TROPICAL YAMS AND THEIR POTENTIAL Part 3. Dioscorea aJaia

Agriculture Handbook No. 495

Agricultural Research Service UNITED STATES DEPARTMENT (^AGRICULTURE in cooperation with Agency for International Development PREFACE The feeding of future generations requires a knowledge of the individual crop plants of the world and their poten- tials. Crops can be recommended for use in particular regions only on the basis of potential yield, the costs of production, the food and feed value of the crop, and the way the crop can be processed or otherwise used. For most of the major food crops of the world, a body of information is already available. However, tropical roots and tubers, which are wide- ly used as staple foods, have been largely neglected. Only in recent years has an awareness been growing of the potential of these crops to supply large amounts of food in relatively small amounts of space. Yams are the second most important tropical root or tuber crop. The annual production, perhaps 25 million tons, places them second in importance to cassava. But yams are better food than cassava, and while they are usually thought to be more difficult to grow, under some conditions yams outproduce cassava. Yams fill an important role in the diet of many areas of the Tropics—a role that can increase in importance. That role and its potential are not, however, well understood. The yam is not a single species. Perhaps 60 species have edible tubers; of these about 10 species can be considered crop plants. The literature concerning these species is wide- spread but fragmentary. This is the third of several Agricul- ture Handbooks in which the major species of yams are individually treated in order to bring the investigator as well as the agriculturalist up to date with respect to the status of these important plants. This is part of a research effort cosponsored by the Agricultural Research Service of the U.S. Department of Agriculture and the U.S. Agency for International Development to introduce, evaluate, and dis- tribute better yam varieties. CONTENTS Page Preface ii Introduction 1 History and origin 1 Geographic distribution 2 Botany 3 Ethnobotany 3 Classification 4 Morphology 4 . Cytology 9 Varieties 10 Culture 15 Environmental requirements 15 Land preparation and planting 18 Fertilization 20 Staking 21 Pests and diseases 21 Harvesting 27 Culinary characteristics 28 Cooking and processing 29 Composition 30 Storage 35 Potential uses 36 Literature cited 38

Washington, D.C. Issued June 1976

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Price 55 cents 25% discount allowed on orders of 100 or more to one address Stock No. 001-000-03552-5 There is a minimum charge of $1.00 for each mail order Also in ''Tropical Yams and Their Potential" series— Part 1. Dioscorea esculenta, USD A Agriculture Handbook No. 457. Part 2. Dioscorea bulbifera. USDA Agriculture Handbook No. 466. TROPICAL YAMS AND THEIR POTENTIAL Part 3. Dioscorea ¿ilatii

By FRANKLIN W. MARTIN, plant geneticist, Mayagüez Institute of Tropical Agri- culture,'^ Agricultural Research Service, U.S. Department of Agriculture, Mayagüez, P.R. INTRODUCTION best yams in the world—in flavor, For many people in the world processing quality, production, the word ''y^ni/' or its translation, and agronomic characteristics. refers to Dioscorea alata L., the most widespread yam species. It History and Origin has been introduced to all parts of In spite of its importance, the the Tropics where growing condi- little that has been written about tions are suitable, and it has fre- the origin of D, alata is mostly quently gone wild and continued speculation. That great numbers to develop new variations. It has of varieties are known from India become accepted in West Africa, to the islands of the South Pacific, where it now sometimes competes many of which are quite local in with two important native species, distribution, suggests that the D. rotundata Poir. and D, cayenen- species was domesticated and sis Lam. It is difficult to say widely distributed very early. whether or not the total production Furthermore, the occurrence of of D. alata surpasses that of its distinct but closely related vari- chief rival, D. rotundata. Whereas eties on somewhat isolated islands the latter is almost restricted to suggests that it has continued to West Africa, D. alata is so widely evolve in areas where it has been distributed that it is impossible to introduced. give an accurate estimate of pro- De Candolle (8)^ hypothesized duction, but it cannot be less than an Indo-Malayan center of origin 10 million metric tons per year. Its for D, alata, based on the wide- agronomic flexibility, widespread spread varieties found there. Bur- occurrence, ready acceptance, and kill (Í), a superb student of the high nutritive value suggest that yam, placed the origin of the spe- D, alata is the most important of cies in Southeast Asia and at- the cultivated species. Further- tempted to trace its distribution more, it has great promise for the throughout the world and gradual future, especially when improved decline in importance as cassava, varieties are distributed. Those sweetpotatoes, and potatoes dis- already discovered are among the placed it. The existence in Burma ^ Formerly, Federal Experiment Sta- 2 Italic numbers in parentheses refer tion. to items in "Literature Cited,'' page 38. AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE of two closely related wild species, amples of the short-tubered, com- D, hamiltonii Hook and D. persi- pact types found in Indonesia milis Prain et Burk., suggested (especially Celebes) and Papua that Z). alata had been selected New Guinea. from these or from their hybrids. It is highly likely that simple Both are characterized by long, or primitive forms of an initial deeply buried tubers that super- species, perhaps well distributed, ficially resemble some cultivated were selected locally to give rise but inferior varieties of D, alata, to the principal varietal types. This Burkill hypothesized that deep could have happened independent- tubers evolved as a protection from ly on widely separated islands. wild pigs and that human selection Interchange and hybridizations in- resulted in the short-tubered, com- creased variation. Inspection of pact varieties. For varieties with varieties from worldwide sources upward-curving tubers, which suggests to the present author that eventually push their way out of at least two distinct species were the soil, he hypothesized another involved in the evolution of D. human selection, since this growth alata, in one of which the stem habit makes harvesting easy. was not winged; study of materials There is little evidence elsewhere on hand is needed to clarify this to support BurkilFs hypothesis. problem. No systematic breeding Wild species related to Z). alata are now appears possible, and existing also found in Papua New Guinea. varieties appear to be ancient Moreover, the variation in D. alata dead-end relics of the evolutionary is enormous, and some varieties process. A remarkable story of are unique to the region. Indo- ennoblement of the species thus nesia also has extremely diverse remains undisclosed. varieties that must have existed for centuries. Geographic Distribution Relying on what is known about Dioscorea alata is a plant of the historical movements of people in hot humid Tropics; it seldom oc- Southeast Asia and on the scanty curs where cool temperatures or information about distribution dry periods prevail during the and variation in yams, Alexander growing season. There is no single and Coursey {1) placed the origin area, however, where D, alata is of D. alata in an area lying be- the chief starchy food; it is almost tween the distribution of Z). hamil- always utilized as one of several tonii (East India and West farinaceous crops (other.yams, Burma) and that of Z). per similis cassava, sweetpotatoes, and (Indochina). The difficulty of aroids) that are used to some ex- their theory is that cultivated tent interchangeably. races in this area (roughly In Southeast Asia Z). alata is Burma) do not include many ex- grown chiefly as a dooryard crop. TROPICAL YAMS AND THEIR POTENTIAL, PART

