potassium fertilizer has demonstrated little effect on vine yield, but has been shown to increase the number and yield of roots. (Hartemink et. al. 2001, p. 703). Inconsistent responses of sweetpotato to inorganic fertilizers is perhaps due to the extreme variations in the physical environment and climate of PNG. A study cited by Peters (2001, p. 21) indicates that sweetpotato yields are very responsive to the application of certain nutrients, particularly nitrogen, but that all such trials were undertaken on soils of recent volcanic origin.
Occurrence and Control of Sweetpotato Diseases and Pests
The sweetpotato weevil: Cylas formicarius, can destroy a root crop almost completely, and has been cited as a serious pest of sweetpotato in PNG, especially where rainfall is less consistent. In the lowland areas of PNG characterized by consistent rainfall, the soil remains more tightly sealed and relatively impervious to the weevils. Deeper rooting varieties are generally less susceptible to weevil damage (Van Wijmeersch 2001, p. 677).
Viral Diseases: Little research has been done on this topic in PNG, although viral diseases are a major constraint to sweetpotato production in several other countries.
Bacterial Diseases: Again, this topic has been the subject of little research in PNG. Bacterial soft rot, caused by Erwinia chrysanthemi, has been associated with sweetpotato rot in the highlands, potentially a serious disease.
Nematodes: A total of 22 genera of plant parasitic nematodes had been identified in PNG as of 1991, but others had not yet been described. Several can cause considerable damage, especially in the highlands, usually root malformation and possible yield loss.
Other Diseases: These are reviewed by Kokoa (2001, pp. 730-735):
- Fungal diseases: Sweetpotato Scab is caused by the fungus Elsinoe batatas, and is widely distributed, especially in areas of cool, wet weather. Generally it is not a great constraint, apparently due to the wealth of resistant varieties. Infection causes small lesions or spots to develop on the stem, petioles, and veins on the underside of leaves.
- Alternaria stem and leaf blight, also fungal, has been reported only since the 1980s. The disease is characterized by lesions, initially small but growing up to 50 mm, and capable of causing dieback of shoots, usually more severe in drier weather.
- Fusarium stem rot is also a recently reported fungal infection which can cause the wilting and death of vines.
- Storage rots can be caused by any of several fungal pathogens. This has not been a problem of roots consumed soon after harvest, but could become a more significant problem as sweetpotato is kept longer in storage and transported longer distances.
VARIETIES AND SEED SYSTEMS
The number of sweetpotato varieties grown in Papua New Guinea has been estimated at 5,000, of which about 1,600 are maintained in ex situ collections: 1,200 at the Highlands Agricultural Experimental Station (HAES) at Aiyura in Eastern Highlands Province, and 850 at the Lowlands Agricultural Experimental Station (LAES) at Keravat in East New Britain Province. There are some duplications between the two stations, but most highland varieties do not yield well in the lowlands (Peters 2001, p. 20).
Evaluation of varieties has been underway only since the 1970s when the first collections were made; the effort is far from complete. Most varieties maintained ex situ have been characterized via descriptors used by the International Plant Genetic Resources Institute (IPGRI), although some question remains as to whether varietal diversity might in some cases be merely superficial variation (Van Wijmeersch, 2001).
A study was carried out by CIP (Zhang et. al. 1998) using random amplified polymorphic DNA (RAPD) to assess genetic variation of 18 cultivars from a widely drawn area of South America (including Colombia, Ecuador, and Peru), compared to 18 cultivars from the highlands and lowlands of PNG. Molecular variance attributed to regional group affiliation (as opposed to individual within-group variance) was estimated at 9.4 percent, sufficient to suggest that varieties currently found in PNG have become genetically distinguishable from their South American ancestors (ibid. ). As noted above, there is also evidence of a strong Mesoamerican component of PNG's genetic sweetpotato diversity.
The effort to screen and select promising varieties is potentially immense, but limited by the modest resources of agricultural research facilities. Varietal trials are undertaken at both the lowland and highland facilities noted above, where cultivars are planted a minimum of three times to identify initially promising varieties, considering factors such as yield, damage caused by sweetpotato weevil, resistance to scab, rat damage, root shape, market appeal, dry matter content, and (last but not least) taste (Van Wijmeersch, 2001).
More information pertaining to the nomenclature and descriptions of particular varieties, as well as the particular uses made of each, can be found in Peters (2001), Cultivar Description and Nomenclature (pages 24-26 of the manuscript). This section pertains to the Dani people of Irian Jaya, the western (and Indonesian) side of the island of New Guinea, but is essentially relevant to PNG as well.
