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O'Hair, S.K. 1990. Tropical Root and Tuber Crops. p. 424-428. In: J. Janick and J.E. Simon (eds.), Advances in new crops. Timber Press, Portland, OR.

Tropical Root and Tuber Crops

Stephen K. O'Hair

  1. INTRODUCTION
  2. PRIMARY CROPS
    1. Cassava
    2. Edible Aroids
    3. White-fleshed Sweet Potato
    4. Yams
  3. SECONDARY CROPS
  4. CONCLUSION
    1. Research Programs
    2. Potential
  5. REFERENCES
  6. Table 1
  7. Table 2
  8. Table 3

INTRODUCTION

The tropical root and tuber crops are comprised of crops covering several genera. They are staple foods in many parts of the tropics, being the source of most of the daily carbohydrate intake for large populations. These carbohydrates are mostly starches found in storage organs, which may be enlarged roots, corms, rhizomes, or tubers. Many root and tuber crops are grown as traditional foods or are adapted to unique ecosystems and are of little importance to world food production. Others such as cassava (Manihot esculenta Crantz) and white-fleshed sweet potato (Ipomoea batatas L.) are known worldwide.

Several of these crops have been termed under-exploited and deserving of considerably more research input. In fact, these crops remained neglected in terms of scientific input until the establishment of the International Center for Tropical Agriculture (CIAT) in Colombia 1967, the International Institute for Tropical Agriculture (IITA) in Nigeria in 1968, and the International Potato Center (CIP) in Peru in 1971.

Although several of these crops have been grown in the U.S. during various periods over the past two centuries, with the exception of potato (Solanum tuberosum L.), they never gained a place of importance in the economy of this country. However, during the past 20 years several new root and tuber crops have appeared in U.S. markets. The increased demand is attributed to massive immigration of people from the tropics to the U.S., mostly from the Caribbean and S.E. Asia, where root and tuber crops are staple foods.

Because of their relatively long growing period of at least eight months, production of all, with the exception of sweet potato, is limited to the warmer regions of the southern-most states. The lack of tolerance to freezing temperatures limits most production of the tropical root and tuber crops to southern Florida.

Since all tropical root and tuber crops are vegetatively propagated and certification is not common, systemic diseases can be a problem. This also presents problems for the importation of germplasm from foreign locations. Tissue culture procedures have been developed to eliminate diseases in vegetative material. However, only a few facilities in the tropics have the expertise and equipment to perform the required procedures.

PRIMARY CROPS

Four root and tuber crops have recently become common in U.S. markets. These are cassava, white-fleshed sweet potato, cocoyam (Xanthosoma spp. Schott), and yams (Dioscorea spp.).

Cassava

Cassava is native to South America and is grown throughout the tropics, with Brazil and Zaire being among the largest producers. It is a perennial shrub of approximately 2 m in height. During the crop production process, it is grown as an annual. It is propagated vegetatively from stem cuttings of approximately 25 cm in length. Because the plants are heterozygous, seeds are only used in the breeding process. Planing is done by hand at densities of 10,000 plants/ha. The most common pests of cassava are weeds and systemic diseases, which are carried from one planting to the next in the cuttings. Starch is stored in enlarged roots with 25 to 40% starch being common. Harvesting begins 8 to 14 months after planting with the entire plant being uprooted by hand. Yields can range from 7 to 30 t/ha. Plants can be left unharvested for more than one season, with the roots becoming larger during the following season. This is not recommended for production in the U.S., since root quality is reduced considerably during the regrowth and aging process. Cassava roots are very perishable with a shelf life of only a few days. Careful handling and storage in high humidity can prolong the shelf life by one or two weeks. Although it is not commonly practiced in the U.S., young tender shoots are consumed as pot herbs in some countries.