It is of special importance in Papua D. alata varieties are found chiefly New Guinea, where more than in the Caribbean, the north shores 30,000 persons depend on yams of South America, and in scattered for their livelihood (19). In south- locations throughout Central ern India D. alata yams are grown America. In localized areas and and appreciated, but are secondary certain islands, especially Trinidad foods. They and other yam species and Barbados, D. alata is of con- are important crops in some but siderable economic importance. not all parts of Indonesia. In Ma- Isolation has undoubtedly played laysia yams are not common. In an important role in establishing the Philippines few varieties of varieties, for the principal varie- D. alata are found, but some of ties differ from island to island them are unique and highly valu- throughout the Caribbean. able. Many varieties of D, alata In South and Central America are found throughout the South D. alata is hardly known and poor- Pacific islands, and the species is ly distributed except for isolated, important, but more so on some usually coastal pockets. A few islands than on others. varieties were introduced in Brazil Dioscorea alata has had to com- years ago, and recently others have pete with native species of yams been brought in. Introduced D. in West Africa, where it has be- alata competes in some areas with come a second or third best yam. introduced African species. Varieties of D, rotundata and D. cayenensis are usually preferred BOTANY because they can be pounded to Ethnobotany produce the typical dish of the re- Although the culture of West gion, a sticky dough called fufu. African peoples has developed There are few areas where D. alata around the exigencies of the yam is unknown, however, and in Sierra (P), in only one area of Southeast Leone D, alata outranks D, rotun- Asia—Papua New Guinea—has a data in importance. Almost every- similar phenomenon occurred. where it is known as water yam, There, D, alata has religious and or the equivalent in local tongues. psychological importance much be- The slight variation among Afri- yond its significance as a food can forms suggests that only a crop. The planting of yams in two few varieties, and certainly not types of gardens has been de- many superior ones, have been in- scribed by Lea (19). Whereas in troduced. Elsewhere in Africa, D. one type of garden yams are pro- alata is even more poorly distrib- duced for food, in the other they uted and not of great importance. are treated with elaborate care to Feral varieties are known in Afri- produce enormous tubers. Planting ca. of the ceremonial gardens is done In the New World, introduced only by men and boys and is regu- AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE lated by numerous taboos. Great might be Asian wing-stemmed pride and masculinity are attached yam. to the production of giant yams. D, alata belongs to the section Although the techniques for pro- Enantiophyllum, a section of the duction of such yams have passed genus known only in the Old to other islands of the South Pa- World, with many species in Afri- cific, little or no special signifi- ca and Asia. The Asian species, cance is attached to their culture about 50, have been described by outside of Papua New Guinea. Burkill (5). Tubers of this section usually grow vertically, and the Classification leaves are simple. The section in- cludes many edible species. Only There are perhaps thousands of Z>. persimilis and D. hamiltonii, D, alata varieties. They differ es- previously mentioned, appear to be pecially in characteristics of the closely related to D. alata, and only tuber and often in characteristics these three species have winged of the foliage. This polymorphism stems. The section also includes has led to attempts to divide D. the principal African species, D. alata into several species or sub- rotundata Poir. and D, cayenensis species or to group varieties into Lam. meaningful categories. (See "Va- In common with other yams, the rieties".) Such attempts have not wing-stemmed yam belongs to the been successful, but have confused family Dioscoreaceae, of which the literature with useless syn- Dioscorea is the principal genus. onyms such as D, atropurpúrea The family is usually classified Roxb., D. purpurea. Roxb., D. glo- among the monocotyledons, al- bosa Roxb., D. rubella Roxb., D. though some evidence of a second ebúrnea Lour, or Willd., D, vul- cotyledon has been found ( 18). garis Miq., D, javanica Queva, D, The family is characterized by colocasifolia Pax, and D, sativa rhizomes, usually reduced to a Del. nodeless structure. The male and In many areas the common name female (fig. 1) flowers, usually on 'Vater yam" is applied to D. alata. separate plants, are small and It is not a good name, for it does often inconspicuous. The inferior not refer to any consistent prop- ovary becomes quite prominent erty of the species. It is used, more- after fertilization. The floral pat- over, for individual varieties or tern is based on sets of three. groups of varieties of D. alata. Other common names are greater Morphology yam, Asian greater yam, winged All varieties of D. alata seen by yam, white yam, and ten-month the author can easily be recognized yam, none of which is entirely ade- as belonging to this polymorphic quate. A useful descriptive name species, and varieties of other spe- TROPICAL YAMS AND THEIR POTENTIAL, PART 3

from the thorns of the Feo type. Wings are believed to help the stem grasp smooth objects in twin- ing. The leaves are often quite large, although those of the better vari- eties may be smaller. They are commonly opposite on the stem, but in a few varieties and on the young vigorous stem of most vari- eties, they are alternate. The leaves are glabrous, with a slight bluish bloom in a few varieties. Many varietal differences can be dis- tinguished (fig. 2). The shape of the leaf is determined principally by the width of the sinus between the leaf lobes. Varieties differ in the folding and undulation of

PN-4437 leaves, in the thickness of the FIGURE 1.- -Female flowers of D. alata. lamina, in green color intensity, and in anthocyanin coloration. The cies can usually be excluded read- wings of the petiole of the leaf are ily. D. alata is a glabrous vine that usually enlarged at the stem, pro- climbs by twining to the right. The ducing a characteristic appear- length of the vine varies from 2 to ance. Stipules are rarely present. 30 meters or more in primitive The flowers of D. alata are pro- forms. duced during the last third of the The fleshy stems are character- growing season, when tubers are ized by wings, typically mem- forming. Not all varieties flower, branous and often ruffled. In some and some flower sporadically, per- varieties the wings are reduced or haps in response to seasonal vari- almost absent, but the stems may ations. It is unknown whether be quite thorny, in which cases flowering can be induced by chang- the thorns occur in straight lines ing environment. representing the wing; the habit Male and female flowers are is associated with two distinct borne on different plants. The tuber types and with a distinct male flowers are small (1-2 milli- leaf shape. In rather primitive meters in diameter) and are pro- varieties with well-developed duced in crowded panicles that wings, the wings of the thickened originate in the axils of the leaves basal stem are often modified into and the tips of the branches. The thick, blunt thorns quite different sessile flowers range from green 6 AGRICULTURE HANDBOOK 495. U.S. DEPT. OF AGRICULTURE

PN-4438 FIGURE 2.—Representative leaves of D. alata varieties. (Two-fifths actual size.) to white. All six stamens are well pecially of the wild or primitive developed. In many varieties the type, appear to be functionally male flowers do not open or open fertile. The female flowers (fig. 1) such a small amount that pollinat- are borne on long racemes from ing insects cannot enter. The pollen the axils of the leaves. The flowers of most varieties, as seen under are characterized by a prominent the microscope, is malformed or trilocular inferior ovary from 8 to aborted, but a few varieties, es- 15 centimeters long. The capsules, TROPICAL YAMS AND THEIR POTENTIAL, PART 3 which develop rapidly after polli- veloping tubers are strictly ad- nation, reach 20 to 30 millimeters ventitious, the tuber being nodeless in length. They mature, dry, and and originating from the stem. The split along the sutures of the wings principal roots arise from what is to release two seeds from each of sometimes called the primary the three locules. Few capsules are nodal complex (fig. 3), or crown, produced, however, even when of the plant, near the surface of the male and female plants are grown ground, but in some varieties, a together. Moreover, the develop- variable number of roots arise ing capsules often contain only a from the body of the tuber itself. single seed, and this frequently When stems touch moist soil they aborts before maturity. Thus, as often develop adventitious roots in the case of males, most female and may even develop large tubers. plants are sterile. The author has The tubers of D. alata are ex- no experience growing D. alata tremely variable, and the number from seeds and believes that con- of possible forms almost endless. ventional breeding of this species Plants normally bear only one would be extremely difficult. tuber, but two to five are common, Only newly germinated seeds even on varieties typically produc- have true roots; the roots of de- ing one tuber. These are normally

FIGURE 3.—Primary nodal complex of rooted cutting of D. alata, showing origin of roots and shoots. (Three times actual size.) 8 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE located vertically in the soil and damp soil, they grow into it rapid- usually descend. Unusual varieties ly. In this way a single plant can are known in which, after growing give rise to many small, rooted down awhile, the tip of the tuber tubers in a short time. Aerial begins to grow upward, producing tubers continue to be produced a long, thin, U-shaped tuber. until foliage dies back at the end Tubers often enlarge as they de- of the growing season. All are scend to form a pyramid, but they edible, but the smaller ones are may also be constricted later in a immature and inconvenient to spindle shape. Tubers of some va- prepare for cooking. rieties are extremely smooth, while Varieties are distinct with re- those of others are extremely ir- spect to flesh color. Anthocyanins regular. Shape is complicated by color some flesh purple, usually tuber branching, which may occur irregularly. The flesh of varieties during early or late growth. There with no anthocyanin varies from may be few or many branches, white or cream to yellow. Unidenti- either the same size as the main fied carotenoids are among the tuber or smaller, and the branches pigments. Purple-fleshed varieties may themselves continue to are favored over white in the branch. Some of the most unusual Philippine Islands because of the shapes are found among the New agreeable color that they lend to Guinea yams, in which the tuber desserts. Nevertheless, anthocyan- can occur as a series of vertical in- ins have no nutritional value, and tersecting plants. in the opinion of many persons, The tubers are renewed annual- distract from the appearance of ly. A tuber produced one year gives the cooked tuber. rise in the next to a shoot, the Both smooth- and coarse-tex- growth of which is usually pre- tured varieties are found. Coarse ceded by the appearance of the tuber texture, which appears curd- primary nodal complex from which like, is attributable to bundles of roots, new tuber, and stem arise. storage tissue in a somewhat clear The new tuber remains extremely matrix. small during the initial months of When a tuber is cut, the flesh growth, but develops rapidly near exudes mucilaginous substances, the end of the growing season. now known to be glycoproteins, Many varieties also produce which can discolor the surface, single or multiple aerial tubers slowly or rapidly, upon oxidation. (fig. 4) in the axils of the leaves Exposed to air, cuts are healed by at the same time underground the formation of layers of dead, dry tubers are being formed. Aerial cells. There is no evidence of suber- tubers often resemble under- in deposition in this species, but ground tubers in shape and dried cells effectively reduce mois- branching habit. When they touch ture loss and fungal penetration. TROPICAL YAMS AND THEIR POTENTIAL, PART 3 9

