Table of Contents
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
- PRIMARY CROPS
- Edible Aroids
- White-fleshed Sweet Potato
- SECONDARY CROPS
- Research Programs
- Table 1
- Table 2
- Table 3
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
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.
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 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.
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 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
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.
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
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.
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.
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
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.
- Byrne. D. 1984. Breeding cassava. Plant Breed. Rev. 2:73-134.
- Cock, J.H. 1982. Cassava: a basic energy source in the tropics. Science
- Hahn, S.K., D.S.O. Osiru, M.O. Akoroda, and J.A. Otoo. 1987. Yam production and
its future prospects. Outlook Agric. 16:105-110.
- Hodge, W.H. 1957. Three native tubers of the high Andes. Econ. Bot.
- Kay, D.E. 1973. TPI crop and product digest. No. 2. Root crops. Trop. Prod
- Krishnan, P. and M.R. Smith. 1983. Evaluation of auger plow for digging wetland
taro. Trans. Amer. Soc. Agric. Eng. 26:1608-1609.
- Leihner, D. 1978. Follow-up evaluation of two harvesting machines. p. 58-59.
In: E.J. Webber, J.H. Cock, and A. Chouinard (eds.). Cassava harvesting and
processing. Int. Dvpt. Res. Cent., Ottawa.
- Leon, J. 1964. Andean tuber and root crops: origin and variability Proc. Int.
Soc. Trop. Root Crops 1(1):121-122.
- Martin, F.W. 1974. Tropical yams and their potential Part 1. Dioscorea
esculenta. USDA Agr. Handb. 457.
- Martin, F.W. 1974. Tropical yams and their potential Part 2. Dioscorea
bulbifera USDA Agr. Handb. 466.
- Martin, F.W. 1976. Tropical yams and their potential Part 3. Dioscorea
alata. USDA Agr. Handb. 495.
- Martin, F.W. 1978. Tropical yams and their potential Part 5. Dioscorea
trifida. USDA Agr. Handb. 522.
- Martin, F.W. and H. Delpin. 1978. New, superior varieties of Dioscorea
alata, the Asian greater yam. J. Agric. Univ. Puerto Rico 62:64-75.
- Moy, J.H., N. Shadbolt, G.S. Stoewsand, and T.O.M. Nakayama. 1979. The acridity
factor in taro processing. Food Process. Preserv. 3:139-144.
- O'Hair, S.K., G.H. Snyder, and J.F. Morton. 1982. Wetland taro: a neglected
crop for food, feed and fuel. Proc. Fla. State Hort. Soc. 95:367-374.
- O'Hair, S.K. and M.P. Asokan. 1986. Edible aroids: botany and horticulture.
Hort Rev. 8:43-99.
- Onwueme, I.C. 1978. The tropical tuber crops: yams, cassava, sweet potato, and
cocoyams. Wiley, New York.
- Rickard, J.E. 1985. Physiological deterioration of cassava roots. J. Sci. Food
- Tracy S.M. 1903. Cassava. USDA Farmers' Bul. 167.
- Wheatley C.C. and W.W. Schwabe. 1985. Scopoletin involvement in post-harvest
physiological deterioration of cassava root (Manihot esculenta Crantz).
J. Exp. Bot 36:783-791.
- Wickham, L.D., H.C. Passam, and L.A. Wilson. 1984. Dormancy responses to
post-harvest application of growth regulators in Dioscorea species. 2.
Dormancy responses in ware tubers of D. alata and D. esculenta.
Agric. Sci. 102:433-436.
- Young, R.A. 1917. The dasheen; its uses and culture. USDA Yearb. 1916.
- Young, R.A. 1924. Taros and yautias; promising new food plants for the South.
USDA Bul. 1247.
Table 1. Major research and breeding programs and germplasm collections
for the tropical root and tuber crops.
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.
|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),
|Cocoyam ||Puerto Rico (R,G), CARDI (R,G), Florida (R,G), IITA (KG), Cameroon (R),
|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)|
yR = general research, G = germplasm collection, and B = breeding.
Table 2. Trends in tropical root and tuber crop production in Dade
zData supplied by the Dade County Cooperative Extension Service.
| ||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|
Table 3. Trends in U.S. imports of tropical root and tuber cropsz.
zData supplied by U.S. Census Bureau.
| ||1983 ||1985 ||1987|
|Crop ||Volume |
|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|
Last update March 18, 1997