The presence of hydrocyanic glucosides (HCN) in all plant parts presents some problems in marketing cassava. Selections have been made from both chance seedlings and in breeding programs which are low in HCN. These are the only types sold in U.S. markets. More than one kg of unprocessed roots would have to be consumed before lethal doses of HCN would be reached. Peeling and boiling in water are common methods of removing a large proportion of the HCN in the roots. Other postharvest problems with cassava include proper handling and storage of cuttings under frost-free conditions.

Roots are usually peeled and boiled or baked. Commercial processing of cassava is limited to packers of frozen, peeled roots, which are marketed in the U.S. in packages like frozen french fries. This convenience pack may have the potential for expanded utilization of cassava in this country and Europe. Deep-fried chips, like potato chips, are produced and marketed in the Miami Florida area. Deep-fried root pieces are offered in Miami Latin restaurants under the name of "Miami fries." Cassava starch, known as tapioca, has limited potential for expansion. Even though cassava flour can be used as a partial substitute for wheat flour in the production of bread, market economics restrict this process to countries where wheat is an import commodity. The future of cassava production in the U.S. is limited by the bulky nature and perishability of the cuttings. The development of cassava which can be grown from seed has great potential. Being a diploid, it should not be difficult to develop material which breeds true to type and germinates uniformly.

Edible Aroids

Two edible aroids are grown commercially in the U.S. The major crop is cocoyam (Xanthosoma sagittifolium) which originated in northern South America. It is known by several common names and in the U.S. is marketed as malanga, its Cuban name. Yautia and tannia are common names for the crop in the Caribbean. Taro (Colocasia esculenta Schott) is grown primarily in Hawaii being most well-known in its cooked form as poi, the traditional staple food of native Hawaiians. Both aroids are known for their love of a humid environment and their ability to flourish in shaded conditions. They are among the most shade tolerant of terrestrial food crops.

Both edible aroids are short-statured perennial plants, grown as annuals. They store starches in large corms at or below the sod surface. Vegetative propagules are taken from the top portion of the corms. Planting is labor intensive with plant populations ranging from 10,000 to 30,000 plants/ha. Harvest begins 8 to 12 months after planting, yielding from 7 to 30 t/ha of edible corms. Lifting devices similar to potato diggers are used as harvesting aids. Considerable amounts of hand labor are involved in this process. Many of the steps in both planting and harvesting could be mechanized further. In the Caribbean and S.E. Asia, young tender leaves are consumed as pot herbs. Limited amounts of leaves are harvested and sold in U.S. markets. The potential for expansion of these markets is considered to be very limited.

Corms which contain 25 to 35% starch, are plagued by the presence of an acrid factor, which causes itchiness and considerable inflammation of tissues. Cooking removes most, if not all, of this factor from domesticated clones. Shelf life of harvested corms varies considerably between taro and cocoyam and depends on the care taken during the harvesting and packaging process. Cocoyam has a considerably longer shelf life of several weeks. This can be extended further with curing and refrigerated storage.

Corms are usually peeled and boiled. Processing is limited to the production of deep-fried chips and poi from taro in Hawaii. Expansion of production for processing in the southern states of the U.S. mainland is possible. However, like cassava, the bulkiness of the propagules and lack of tolerance to freezing temperatures, presents problems. One area that deserves additional investigation concerns the digestibility of the starch. In Cuba, babies and people with mild ulcers are placed on diets of cocoyam.

White-fleshed Sweet Potato

White-fleshed sweet potato is a trailing perennial vine, grown as an annual. Originating in South America, its production has spread throughout the tropics and it is a staple food in countries of Africa and the South Pacific. The main differences between the white-fleshed and the familiar orange-fleshed sweet potatoes grown throughout the southern U.S. are that the white-fleshed types: tend to have a higher dry matter content with 25 to 40% starch and sugar content, are usually less sweet, generally are allowed to grow to a larger size, have variable root shape, and have a skin color that ranges from red to white. In addition the white-fleshed types are usually grown from stem tip cuttings of 30 to 40 cm in length, whereby little attention has been paid to the qualities and yield potential of the enlarged storage roots below the soil. Planting densities are approximately 30,000 plants/ha and harvest begins from four to six months after planting. Although planting is not mechanized, vegetable transplanters could be modified to accommodate the cuttings. Harvesting aids used for cocoyam are also used for white-fleshed sweet potato harvest in Florida.