PN-4440 FIGURE 4.—Typical aerial tubers of D. alata. (One-half actual size.) Cytology polyploidy is normal in the species and probably accounts for its high Numerous counts have been sterility. made of the chromosomes of D. Variations in chromosomal num- alata. All numbers reported, ber within the cells of a single whether based on pollen-mother- variety have been reported (28). cell or root-tip observations, are Such variation can be fixed by based on multiples of 10 (21 ). The asexual propagation and in fact occurrence of 2n numbers such as probably accounts for the occur- 30, 50, or 70 can only be accounted rence of sibling varieties—that is, for through the hybridization of varieties that differ only in minor varieties with different numbers details. Odd chromosome numbers of sets of 10 chromosomes. Thus, due to the presence of one or two 10 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE extra chromosomes are uncom- anthocyanin coloration of tuber mon, and such counts are not re- and foliage, but also on minor liable. Aneuploidy, when it occurs, morphological features such as would contribute to the sterility of number of wings on the stem. The the species. Extra chromosomes materials available to him appar- have been attributed to a sex-de- ently did not represent the full termining mechanism. There is, variation in the species. Ochse and however, no concrete evidence to van den Brink (25) set aside a suggest an XO sex-determining group of highly improved varieties system. Furthermore, Martin (20) from a much larger, more hetero- has shown how polypoidy can ex- genous primitive group. Burkill, plain the unusual sex ratios found a careful observer who had an ex- in most species. Males predominate tensive Southeast Asian collection, over females in most controlled based his classification on four crosses, in predictable ratios as- kinds of tubers (3): long and deep- sociated with numbers of sex-con- ly buried tubers, half-long tubers, trolling chromosomes present. short tubers, and nonburying As previously noted, male and tubers. He then broke each class female flowers are usually sterile, into a number of subclasses based and in varietal collections seeds are on characters such as branching seldom produced. No efforts to tendency, presence of a neck, and breed the wing-stemmed yam have tendency to curve. While his classi- been reported in the literature. It fication groups certain related is difficult to escape the conclu- varieties together, it is an over- sion that existing varieties are simplification of the varietal pic- very old and perhaps have di- ture and is useful principally in verged from their progenitor making rough order out of appar- varieties by somatic mutation. ent chaos. Nevertheless, a sexual system must Gooding (15) developed a key to have been present at one time to distinguish some West Indian have produced the variation ex- varieties of D, alata. His system, hibited by this species. Primitive though useful, cannot adequately man presumably practiced selec- deal with a large collection. Martin tion and gradually produced the and Rhodes (22) studied the cor- fine varieties now known. It ap- relations among 100 character- pears that this process of improve- istics of 30 D, alata varieties and ment is no longer possible. found that certain tuber and VARIETIES foliage characteristics tended to The variation of D, alata has occur together. Three groups of been dealt with in several classifi- varieties were delimited: Feo (fig. cations. Roxburgh (27) divided D. 5), primitive, and choice. Feo was alata into five species (see "Clas- judged to be a natural grouping; sification''), chiefly based on the choice, highly artificial; and primi- TROPICAL YAMS AND THEIR POTENTIAL, PART 3 11

PN-4441 FIGURE 5.—Irregular tubers of a Feo group yam. (One-fourth actual size.) tive, intermediate. In another acteristics found by the authors to study Rhodes and Martin used the be useful in classifying varieties techniques of numerical taxonomy and their method of rating are to define groups in a less arbitrary given in table 1. fashion (26). The primitive group The classification of D. alata was divided into two easily dis- varieties into groups is probably tinguished subgroups. In this always artificial and serves best study the difficulty of grouping the needs of particular investiga- was revealed by the special tech- tors. The family tree of the species niques used. On a multidimensional would probably consist of a net- graph made possible by the prin- work of anastamosing branches, a cipal-components method, few va- few small limbs, and many twigs rieties were found to occur in now separated from the parental recognizable clusters. The char- trunk. Thus, the bewildering array 12 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE of varieties will probably continue eases, and low in anthocyanin to confound. coloration. Tubers should be short, Although there is regional dis- compact, uniform in shape, smooth agreement about what traits a skinned, and free of large roots, good variety should have, the fol- Flesh should be essentially free of lowing characteristics would gen- pigments, low in oxidation tend- erally be considered desirable. The ency, and uniform, with a very fine foliage should be vigorous, rela- granular structure. After cooking tively resistant to leaf-spot dis- the color should be white or cream. TABLE 1.—Suggested characters for D. alata analysis

Character Method of rating^

Plant characters: Anthocyanin in old stem2 0 to 6. Anthocyanin in lower petiole^ 0 to 6. Anthocyanin in upper petiole^ 0 to 6. Angle of basal veins Estimated, 15° intervals. Widest region of leaf Observed. Green color intensity of leaf 1 to 6. Rugosity of leaf 1 to 6. Development of spines 0 to 6. Development of wings 1 to 6. Petiole length Measured. Leaf blade length Measured. Width/length ratio of leaf Calculated. Presence of aerial tubers 0 to 2. Tuber characters: Average weight Weighed. Type 1 to 5, solitary to multiple. Shape 1 to 4, spherical to long. Tendency to have neck 0 to 3. Tendency to branch 0 to 5. Site of branching 0 to 5, head to tip. Length Measured. Diameter/length ratio Calculated. Pinkness of cortex, stem end 0 to 6. Anthocyanin^ 0 to 6. Yellow color 0 to 3. Tendency to oxidize 0 to 3. Grainy appearance 0 to 3. Sting of flesh 0 to 3. Acceptability after cooking (appearance) • 0 to 3, not acceptable to pleasing. Erosion in cooking 0 to 3. Flavor 1 to 3, poor to excellent.

1 In the numeric scales indicating range of variation within a character, 0 designates none or not present and 1 designates lowest or least, unless otherwise specified. - Judged by red color. TABLE 2.—Superior cultivars of D. alata and their attributes O ^"'^^^^^ Recent source Advantage Disadvantage g

W ""'..■::.■.■::::;::::.■:: SfÍMca Unifo™ shape and cdor Poorkitchenquamies. ^ 'rprTw t Vigor, good quality and yield Susceptible to leaf spot. § ,^^'"*'^°'., do Perfect shape, multiple tubers. ° downl;.' M-^' '• Good shape, thick bark Polyphenolic oxidation. = '?r«?«" f'^^^'n ■•; Excellent shape and flavor Pooryields. S ^^"^'^ IvoryCoast Good shape, color, and yield Quality not as high as ^ 'Pnto' /-ci. desired. O <^ .,'■ ;,'/■■.■; ^''*"* Good shape and flavor Lowviabilitv á 'SmoohStatm' Caribbean Excellent shape and flavor SusceSlet'o virus. |

'White Lisbon' Trinidad Best cooking characteristics PoorÏhlpe. > ^""^""^^ Nigeria Heavy yields, good flavor Irregular shape. H