CONSUMPTION, STORAGE, AND MARKETING
Per capita sweetpotato consumption in rural PNG could well be the highest in the world, though much of the crop is consumed indirectly, via pigs. Peters (2001, p. 29) cites a survey indicating that in the highlands, sweetpotato never accounts for less than half of the local diet, and can be as high as 90 percent. Other surveys estimate that sweetpotato provides from 50 to 70 percent total energy and from 35 to 70 percent of total protein.
Rural and urban food consumption patterns are very different. Of a total population estimated at 5,712,000 in 2003, approximately 756,000 people (or just over thirteen percent) are urban (FAOSTAT). In 1996, an average urban resident consumed 42 kilograms of sweetpotato and 66 kg of rice (and several other staples), while the average rural resident consumed 299 kg of sweetpotato and 24 kg of rice (Gibson 2001, p. 41).
Rice remains mostly an imported product. The government of PNG has encouraged rice production for several years, but so far with little apparent success. Yet, given the high costs of marketing and transportation in PNG, imported rice is very cost competitive with locally grown sweetpotato (and other products) in cities, especially the capital city of Port Moresby. Surveys have indicated that the primary factor favoring rice consumption is price; even in cities, people do not consider sweetpotato to be an "inferior good" (Gibson 1995).
Sweetpotato is not only the primary food source for the people of PNG, but for pigs as well. Peters (2001, p. 31) cites a study undertaken in 1978 that estimated that forty percent of PNG's sweetpotato production is fed to pigs. The importance of sweetpotato as a source of food for pigs can be appreciated only with an understanding of the importance of pigs, both nutritionally and culturally, for PNG. That topic is beyond the scope of this chapter, but is discussed in greater depth by Peters, especially in section 4.
Although sweetpotato is a low-cost component of animal feed in its raw form, it has to be processed, usually cooked, to reduce trypsin inhibitors which limit protein digestibility. This constraint can also be addressed by providing protein supplements such as soy meal, the optimal balance between relatively expensive protein supplements and the cost of cooking sweetpotato being a function of local costs and objectives (Danbaro et. al. 2001). Locally grown corn can, in some areas, provide part of the protein component as a cost effective alternative to commercial feeds (Duks et. al. 2001). Sweetpotato is very significant as a component of pig feed in Vietnam, a subject of current research; please refer to the Vietnam chapter for more information and links.
Since sweetpotato production is possible in many areas of PNG at any time of year, there has not traditionally been much need for longer term storage. However, as noted above, improved storage could be a major factor of marketing in urban markets, allowing sweetpotato to provide an alternative to rice.
Long-distance marketing of sweetpotato is still minimal, but growing. From 1989 to 1999, the amount of sweetpotato marketed to Port Moresby increased from 600 tons to over 2,000 tons annually, most shipped from the highlands (Bang and Kanua 2001, p. 669).
RESEARCH FACILITIES AND CONTACTS
Contact information for the National Agricultural Research Institute is:
Dr. Segie Kopen Bang, Director of Research
National Agricultural Research Institute, Head Office
Kana Aburu Haus, Sir Alkan Tololo Research Station, Lae
P.O. Box 4415, LAE 411
Papua New Guinea
Tel: 675 475 1444/45, Fax: 675 475 1449/50
Email: email@example.com; firstname.lastname@example.org
Graphs pertaining to several of the topics covered here are included in Sweet Potato in Papua New Guinea, by Mike Bourke of the Australian National University.
A resource for more general information about Papua New Guinea is the World Wide Web Virtual Library for PNG.
Kelly Theisen is the principal contributor to the initial (2006) Papua New Guinea chapter of the Sweetpotato Atlas.
Allen, Bryant J. and R. Michael Bourke. 2001. The 1997 Drought and Frost in PNG: Overview and Policy Implications. In: Bourke, R.M.; M.G. Allen; J.G. Salisbury (eds.). 2001. Food Security for Papua New Guinea. Proceedings of the Papua New Guinea Food and Nutrition 2000 Conference, PNG University of Technology, Lae, 26-30 June 2000. Australian Centre for International Agricultural Research (ACIAR) Proceedings 99. ACIAR. Canberra, Australia.
Allen, Bryant J.; R. Michael Bourke; Luke Hanson. 2001. Dimensions of PNG Village Agriculture. In: Bourke, Allen, and Salisbury (op. cit).
Bang, Sergie and Matthew 'Wela B. Kanua. 2001. A Sweet Potato Research and Development Program for PNG. In: Bourke, Allen and Salisbury (op. cit).
