Sweetpotato

Contributor: Wanda W. Collins

Common Names
Scientific Names
Uses
Origin
Crop Status
Anti-nutritional factors
Botany
1. Taxonomy
Crop Culture
Germplasm
1. Collections
2. Commercial "Seed" Sources
Key References
Selected Experts

Common Names

English: sweetpotato
Spanish: batata, boniato, camote
Polynesian: kumara

Scientific Names

Species: Ipomoea batatas (L.) Lam.
Family: Convolvulaceae

Cultivated primarily for edible storage roots; vines are used as vegetables in some parts of the world. Both starchy roots and vines can be used as animal feed or feed supplement. Various products such as candy, pastas, flour, drinks are produced in local industries.

Origin

New World; northwestern South America

Crop Status

A tropical perennial cultivated as an annual in temperate climates; grown in more than 100 countries in tropical, sub-tropical and temperate climates, it is used as a major food staple in a few countries, as an alternative staple in many countries, and as an incidental or luxury addition to the diet in many countries. It is one of only seven world food crops with an annual production of more than 100 million metric tons per year ranking thirteenth globally in production value among agricultural commodities. It is cultivated primarily for the enlarged edible storage roots which provide high amounts of starch to staple diets. Use as an export crop is rare and production is usually to meet local or national needs. Asia accounts for over 80% of world production (most of that is in China), Africa for about 15% and the rest of the world about 5%. The US accounts for less than 1% of world production.

Although variation in storage root skin color and flesh color is abundant, two major types exist which delineate usage. In most developing countries where sweetpotato is used as a staple or alternative staple food, the type of root which is produced has white to cream colored flesh and a bland, non-sweet flavor. It is dry in texture and often has a high dry matter content. The energy content which it can provide in staple diets depends on the dry matter content. In contrast, the type most used in developed countries, where use is normally as a vegetable or dessert, has root flesh color of yellow or deep orange, moist texture, a very distinct flavor and high sugar content. The yellow or orange flesh color is directly related to beta-carotene, a precursor of Vitamin A. Other nutrients supplied by sweetpotato are Vitamin C, iron and potassium. Protein content is very low in storage roots and does not add substantially to daily supply of protein.

Anti-nutritional factors

Sweetpotatoes contain trypsin inhibitors which may reduce ability to utilize protein if eaten raw. However, trypsin inhibitors do not survive cooking and are of no consequence in cooked roots.

Botany

Taxonomy

Sweetpotato is a member of the family Convolvulaceae, Genus Ipomoea, section Batatas. It the only hexaploid (6x = 90) in this section and its origin is unknown. Section Batatas continues to undergo revision but contains approximately 12 other species, most of which are diploid (2n = 30), with a few tetraploids (4x = 60). Tetraploid sweetpotatoes have been collected in the wild although rarely. Ipomoea trifida, a diploid, is purported to be one of the likely progenitors of sweetpotato. Species in section Batatas have been shown to contain unreduced gametes but the derivation of the hexaploid sweetpotato remains a mystery. Different theories of the evolution of I. batatas have been advanced by Japanese researchers, who consider it an autopolyploid derivative of I. trifida, and American researchers, who favor an allopolyploid origin involving I. trifida and an unidentified tetraploid parent. Introgression of traits from related species is normally prevented by crossing barriers between species. Some success has been reported through using massive numbers of crosses and embryo culture. However, introducing genes by this method is still generally not viable in a breeding program even though a few traits of considerable interest have been identified in related species.

The major center of diversity for I. batatas is in northwestern South America (northern Peru, southern Ecuador) but other very important centers of diversity exist in sub-Saharan Africa, Papua New Guinea, and Indonesia.

Crop Culture

Ecology

Sweetpotato is cultivated as a perennial in tropical and subtropical lowland agroecologies although it is well adapted to other zones and can be grown over widely different environments. The crop will grow with temperatures between 15°C and 35°C; however, the lower and higher temperatures have detrimental effects on yield. Storage roots are sensitive to changes in soil temperature depending on stage of root development. Sweetpotato responds well to increasing moisture but is considered a drought-tolerant crop because it is deep rooted and capable of developing storage roots under very dry conditions. Excessive moisture inhibits storage root development in early growth stages and causes decay of storage roots in later growth stages. Sweetpotatoes grow best in a sandy loam, well-drained soil. They have been produced at altitudes in excess of 2000m and as far north as Canada.

Cultivars

Hundreds of cultivars and land races are used throughout the world and are unique to countries or to smaller regions within countries. In industrialized countries which grow sweetpotatoes, limited numbers of cultivars are grown. In the United States, growers tend to grow only one or two major cultivars for regional and national markets but may grow several cultivars in small amounts for local markets. Despite the tremendous genetic variability available to breeders in almost all traits of sweetpotato, the crop is considered to be genetically vulnerable due to this heavy concentration in only one or two cultivars. The two cultivars which account for most of the current US acreage are 'Jewel' and 'Beauregard'.