CO 14 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE

PN-4442 FIGURE 6.—Typical tuber of the select variety 'Florido'. (Two-fifths actual size.) the flavor rich and free of bitter- and good precooked, dried flakes. ness, and the texture smooth. The It appears that D. alata varieties tuber should be resistant to insects with good flavor and acceptable and fungi in the ground and in cooking characteristics are low in storage and should have a long phenolic compounds, including storage life before germination anthocyanins. Although phenolic begins. Germination in season compounds (and polyphenolic oxi- should be dependable and rapid. dation) account for undesirable A good yam can be processed into flavors and colors, these com- good fries, good chips, good flour, pounds may be also associated with TROPICAL YAMS AND THEIR POTENTIAL, PART 3 15 disease resistance, since varieties and 2 years may be required to with low phenolics seem more change seasonal response. The fac- susceptible to diseases. External tors controlling plant response are tuber characteristics appear to be not known, but are suspected to unrelated to cooking quality. include day length, temperature, Varieties selected by the author and water availability. and their sources and character- When a collection of varieties is istics are given in table 2. All are assembled and grown in one loca- available for distribution.^ Other tion, differences among their superior varieties are expected to growth cycles become obvious. come out of the author's program. Some varieties mature consist- 'Florido' (fig. 6) is representative ently early, while others are of a very good D. alata variety, but consistently late. The tubers of it is susceptible to leaf-spot dis- early-maturing varieties may also ease. 'Gemelos' (fig. 7) usually germinate early. Thus, earliness is gives excellent yields and has very not necessarily related to shorter high quality. The best variety for maturation time (which also var- kitchen and processing quality, ies from 7 to 10 months), but to a 'Forastero' (fig. 8), bears some- season of dormancy unique for what irregular tubers, but its each variety. superior cooking qualities com- In addition to differing in sea- pensate for this defect. son of dormancy, varieties also differ with respect to length of CULTURE dormancy. Natural dormancy is Environmental closely related to storability. Vari- Requirements eties with tubers that store poorly The growth cycle of the wing- are precisely those with short stemmed yam, a plant of shady periods of dormancy. rain forests, is intimately tied to a Changing germination season, long rainy season followed by a time to maturation, and matura- short dry season. In perhaps all tion season is extremely difficult. areas where this species has been Planting earlier than normal, for successful, the dry period corre- example, does not necessarily sponds to the time of short days stimulate earlier germination by and lower-than-normal tempera- the same amount. On the other tures. When varieties are moved hand, tubers germinate at the ap- from one hemisphere to another, ad- propriate season for the variety justment difficulties are marked, even when conditions for growth 3 Free tuber pieces distributed dur- are unsuitable. This seasonal re- ing January and February upon request sponse of D. alata is one of its to Mayagüez Institute of Tropical Agri- greatest limitations as a crop culture, Agricultural Research Service, U.S. Department of Agriculture, Maya- plant. güez, P.R. 00708. The wing-stemmed yam is 16 AGRICULTURE HANDBOOK 495, U.S. DEFT. OF AGRICULTURE

PN-4443 FIGURE 7.—Tubers of the variety 'Gemelos', somewhat more branched than normal. (Two-fifths actual size.) adapted to growth in the forest. site for optimal growth. Experi- In nature the relatively leafless ments have demonstrated that new vine climbs rapidly through yields are reduced by drought. undergrowth to the tops of trees, However, excessive rainfall can where it spreads its foliage. The contribute to the spread of foliar climbing habit makes it necessary diseases in susceptible varieties. to stake the vines for maximum Furthermore, the plants cannot growth and high tuber production. tolerate excessive moisture around When vines are not staked, accept- the roots and tubers. able, but not maximum, yields are If drainage is provided, D. alata sometimes obtained. tolerates a wide range of soil types Long rainy seasons are a requi- and pH conditions. Deep, loose. TROPICAL YAMS AND THEIR POTENTIAL, PART 3 17