The main problem in production is sweetpotato weevil (Cylas formicarius Fab.) infestation in the roots, which can result in total crop loss if left uncontrolled. Unlike the other root and tuber crops, sweet potato importation into the U.S. is prohibited due to concern over importation of exotic diseases and insects. Thus, nearly all of the white-fleshed sweet potatoes marketed in this country are grown in southern Florida.

Roots are either baked or boiled. There is little processing potential for white-fleshed sweet potato as it is currently known. Limited amounts of research efforts have been focused on selecting types with little or no sugar content. Such types could be grown in warm climates and used as potato substitutes.

Yams

Several species of yams are grown in the tropics and subtropics. Some, which will not be discussed here, are grown only for medicinal purposes. Of the edible species, Dioscorea alata L., known as the Greater Yam, D. cayenensis Lam., the Yellow Yam, and D. rotundata Poir., the White Yam, are the most common. D. alata originated in S.E. Asia and the latter two are native to Africa. All are herbaceous vines which must be trellised for maximum yield. Yams are grown as annuals at planting densities of 10,000 plants/ha and produce a tuber which is harvested to 12 months after planting. Tuber flesh varies from white to yellow and is from 15 to 40% starch. Tubers have a distinct dormancy period, which can be extended with curing and the application of gibberellic acid. This makes yams ideal for long distance shipment and export. Alternatively, this causes large fluctuations in availability of fresh yams, since the crop is not planted until the dormancy period, which coincides with the dry season, has ended. Once sprouting begins, tuber quality decreases rapidly Thus, good quality yams are in short supply until the next harvest begins. Types have been selected to produce "out of season" crops and may have potential for alleviating these problems.

Propagules are portions of the tuber, with larger propagules producing larger tubers at harvest. None of the cultural practice operations are mechanized. However, potato equipment could readily be adapted for yam culture, especially if types are selected which require no trellising. Because of the expense of trellising, there is no commercial yam production in the U.S.

Yams are usually baked or boiled and mashed. Unless the production expenses are reduced, little potential exists for commercial processing of yams into items like potato chips or french fries.

SECONDARY CROPS

In comparison to the above crops, the potential for the remaining tropical root and tuber crops are limited to a few hectares in the U.S. and are placed in the category of specialty vegetables. Several of these lesser-known root and tuber crops are discussed in accompanying papers by N. Vietmeyer, M. Lamberts; C.R. Sterling and S.R. King; and M. Yamaguchi and will not be discussed further here.

One specialty crop that has potential in much of the southern parts of the U.S. is the Chinese water chestnut (Eleocharis dulcis Trin.). This annual sedge grows in flooded conditions, requiring seven to eight months to produce a crop of up to 28 t/ha. Its greatest potential in the U.S. is as a fresh vegetable in salads as well as in the traditional Chinese cuisine.

CONCLUSION

Research Programs

Unfortunately the majority of the research on root and tuber crops remains limited to a few institutions (Table 1). Of major concern is the scarcity of germplasm collections and the even greater lack of breeding programs. Worldwide, there are more major breeding programs for potato than for all the other roots and tubers combined.

Although there is room for considerable amounts of breeding and selection in the root and tuber crops, progress may be slow in some instances. Genetic engineering holds promise for incorporating genes for virus immunity into most of these crops. Once accomplished, yields should increase dramatically. However, there is one note of caution. Immunities will most likely be limited to single genes. Therefore, concern for the break down of such immunities should be taken into consideration.