Benjamin, A.K.; I. Mopafi; T. Duke. 2001. A Perspective on Food and Nutrition in the PNG Highlands. In: Bourke, Allen, and Salisbury (op. cit).
Bourke, R.M.; M.G. Allen; J.G. Salisbury (eds.). 2001. Food Security for Papua New Guinea. Proceedings of the Papua New Guinea Food and Nutrition 2000 Conference, PNG University of Technology, Lae, 26-30 June 2000. Australian Centre for International Agricultural Research (ACIAR) Proceedings 99. ACIAR. Canberra, Australia.
Brook, Robert M. 2000. Hedgerow Intercropping with Sweetpotato in the Humid Lowlands of Papua New Guinea. Tropical Agriculture Volume 77 (3):137-145.
Danbaro, Gariba; G. Vegofi; A. Kila. Use of Sweet Potato and Soybean for Feeding Exotic-Type Pigs. In: Bourke, Allen, and Salisbury (op. cit.)
Diamond, Jared. 2005. Collapse:How Societies Choose to Fail or Succeed. Viking Penguin. New York.
Duks, J.B.; M. Moat; C. Dekuku. Growth and Feed Efficiency of Pigs Fed Common Staples and Protein Supplements. In: Bourke, Allen, and Salisbury (op. cit)
FAOSTAT. (Agriculture, Agricultural Production, Crops Primary, Sweet Potato; Population, Annual Time Series.
Gibson, John. 1995. Food Consumption and Food Policy in Papua New Guinea. PNA, Institute of National Affairs Discussion Paper No. 65.
Gibson, John. 2001. The Economic and Nutritional Importance of Household Food Production in PNG. In: Bourke, Allen and Salisbury (op. cit.).
Gwaiseuk, William. 2001. The Role of Agriculture in the PNG Economy. In: Bourke, Allen, and Salisbury (op. cit.).
Hartemink, Alfred E.; S. Poloma; J.N. O'Sullivan. 2001. Integrated Nutrient Management on Sweet Potato at Hobu, Morobe Province. In: Bourke, Allen, and Salisbury (op. cit.).
Igua, Passinghan Bukley K. 2001. Food Security Strategies for Papua New Guinea. Working Paper 56. Regional Coordination Centre for Research and Development of Coarse Grains, Pulses, Roots, and Tuber Crops in the Humid Tropics of Asia and the Pacific (CGPRT Centre). CGPRT Centre. Bogor, Indonesia.
Kokoa, Pere. 2001. Review of Sweet Potato Diseases in PNG. In: Bourke, Allen, and Salisbury (op. cit.).
Lutulele, Robert P. 2001. Sweet Potato Variety Development in the PNG Highlands: Implications for Future Research and Extension Focus. In: Bourke, Allen, and Salisbury (op. cit.).
Ohtsuka, Ryutaro. 1995. Agricultural Sustainability and Food in Papua New Guinea. Chapter Five of: Population, Land Management, and Environmental Change. The United Nations University Global Environmental Forum IV.
Ossum, E.M.; P.F. Pace; R.L. Rhykerd; C.L. Rhykerd. 2001. Effect of Mulch on Weed Infestation, Soil Temperature, Nutrient Concentration, and Tuber yield in Ipomoea batatas (L.) Lam. in Papua New Guinea. Tropical Agriculture Volume 78 (3):144-151.
Peters, Joe. 2001. Local Human-Sweet Potato-Pig Characterization and Research in Irian Jaya, Indonesia, with Limited Reference to Papua New Guinea. International Potato Center (CIP) and the Australian Council for International Agricultural Research (ACIAR).
Purseglove, J.W. 1968. Tropical Crops: Dicotyledons. Longman Group Limited. Essex, England
Van Wijmeersch, Paul. 2001. The Status of Sweet Potato Variety Evaluation in PNG and Recommendations for Further Research. In: Bourke, Allen, and Salisbury (op. cit.).
Yen, D. E. 1982. Sweetpotato in Historical Perspective. In: Villareal, R.L. and Griggs, T.D. (eds.). Sweet Potato: Proceedings of the First International Symposium. AVRDC Publication 82:172, pp. 17-30. Tainan, Taiwan.
Zhang, Dapeng; Genoveva Rossel; Albert Kriegner; Robert Hijmans. 2004. AFLP Assessment of Diversity in Sweetpotato from Latin America and Pacific Regions: Its Implications on the Dispersal of the Crop. Genetic Resources and Crop Evolution 51: 115-120. Kluwer Academic Publishers. The Netherlands.