PN-4444 FIGURE 8.—Typical tuber of the variety 'Forastero'. (Two-fifths actual size.) permeable clay soils are preferred, cause they seldom retain enough Sandy soils are not tolerated be- moisture and frequently are not 18 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE fertile. Drainage is best when the cumulate organic materials in a soil itself is permeable, but ridges hole or trench and to plant the and mounds are also useful in pro- seed yam directly in the hole. When tecting plants from flooding. A very wide spaces are used in con- high level of organic material in junction with tall vine supports the soil is conducive to good drain- and large seed pieces, very large, age and aeration as well as to good even giant, tubers are produced. nutrition. Such practices are followed in New Guinea, other islands of the Land Preparation Pacific, and Trinidad to produce and Planting large tubers for competition or for Because varieties of D, alata are ceremonial or display purposes. grown in such widespread areas New yams are always planted and by peoples with differing cus- from existing tubers, pieces of toms and skills, many techniques tuber, or aerial tubers. Although are used in their culture. However, most varieties of D. alata can be the requirements of the varieties propagated from a one-node stem are the same. In this discussion the cutting (fig. 3), the technique is most advanced techniques are em- slow, and the resulting plants need phasized, although it is recognized at least a second year to mature. that most yams are now grown, To stimulate root and tuber pro- and may continue to be grown, by duction of small cuttings, young simpler techniques. stems are cut into pieces with one In Puerto Rico excellent yields node and part of the stem on either of yams can be obtained in deep, side. The stem and node are buried well-drained clay soils without in sand or other porous rooting plowing or forking. In most soils medium and treated intermittent- the practice is to build ridges or ly with mist spray for 1 or 2 mounds of soil by hand or machine. months until rooted. Normally, the Since yams benefit from the addi- first tissue produced at the axil of tion of organic material to the soil, the leaf is undif f erentiated but be- such materials can be spread over comes the primary nodal complex the soil first and then mixed in from which roots, tubers, and and formed into the ridge with a shoots arise. disk plow. This method was de- When aerial tubers are avail- veloped from primitive hand- able, they can be used as seed mounding, still used throughout pieces. They usually germinate most of the Tropics. A second uniformly and produce vigorous method is to place the organic ma- plants. However, aerial tubers are terial in trenches and to build the usually small, and many are too ridges over the trenches. small to produce plants of suffi- A particularly effective method cient size in one growing season. for growing large yams is to ac- In replicated trials aerial tubers TROPICAL YAMS AND THEIR POTENTIAL, PART 3 19 have produced more tuber yield Seed pieces are usually made by than comparably sized pieces of cutting the yam crosswise. The large underground tubers. piece including the plant crown Small, whole, underground tu- has preformed buds and germi- bers OÍD. alata are sometimes used nates readily. The tender growing for seed. Although this practice tip of the tuber often does not might appear similar to planting germinate very well. (However, an aerial tuber, small underground there is disagreement on this tubers often give poor results be- point.) The midsections usually cause their smallness may be the germinate well after new buds result of abnormal growth. Studies have developed. To avoid the prob- in Puerto Rico have shown that a lems of middle and tip sections, the large proportion of small tubers crown ends of tubers to be used for come from virus-infected plants, food can be cut off and saved for and plants grown from them are seed. These pieces, which are usually infected with virus. tougher and more fibrous than the The vast majority of yams are remainder of the tuber, are likely planted from pieces of mature to have low value as food anyway. tubers. Yams for seed should be Clarification of the relative ease selected from the largest and of germination of the various sec- healthiest and stored at moderate tions as related to tuber age and to cool temperatures and moderate cutting time is desirable. to low humidity. Seed pieces cut The germination of seed pieces from tubers should weigh 100 to can be stimulated to some extent. 500 grams. Planting is usually High temperatures (40°-60° C) timed to coincide with the begin- for 1 hour or more advance germi- ning of the rainy season. When nation 1 or 2 weeks. Planting sprouting begins in a few tubers, stimulates germination, probably tubers should be sectioned and by furnishing warm and humid planted a few days later. When conditions. Cool, dry storage in- cutting tubers for seed pieces, rot- creases the dormant period. There ten portions should be discarded. are limits to these techniques, but Cutting tubers weeks in advance they have not been investigated or permitting sprouting before systematically. The biological cutting are common practices but clock mechanism of the variety is are not recommended. The cut sur- probably the most important faces should be dusted with wood factor determining time of germi- ashes or a dry fungicide mixture nation. and allowed to dry a little before Chemical treatments have been planting. This procedure prevents useful in stimulating germina- rot in the soil, preserving the nu- tion. An 8 percent solution of ethyl- trients stored in the tuber piece for ene chlorohydrin (2-chloroetha- the growing plant. nol) used as a dip 4 to 6 weeks 20 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE after harvest is said to cause Fertilization germination of most tuber pieces in 2 to 3 weeks (6). The concentra- Considerable controversy exists tion necessary to stimulate germi- with respect to the fertilization of nation decreases as the season of D. alata. Common observations dormancy passes. An ethylene- have shown that organic material releasing substance, ethephon, is in large amounts is useful, but that also useful in stimulating germi- it should be partially rotted before nation. It is effective in concentra- use. Organic materials are added tions as low as 0.4 percent in water before or at the time of planting. when applied in a 5-minute dip. Mineral fertilizers have given Seed pieces should be handled equivocal results and at times with great care. They are planted have actually reduced yields. by opening a hole in the soil with a Nevertheless, a pattern of needs small hand hoe and placing the has emerged from many experi- tuber piece in it. Some persons feel ments. During the first 6 weeks of that the orientation of the tuber growth, plants rely heavily on the piece in the hole is very important nutrients störend in the seed piece. and that the cut surface should Then, during the first half of the face upward. The seed is covered growth period, they need large with about 10 centimeters of soil quantities of nitrogen to sustain to prevent its drying. Occasionally, vegetative growth. When tuberi- a mulch of dried plant material is zation occurs, much potassium is required. placed over the planting site. In Among the fertilizer recommen- West Africa mulching retains soil dations given are the following: moisture in the root zone and sig- Nigeria, 12:12:18, 60 grams per nificantly increases yields. Plant- plant (9); Trinidad, nitrogen, 100 ing can be simplified by opening a kilograms per hectare applied 3 furrow with machinery, dropping months after planting (16) ; Domi- in the seed pieces, and then cover- nica, 3:9:18, 30 grams per plant ing the furrow. 4 to 6 weeks after planting (un- Optimum planting distances de- published) ; Barbados, ammonium pend on many factors, and local sulfate and muriate of potash, trials should always be made. For about 200 kilograms of each per hectare (13), With such a wide culture of giant tubers, spacing of variety of recommendations exact 2 or 3 meters is justifiable. One advice cannot be given; mineral meter between plants is usually fertilization trials will always be adequate. When yams are planted necessary for particular locations in machine-made rows, the rows and cultural practices. When inex- should be 2 meters apart and the pensive organic material is avail- plants, 0.5 meter in the row. able, it will probably always be TROPICAL YAMS AND THEIR POTENTIAL, PART 3 21 aavantageous to use it, especially which are passed over the wire. at heavy rates (5 to 50 metric tons Sufficient space is left between per hectare). rows of poles to provide access to the vines. In a variation of the Staking technique a lower wire is also used, As a rule, stakes for climbing and strings are passed between the guarantee better yields. However, two wires. Details of this well the gain in yield must be balanced thought-out technique merit study. against the cost of the staking sys- It is not unusual to find the tem. In most locations it is profit- wing-stemmed yam growing with- able to stake, and some inexpensive out support (fig. 9). On Barbados, staking method should be sought. it is normally grown between crops When necessary, however, the of sugarcane. Before the cane is wing-stemmed yam can be made planted, yams are planted on the to produce without staking. ridges and usually given some Staking plants with banana, fertilizer. They soon cover the corn, pigeon peas, and other crop ground. Under this condition plants has not proved beneficial. weeds are somewhat difficult to The support plants compete for control, and the vines do not reach nutrients with the yam crop and their maximum development. With usually reduce yield considerably. careful attention to details, stake- The staking system used should less culture can yield up to about rely on local materials. The stakes 10 metric tons per hectare, about should be about 2 meters tall. one-third of the yield of staked Taller stakes drastically increase yams under optimum conditions. installation costs, are difficult to manage, and may not increase Pests and Diseases yield sufficiently to justify their cost. Individual stakes are pre- Dioscorea alata foliage is usual- ferred over systems of strings or ly free from serious insect pests. wires, which encourage the vines Susceptibility is closely related to to become tangled. variety. The work of leaf-cutting One of the best staking systems insects (fig. 10) detracts from the was developed in Trinidad (SO), appearance of vines but does little Twelve-foot teak poles are placed damage. Mites, mealybugs, scale in holes every 30 to 50 feet in a insects, and caterpillars are some- row and supported by guy wires. times seen. The best review of in- The poles are strung with 12-gage sect pests is Coursey's (9), galvanized wire 6 to 8 feet off the On the other hand, various spe- ground. Two rows of yams are cies of beetles can severely damage planted, one on each side. When the tubers in the ground or after the vines are sufficiently devel- harvest. The worst of these, oped they are tied with strings, Heteroligus meles (Billb.), the 22 AGRICULTURE HANDBOOK 495, U.S. DEFT. OF AGRICULTURE

PN-444B FIGURE 9.—D. alata growing without stakes. greater yam beetle, passes through lonema bradys (Steiner and Le its mating and reproduction cycle Hew) Andrassy, Meloidogyne in swamp ground but migrates to spp., and Pratylenchus spp. Dam- yam plantings for feeding. In age occurs chiefly to the tubers, Puerto Rico white grubs, Lach- in the form of superficial lesions nosterma spp. and Diaprepes abre- that permit the entrance of fungi. viatus, often cause extensive dam- Where yams are grown in sandy age to the tubers. These beetles soils, nematodes constitute a ser- have been controlled by the appli- ious problem and control is diffi- cation of insecticides to the soil, a cult. Practical control includes the risky practice. In most parts of the use of clean seed materials and world yams are not treated in any planting in heavy soils, where way to protect them from insect nematodes might not occur. In pests. some cases hot-water treatment A serious plague limiting yams kills the nematodes in the tissue. in some areas is the nematode. Al- Extended cool treatment in a re- though these small organisms have frigerator is also useful. Nema- been chiefly associated with Afri- todes are often present in harvest- can species, they can also be very ed yams and may continue to live detrimental to D. alata. The spe- and multiply in stored tubers (29). cies attacking yams are Scutel- They are suspected to be associated TROPICAL YAMS AND THEIR POTENTIAL, PART 3 23

PN-4446 FIGURE 10.—Insect damage and angular leaf spots on leaves of a particularly sensitive D. alata variety. (Two-fifths actual size.) with dry-rot disease, which affects more study because they constitute tubers in the ground or in storage, perhaps the chief risk to the ex- The pests attacking yam merit pansion of yam production in new 24 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE areas, and in most areas, rotting fections generally occur late in the and loss of quality are inevitable growing season, when the yams results of infestation. Since the are switching from the production biology of some of the species is of vegetation to the production of complex, no easy solutions can be tubers. At this stage, aided by expected. rains, the disease can spread rapid- The most serious foliage disease ly, affecting much of the foliage. of the wing-stemmed yam is leaf The vines wilt in severe cases. spot (fig. 11). The disease appears The most effective control of as very small brown or blackish leaf spot is the use of resistant spots on the leaves and stem. The varieties. Many have been found, spots enlarge, the uninfected leaf but most of these do not have de- tissue tends to yellow, and even- sirable agronomic characters. Se- tually the leaves die, but may hang lection of varieties from a broader on the vine for long periods. Pre- base of germ plasm appears mature dieback of leaves and stems promising, however. When accept- drastically reduces yield. The dis- able resistant varieties are not ease is generally attributed to a available, the disease can be con- species of Colletotrichum, but trolled by the use of copper-based other fungi might be involved, per- fungicide. In addition, zineb and haps in secondary roles. ferbam sprays at 10-day intervals Although leaf spot can be found have been reported to be useful. on the leaves of almost all D. alata If rains are frequent, control is varieties at all times, severe in- seldom effective, and repeated ap-

PN-4447 FIGURE 11.—Leaf-spot disease of D. alata leaves. (Two-fifths actual size.) TROPICAL YAMS AND THEIR POTENTIAL, PART 3 25