Potential

The potential for continued expansion of root and tuber crops in U.S. markets depends on several factors: continued immigration of people from tropical regions, purchase of these foods by offspring of immigrants, and acceptance of these foods by the general public. In the first instance, trends for additional immigration from the Caribbean region will likely continue. Over the past 20 years production of these crops has grown from none to production valued at nearly $30 million (Table 2). Furthermore, imports of tropical root and tuber crops have grown to a value of more than $42 million (Table 3). The continued purchase of cassava, cocoyam, white-fleshed sweet potato, and yam by the offspring of immigrants will in part depend on economics. Historically the market price of these crops is two to four times the price of potato. Therefore, unless the price can become more competitive with potato, purchases will most likely be reserved for special occasions. In the final case, acceptance of cassava, cocoyam, white-fleshed sweet potato, and yam by the general public will be rather limited, due to the price differential with potato and by difficulty in persuading people to try new products. Previous efforts to persuade the U.S. public to use edible aroids as potato substitutes have failed. However, the situation is slightly different now and may hold some surprises.

The potential of the roots and tubers being processed into snack foods again depends on economics and public acceptance. Unless the costs of production can be reduced dramatically through mechanization and selection of earlier maturing clones, the future is not bright. Both obstacles are not insurmountable. However, a decided commitment to research and development must be made in order for this to happen.

REFERENCES

Table 1. Major research and breeding programs and germplasm collections for the tropical root and tuber crops.

Crops Institutionz or local programs (types)y
Cassava CIAT (R,G,B), IITA (R,G,B), Brazil (R,G,B), Nigeria (R), Zaire (R,G,B), India (R)
Cocoyam Puerto Rico (R,G), CARDI (R,G), Florida (R,G), IITA (KG), Cameroon (R), INRA (R,G)
Taro IITA (KG), Hawaii (KG), Philippines (R,G), South Pacific (R,G,B)
White-fleshed IITA (R,G,B), CIP (R,G,B), AVRDC (R,G,B), China (R,G,B)
sweetpotato No. Carolina (R,G,B), Florida (R), Puerto Rico (R)
Yam INRA (R,G,B), IITA (R,G), Nigeria (KG), CARDI (R), UWI (R)
zCIAT = International Center for Tropical Agriculture, Call, Colombia; IITA = International Institute for Tropical Agriculture, Ibadan Nigeria; CARDI = Caribbean Agricultural Research and Development Institute, St. Augustine, Trinidad; INRA = National Agricultural Research Institute, Guadeloupe, France; CIP = International Potato Center, Lima, Peru; AVRDC = Asian Vegetable Research and Development Center, Tinan, Taiwan; UWI = University of the West Indies, St. Augustine, Trinidad.
yR = general research, G = germplasm collection, and B = breeding.

Table 2. Trends in tropical root and tuber crop production in Dade County, Florida.z

Value ($1,000)
Crop 1983 1984 1985
Cassava 1,100 1,400 1,600
Cocoyam 9,800 12,400 14,000
White-fleshed sweetpotato 10,500 12,600 14,200
Total 21,400 26,400 29,800
zData supplied by the Dade County Cooperative Extension Service.

Table 3. Trends in U.S. imports of tropical root and tuber cropsz.

1983 1985 1987
Crop Volume
(thousand t)		 Value
(million $)		 Volume
(thousand t)		 Value
(million $)		 Volume
(thousand t)		 Value
(million $)
Cassava 5.6 2.2 6.2 2.7 7.1 3.3
Cocoyam 17.0 6.5 25.3 9.1 17.4 7.7
Jicama 5.1 1.7 5.9 1.9 8.9 3.2
Yam 8.3 4.9 11.4 6.2 14.4 8.4
Chinese water chestnut 20.0 18.0 20.6 17.2 27.0 21.6
Total 56.0 33.3 49.4 37.1 74.8 44.2
zData supplied by U.S. Census Bureau.

Last update March 18, 1997 by aw