Production Practices

In temperate climates sweetpotatoes cannot survive freezing temperatures and storage roots must be stored overwinter and used as "seed" roots the next spring. Overwintered storage roots are presprouted while still in storage by raising the temperature to 20-30°C for 2-6 weeks. Roots are then placed in ground beds in late winter, covered with up to 5cm of soil and covered with plastic. Exact conditions and types of beds vary with location. Plastic is removed after the threat of frost is over. Typically roots will produce enough sprouts to begin planting in 5-6 weeks. Sprouts approximately 25cm in length are either pulled from the mother roots or cut (cutting prevents spread of some diseases but is costly in terms of labor) and planted in field rows using mechanical 4 or 8 row transplanters. Optimum plant density depends on cultivar but is usually around 40,000 plants/ha. Rows may vary from 1m to 1.25m apart; in row spacing is usually 25-30cm.

Fertilizer is usually incorporated in the soil with an additional application approximately 6 weeks after planting. Requirements depend on soil testing but are normally in the range of 60kgN/ha and 120kgK/ha. Phosphorous is also required but is only added in phosphorous deficient soils. Boron is usually added to prevent a surface defect known as blister. Requirements for water vary with soil type but can be generally estimated as 18-20mm/week early in the season, 40-45mm/week during the middle part of the season when storage roots are enlarging rapidly, and a reduction to about 20mm late in the season. Excessive moisture early in the season delays storage root development and enlargement; late in the season, it induces cracking and/or rotting of roots. Herbicides are normally necessary for sweetpotato production but recommendations vary in different states. Few herbicides are labeled for use with sweetpotato. Nematicides may also be necessary. The severity of nematode damage varies from state to state and thus all released cultivars may not have nematode resistance. As with herbicides, choices for nematicides are extremely limited.

Harvesting and Storing

Storage roots are harvested when the production of the Number 1 grade (5-9cm in diameter) is maximized. This depends on cultivar but is usually between 90-120 days. Harvest is usually done mechanically with a number of different types of harvesting aids. Typically vines are cut off and roots are thrown to the soil surface using tractors with various types of dish plows; the roots are picked up by hand and transferred to bulk bins in trucks. Mechanical harvesters which lift the roots onto a belt and into bulk bins are available but are slow and expensive. Roots harvested early are usually washed, graded and sent directly to market because of high prices. Subsequently harvested roots are placed in buildings to cure (30-35°C, 90%RH) and then are stored (10-15°C; 85-90%RH) until needed for market. Curing promotes wound healing and provides a barrier to prevent bacteria and fungi from entering wounds received during harvest and handling. Properly cured roots will store for 12 months or longer with 15-25% losses under the best conditions. Temperature must not drop below 12°C in order to prevent physiological cold damage to which sweetpotatoes are particularly susceptible. Relative humidity should remain between 80 and 90% to prevent dehydration as the living storage roots continue to respire. As they are needed for marketing, roots are removed from storage rooms, processed through a mechanical washer/grader and packed into boxes of about 15kg. Wash water may contain chlorine or other approved fungicides to reduce infection of wounds generated by the grading procedure.

Roots which are designated as the next year's "seed" roots are usually harvested late and are kept separate from the commercial stock. Most states have some form of voluntary certified seed program to assure quality of stock used for propagation.

Processing

Most of the sweetpotatoes which are processed in the US are canned. Because the best prices are received for roots on the fresh market, those going to canning processors are the smaller roots (2.5-5cm in diameter). Roots larger than those desired for fresh market (>9cm in diameter) are usually sent to baby food processors.

Germplasm

Collections

US Germplasm Repository, Plant Introduction Station, Griffin, GA (Curator R. L. Jarrett)

World Collection: International Potato Center, Lima, Peru (Curator Z. Huaman)

Commercial "Seed" Sources

Contact North Carolina Crop Improvement Association, 3709 Hillsborough St., Raleigh, NC 27607 Tel: 919 515 2851 FAX: 919 515 7981

Key References

Edmond, J. B. and G. R. Ammerman. 1971. Sweet Potatoes: Production, Processing, Marketing. The AVI Publishing Co., Inc., Westport, CT, 334p.
International Potato Center, 1987. Exploration, maintenance and utilization of sweet potato genetic resources. Report of the First Sweet Potato Planning Conference. Lima, Peru. 369 p.
Woolfe, Jennifer A. 1992. Sweet Potato: An Untapped Food Resource. Cambridge University Press, Cambridge, UK. 643 p.
Bouwkamp, John C. 1985. Sweet Potato Products: A Natural Resource for the Tropics. CRC Press, Inc., Boca Raton, FL.271 p.
Asian Vegetable Research and Development Center. 1982. Sweet Potato: Proceedings of the First International Symposium. AVRDC Press, Shanhua, Tainan, Taiwan, ROC. 481 p.
Yen, D. E. 1974. The Sweet Potato and Oceania: An Essay in Ethnobotany. Bishop Museum Press, Honolulu, HI. 389p.

Selected Experts

Wanda W. Collins, Department of Horticultural Science, Box 7609, North Carolina State University, Raleigh, North Carolina 27695-7609 Tel: 919 515 1205; FAX: 919 515 2505; e-mail: Wanda_Collins@ncsu.edu

Jonathan Schultheis, Department of Horticultural Science, Box 7609, North Carolina State University, Raleigh, North Carolina 27695-7609 Tel: 919 515 1225; FAX: 919 515 2505; e-mail: Jonathan_Schultheis@ncsu.edu

J. M. Cannon, Resident Director, Sweetpotato Research Station, Louisiana State University Agricultural Center, Chase, LA 71324. Tel: 318 435 2155

[Contributor: Wanda W. Collins, Department of Horticultural Science, North Carolina State University]

Last update Tuesday, February 24, 1998 by aw