PN-4448 FIGURE 12.—Mosaic disease of D. alata leaves. plications become expensive. have been attributed to virus, but Therefore, good varieties of D. the range in type and intensity of alata susceptible to leaf spot are the symptoms suggests that more often grown only in areas fairly than one virus is involved. In a few free of the disease or where cases the presence of a virus has weather conditions are not con- been verified. ducive to its full development. The virus diseases of D. alata In addition to leaf spot, mosaic have not been well studied, but (fig. 12) diseases of the leaves certain generalizations based on often cause severe losses in plant- experience can be made. Once a ings of the wing-stemmed yam. virus disease occurs in a planting, Leaf symptoms include dwarfing, it tends to extend itself rapidly, narrowing, crinkling, and other and new plantings tend to be more distortions of shape; mottling; and diseased than previous plantings. vein banding. Such symptoms The tubers of heavily infected vary not only among plants of a plants are smaller than those of single clone but also among the lightly or apparently uninfected branches and the leaves of a single plants. When planted, they pro- plant. In most cases the symptoms duce heavily infected plants. Tu- 26 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE bers from symptomless plants may ciated with dry rot before and be symptomless but nevertheless after storage. The most important virus infected. Occasionally, symp- organisms in the rotted tissue are tomless plants grow from the Rhizoctonia solani and Fusarium tubers of lightly infected plants, oxysporum. From other rots the and thus selection of seed stock organisms Pénicillium oxalicum with low virus symptomolgy is pos- and Aspergillus nig er have been sible. isolated. A common treatment to Regular rogueing appears to be avoid rot is to dust the tuber, es- the best protection against mosaic pecially its cut surfaces, with wood disease. Treatment of infected ashes or other alkaline substances. tubers by hot air over long periods Avoiding damage at harvest is the has also rid them of disease. In best preventative measure. yams grown for medicinal pur- A brown spot found within the poses, similar symptoms have been living tuber of D, alata affects shown to be carried by aphids. food and keeping quality (10), The Control of insect pests appears to brown nodules are concentrated be an effective way of reducing near the crown end of the tuber. spread of the disease. Plants grown from tubers with Rotting of tubers in storage is this condition show virus symp- common and accounts for most toms of the foliage, and bundles of losses after harvest. Rotting is filamentous particles can be seen often associated with wounds in- in the tissues with the electron flicted at harvest and can be re- microscope. The virus appears to duced by careful harvest. A curing be of the class found in cacao swol- period of about 1 week in warm, len shoot and may thus be trans- dry conditions leads to the forma- mitted by mealybugs as well as tion of a cap of dry cells that pro- other insect vectors. tect wounds against fungal Because the virus diseases of entrance. Afterwards, a cool tem- yams are not well understood, in- perature and moderate humidity troduction of varieties from one are required for maximum storage region to another is hazardous, life. Rotting is also associated with even though it appears that virus insect and nematode lesions and is universal in its culture. Unless with a history of poor drainage needed for study purposes, virus- around the tubers. infected plants should be rooted Many organisms are associated out of plantings as soon as detected. with storage rots. Soft rot is caused Large tubers from healthy mother principally by Botryodiplodia theo- plants should be selected as seed bromea. Indoleacetic acid solutions material. In this fashion it should applied to the yam tubers before be possible to reduce virus occur- storage are useful in suppressing rence in commercial fields to a the disease. Nematodes are asso- tolerable level. TROPICAL YAMS AND THEIR POTENTIAL, PART 3 27 Harvesting to remove vines and staking sys- Near the end of the growing sea- tems first. Needless to say, me- son, which generally coincides chanical harvesting is facilitated with the beginning of the dry sea- by planning spacing in advance son and shorter days and cooler and by using ridges of soil from temperatures, vegetative growth which tubers can easily be re- ceases in most varieties of wing- moved. stemmed yams. Flowers and aerial Poor care, bad weather, and dis- tubers are sometimes produced. At ease can reduce yields to zero. On this time the underground tubers the other hand, favorable combi- are also produced. These develop nations of variety, cultural tech- rapidly until the foliage dies back. nique, and weather can produce It is not known what triggers sea- exceptional yields. Unfortunately, sonal dieback, but once begun, high rather than average yield fig- fungi often kill the foliage rapidly. ures tend to be made available, so This terminal fungal infection of it is difficult to know what typical the leaves, not invariable, should yields are. not be confused with premature Exceptional weights of indi- dieback associated with Colleto- vidual tubers have been recorded. trichum. Tuber growth cannot con- Haynes and Coursey (16) ^ in a tinue after the foliage has dried study of giant yam production in up, but maturation of the tissue Trinidad, mention tubers up to 81 may continue, and in particular kilograms. This weight compares the cork cap over the tuber will favorably to that of the largest continue to develop. Therefore, it yams produced in Papua New may be advantageous to harvest Guinea. These giant yams are pro- several weeks after foliage die- duced by giving them individual back. attention not compatible with com- Tubers are frequently dug with mercial production. special sticks that look somewhat An experiment station's yield like large spoons. Conventional from small plots sometimes reaches spading forks and shovels are 40 to 50 metric tons per hectare. more useful. Normal yields under very good Most varieties of Z). alata pro- agronomic conditions range from duce large, deep tubers that must 15 to 20 metric tons. In Barbados be dug with some care. On the yields obtained without stakes other hand, a few varieties produce range up to about 18 metric tons compact, often multiple tubers per hectare under the very best near the surface of the soil. Con- conditions (13). In Fiji yields ventional machinery for the har- range from 4 to 25 metric tons per vest of potatoes and sweetpotatoes hectare, depending on staking and can be adapted to such varieties. size of seed piece (2), Coursey In these cases it may be necessary (9) has summarized typical yields 28 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE as follows: Southeast Asia, 12 to color with the oxidation of poly- 25 metric tons; West Indies, 20 to phenolic substances and may 30 metric tons. These estimates ap- become gray or ugly brown. Pro- pear too high. tection of the surface from oxygen In summary, very excellent by cellophane or any impermeable yields can be obtained when cul- membrane decreases oxidation tural conditions are appropriate, tendency. Polyphenolic oxidation and exceptional varieties of yams is associated with off-flavors, in- can outyield most other farina- cluding bitterness, and should be ceous crops. avoided. There is some suggestion that spots that oxidize readily are CULINARY associated with virus infection. CHARACTERISTICS The cut surface may sting human Many varieties of the wing- flesh, especially tender areas such stemmed yam present problems in as the inside of the wrist. This the kitchen. First, one tuber is sting, associated with calcium usually too large for a single meal, oxalate crystals, usually disap- so a part is used and a part saved. pears in about 10 minutes. Since the cut surface is often The flesh of the yam varies in dirtied or dries and cracks, using texture. Coarse texture, which a tuber for several meals increases looks a little like soured milk, is wastage. Second, many tubers are caused by the association of vessels irregular in shape and are there- in thick bundles. In some varieties fore hard to peel. Third, the bark the texture is very fine, and such is sometimes thick and corky and bundles cannot be seen. difficult to remove; this character- The flesh also varies in color. istic is an advantage, however, for Yellow, unusual but not unknown it protects the tuber against in D, alata, is sometimes due to the wounds. Cleaning and preparing presence of still unidentified such yams for cooking can be an carotenoids. Yellow color is dis- arduous chore best avoided by use tributed evenly and is usually of better varieties. Very few vari- maintained during cooking. When eties of D, alata produce small tu- associated with polyphenolic oxi- bers that can be boiled or baked dation, however, the resulting without peeling. On the other hand, brownish gray is very unappetiz- many good varieties yield tubers ing. The purple color of antho- of regular shape that are con- cyanin is usually not distributed venient to use. These, however, are uniformally through the tissue, poorly distributed and not known and except to the eyes of those who in much of the yam-growing world. prefer such yams, the appearance When the tuber is cut open, is not appealing. The flesh of the sticky proteinaceous gums exude. best varieties is very white, The cut surface begins to change smooth, free of the appearance of TROPICAL YAMS AND THEIR POTENTIAL, PART 3 29 texture, and low in oxidizing tend- in products superior to similar po- ency. tato products. Varieties especially It is difficult to distinguish the suited for fried products have been taste of D, alata tubers from those selected (23). of other edible species. Neverthe- The preparation of instant yam less, considerable variation exists, flakes has been investigated in and some yams are recognized as Puerto Rico, Trinidad, and Bar- tasting better than others. In areas bados (l-i)- The processes are where the fine-textured, white- washing, lye peeling, trimming, fleshed varieties are not seen, slicing, cooking, maceration or coarse texture and strong flavor finer cutting, mixing with water, are very much appreciated. These drum drying, and packaging. The coarse yams are not very good for flakes store well, are easily pre- processing. Among persons just pared for the table, and have good learning about yams, the palate is flavor. Quality is strongly influ- rapidly accustomed to the fine, enced by variety, but rather coarse white types. It is a general belief yams with a tendency to oxidize that such types were selected and may yield very acceptable instant ennobled by early man. Good yams flakes if an antioxidant is used, be- for eating have rich characteristic ginning with the slicing operation. flavors that are well accepted by The costs of processing would almost all persons. They may be probably make exportation, but slightly sweet, especially after long not domestic consumption, profit- storage. able. Flours have been made from COOKING AND yams by simple processes by many PROCESSING peoples. Flours made from dried By far the greatest use of the yam must be distinguished from wing-stemmed yam is as a boiled the use of fresh yam as a substitute vegetable. The tuber is served in for flour in baking. Whereas the pieces or mashed or in soups, former process permits storage of where, finely divided, it serves as the yam, the latter may be con- a thickener. Tubers are sometimes sidered as just another way to use roasted, or the mashed yam is fried fresh yam. The usual technique as a cake. In these forms yams are has been to peel and slice the tuber, often eaten with an oil or a sauce. dry the slices in the sun, and then A special potential for yam cook- grind the slices into meal. If mois- ery is french fries and chips. The ture content is lowered sufficient- best varieties for these purposes ly, dried powdered yam can be have fine, dense flesh, low poly- stored for a long time. Such flour phenolic oxidation, and no antho- can be cooked to yield a thick soup cyanin. Frying at 190° C in corn or mashed yam or can be substi- oil or corn oil-lard mixtures results tuted for all or part of other flours 30 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE in baking. Flours prepared by such wheat flour mixtures needs study. techniques are often gray to black, Since the protein content of the from mold growth and polypheno- two simple flours is about the lic oxidation, and are not favored same, the total protein may not except when fresh yams are not change by combining them. Never- available. theless, the relatively high lysine With modern techniques a higher content of yam flour and the com- grade of flour can be produced. In plementary content of methionine an ideal process the yams would in wheat flour should result in a be washed, lye-peeled, trimmed, mixture of high nutritional value. cut into pieces, dried with hot air, The realistic use of yam flour ground into a fine flour, sieved to calls for a cookery based only on appropriate size, and then pack- it, not on a mixture of flours. Such aged. Laboratory experience shows a cookery needs to be developed that the flour potential of D, alata not only for homemade flour but varieties differs. Pigments in the also for commercially produced flesh or a tendency towards poly- and marketed flours. The potential phenolic oxidation results in off- of yam flour in the processed-food colored flours or unappetizing industry merits investigation. If odors or flavors. The best yams for yam flour is to become widely used, flour have fine, dense, perfectly techniques to produce yams more white flesh, the same varieties pre- cheaply, probably through mech- ferred for boiling and frying. anization, need to be developed. In contrast to flours of other COMPOSITION farinaceous roots and tubers, yam The dry weight of the edible flours can be substituted for wheat portion of the yam is about 15 to flours at a higher percentage, per- 35 percent of the fresh weight. haps as much as 60 percent. The High dry weights are associated success of this substitution pos- with fine structure, dense feel, and sibly is related to the high protein high quality. Maximum dry content of the tubers or perhaps weights are achieved near the end to its mucilagenous character. of the dry season. High density is However, as the percentage of yam a varietal character that is not flour increases, certain problems changed much by environmental are encountered, among them, poor influences. crumb quality and poor rising of The composition of five varieties the dough. Taste panels have of J9. alata was reviewed by Wilde- judged products made from yam man (table 3). Water is the chief flour, including pancakes, cup- ingredient, of course, followed by cakes, conventional bread, and starch. The lowest starch figure, hard bread rolls, acceptable to 15 percent, represents the content delicious. of an immature tuber, and few The nutritional value of yam- varieties would assay less. As a TROPICAL YAMS AND THEIR POTENTIAL, PART 3 31

TABLE 3.—Tuber composition of five D. alata varieties^ [Percent of whole-tuber weight]

Variety- Substance

Water 77.60 65.95 67.12 72.42 76.15 Starch 15.60 19.50 23.87 17.71 19.34 Mucilage or protein 2.10 5.68 1.36 6.87 1.31 Cellulose 1.10 7.50 3.15 2.53 .65 Sugar (2) 1.19 .50 1.00 (2) Minerals 96 (2) {') (^) .73 Fats 23 .18 .11 .04 .16 Not determined 2.41 .... 3.89 .43 1.66 1 From Wildeman (31), - Not given.

group the varieties contain less Starch prepared in this fashion starch than usually reported; it is contains few other materials, but not uncommon to find tubers with can be expected to have 13 to 18 28 percent starch. Little carbohy- percent moisture and 1 to 4 per- drate occurs as sugars. The amount cent protein. of cellulose is low; in fact, very The particles of D. alata starches few tubers contain enough cellu- are mostly large and pyramidal in lose to present a fibrous appear- shape. One point of the pyramid ance. Fats are negligible. Vitamin represents the small grain on the C in useful quantities has been re- side of which successive layers of ported. Minerals are not important starch are deposited. The mature components. grains vary from 1 to 7 micro- Starches, which make up 13 to meters in diameter, although much 32 percent of the tuber, are rela- larger grains have been reported, tively difficult to extract from D, making the grain considerably alata. During peeling and cutting, larger than that of cassava, a the equipment is likely to become major source of industrial starch. clogged with released water-sol- The large size might make D. alata uble gums, which must be dis- starch difficult to digest, especially persed with large amounts of when uncooked. water. Once the gums are dis- The amylose content of D. alata persed the larger particles of starches ranges from 19 to 20 per- starch settle to the bottom of the cent, somewhat higher than that container in about 24 hours. The of Z). esculenta, but not significant- starch must then be resuspended ly different from that of cassava. in water two or three times and The viscosity of D, alata starches allowed to settle. The resulting has been reported to be moderate, starch cake can then be dried. although there is much variation 32 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE among varieties. Probably none protein and as much as 15 percent approximates the viscosity of the has been recorded. starches of the African species D, The protein of D. alata is not rotundata. The starch gel is well balanced (table 4). However, opaque. usually only the sulfur-bearing The properties of D. alata amino acids methionine and starches need more study in refer- cystine are particularly short. The ence to industrial use. Sufficient percentage of these desirable differences have been found among amino acids has not been carefully varieties to suggest that selection estimated but varies from 20 to 50. for particular starch types might It should be possible to select vari- be possible. Yam starches would eties with fairly balanced protein, probably be useful in a number of for strong varietal differences industrial processes, but their cost exist, not only in protein content is high compared to the cost of but also in sulfur-bearing amino corn and cassava starches. acid content (table 5). In the ab- D, alata tubers contain surpris- sence of selected varieties, pro- ingly large amounts of protein teins high in methionine should (2JÍ). About 85 to 95 percent of supplement staple yam diets. the nitrogen can be recovered as Lysine is often present in greater- amino acids. Varieties that contain than-normal quantities in D. alata only 6 percent protein, a low fig- tubers. ure for yams, thus contain three to The red and purple pigments of four times more protein than their D. alata are anthocyanins of rival staple, cassava. Moreover, cyanidin. Purple forms occur in many varieties contain 8 percent the parenchyma of the tuber tissue,

TABLE 4.—Essential amino acid content of D. alata cultivars compared with FAO reference protein [Grams amino acid per 100 grams protein]

Cultivar Amino ^^^ _ __• j reference ,^, ., , ^White 'Smooth ^"** proteinx 'blondo' Lisbon' 'Forastero' 'Barbados' Statia'

Valine 4.2 4.9 4.8 5.2 4.7 4.8 Isoleucine 4.2 4.2 4.2 4.7 4.0 4.1 Leucine 4.8 7.7 7.8 7.4 8.1 8.1 Threonine 2.8 4.2 3.9 7.1 3.9 4.2 Methionine 2.2 1.6 1.5 1.6 1.5 1.7 Cystine 2.0 .4 .4 .2 .4 .3 Phenylalanine •• 2.8 5.8 5.8 5.9 6.1 6.1 Tyrosine 2.8 2.8 2.7 2.2 3.1 3.0 Lysine 4.2 5.1 5.0 4.8 5.0 5.4 1 FAO Nutritional Studies 24 (12), TABLE 5.—Protein, total amino acids, and sulfur-hearing amino acids in D. alata cultivars I ' Total Sulfur-bearing amino acids^ Cultivar proteini ^^j^^j Total Methionine Cystine (g/lOOg) (g/lOOg) (g/lOOg) g/lOOg Percent g/lOOg Percent 'Ashmore' 7.21 5.60 0.10 0.09 1.57 0.01 0.16 H ^Barbados' 8.12 6.36 .12 .09 1.47 .03 .41 O 'Bottleneck Lisbon' 7.25 5.65 .11 .09 1.54 .02 .40 ^ 'Brazo Fuerte' 6.28 4.69 .23 .16 1.57 .07 .68 > 'Corazón' 9.16 6.03 .12 .09 1.55 .03 .43 r 'Cuello Largo' 11.22 8.13 .19 .14 1.70 .05 .57 K: > 'De Palo' 8.88 7.34 .14 .12 1.63 .02 .27 'Espada' 10.82 7.11 .12 .11 1.59 .01 .17 c/^ 'Farm Lisbon' 8.25 6.43 .12 .10 1.51 .02 .36 > 'Feo' 8.16 6.68 .16 .12 1.86 .04 .54 C 'Florido' 10.47 8.22 .17 .13 1.58 .04 .43 H .09 1.60 .01 .17 ffi 'Forastero' 7.25 5.64 .10 W 'Gordito' 8.47 6.61 .13 .11 1.67 .02 .29 B 'Hawaii Branching' 8.28 6.07 .11 .09 1.56 .02 .28 ^ 'Hunte' 9.22 6.96 .12 .11 1.62 .01 .21 o 'Macoris' 7.68 6.08 .12 .09 1.54 .03 .15 H 2: 'Morado' 8.04 5.99 .11 .09 1.48 .02 .28 H 'Oriental' 7.72 6.32 .13 .11 1.72 .02 .27 > 'Prolific' 9.44 7.38 .14 .11 1.50 .03 .35 r 'Purple Lisbon' 7.66 6.16 .10 .09 1.50 .02 .36 ^ 'Pyramid' 9.41 6.03 .13 .10 1.61 .03 .48 > 'Seal Top 7.84 5.73 .12 .09 1.57 .03 .58 H 'Smooth Statia' 6.56 5.23 .11 .09 1.67 .02 .29 w 'Sweet' 6.84 4.81 .12 .09 1.90 .03 .53 'Vino Blanco' 8.06 6.50 .11 .09 1.45 .02 .34 'Yellow Lisbon' 7.91 5.99 .13 .10 1.60 .03 .42 1 Percent of whole-tuber weight, dry basis. - Grams per 100 grams protein, dry weight. ^ Grams per 100 grams proteinn (dry U> weight) or percent of total amino acids, as indicated. 00 00

> o » O G.

G W M X > ■z. O to O O

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in ö n H O > O » O G PN-4449 r FIGURE 13.—The cut tubers on the left were cured in hot, dry air. The tubers on the right were uncured and are beginnitig to rot. H G (One-half actual size.) ta M TROPICAL YAMS AND THEIR POTENTIAL, PART 3 35 whereas red forms occur chiefly soon as possible and should not be in the foliage and in the cortex of stored. the tuber. The occurrence of antho- The first week after the harvest cyanins is somewhat related to is crucial to the storage life of taxonomic grouping of the vari- tubers. Curing under humid condi- eties. More advanced and highly tions invariably results in rapid selected varieties contain little or fungal growth. Curing by hot, dry no anthocyanin. When foliage is air permits the development of a free of anthocyanins, so are the layer of dead cells that protects tubers. Varieties with high antho- against fungal infection (fig. 13). cyanin content are often prone to Cut surfaces do not develop a new polyphenolic oxidation, and their meristematic layer, and a wound tubers are usually much branched periderm is not formed. The dry and agronomically inferior. cells are not suberized. If the dry- Although the yellow pigments of air treatment is too long, excessive D, alata have not been well studied, moisture loss leads to cracking, there is no reason to believe they permitting severe fungal infection. are different from those in other After curing, moderate humidities species of yams, which contain and cool temperatures (15°- principally xanthophylls and their 17°C) can give a storage life of esters, although the vitamin A con- a year or more. tent is sometimes high enough to Storage practices vary. Storage be of nutritional significance. in the ground (delayed harvest) is Good yellow-fleshed varieties of D. common, but insect damage in- alata would be desirable for vita- creases, the risk of loss from rats, min content. pigs, and humans must be con- fronted, and early rains may im- STORAGE pede later harvest or lead to Some say wing-stemmed yams premature sprouting. Neverthe- store well; others say they have less, the quality of tubers stored poor storage characteristics. These in the ground is as good as that of conflicting points of view probably tubers harvested and stored else- reflect experiences with different where. varieties or with different storage Tubers are often stored by piling methods. It is well recognized that them in pits, tying them to walls, injured tubers can be readily in- or placing them on shelves in yam fected with fungi and that fungus barns. They can keep well under diseases can destroy entire tubers, these conditions. Free circulation often in a short time. Further- of air around the tubers helps more, once started in a tuber, fungi avoid rot. Under these conditions are almost impossible to eradicate. tubers begin to germinate when Therefore, tubers that are dam- their normal season of regrowth aged at harvest should be used as occurs. Sprouting rapidly di~ 36 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE

minishes the starch reserves of the jury (ii). When tubers are re- tuber and reduces flavor. turned to normal temperatures, The storage of yams in piles is death of tissue, internal break- common but should be discouraged. down, foul odor, and decomposition In piles, rotting can spread rapid- follow rapidly. The cause of the ly, and since it is hidden, it cannot disorder has not been determined. be controlled. Leaving the tubers A problem in marketing, in in- in the sun for extended periods dustry, in the kitchen, and in the severely reduces storage life. design of scientific experiments is Storage life can be increased by the great variability of D. alata chemicals. The most effective is tubers, not only among varieties of MENA, the methyl ester of alpha- the species but also within the naphthaleneaceticacid (7).Tubers same variety. Undoubtedly, this are packed in papers treated with variation has led different investi- the chemical at the rate of 100 gators to different conclusions. milliliters reagent in 100 milliliters The problem of variation is asso- acetone applied to 250 grams of ciated with many factors, such as packing paper. Although rate of source and size of the seed piece water loss is not affected to any and virus disease in the tuber. In great extent, tubers remain in a few cases clonal selection has re- edible conditions from 114 to 2 duced variation, probably by elimi- months longer than normal. The nation of heavily diseased tubers. tubers develop a warty surface The best assurance of high uni- after 6 months. Results are about formity, however, is growing the same if the tubers are treated plants under optimum conditions at harvesttime or 2 or 3 months in loose soil, where they can reach thereafter. The cost of the treat- their full potential in size and ment, however, may limit its use. shape. In addition, preliminary small- scale experiments have shown that POTENTIAL USES dipping tubers in weak solutions The wing-stemmed yam has a of gibberellic acid can delay germi- bright future throughout the Trop- nation 2 to 3 months. Experiments ics wherever subsistence agricul- are needed with this hormone, ture is practiced and rainfall is which acts powerfully even at low sufficient. Because of the excellent concentration, to see if some type flavors of yams, they will probably of treatment similar to that with always be more popular than other MENA can be devised, in which roots and tubers. Nevertheless, be- the active chemical is constantly cause they need special attention, available to the tuber. especially staking, they may not Storage of wing-stemmed yams complete well with cassava, sweet- at 10° to 12° C or less results in a potatoes, and aroids. The wing- characteristic low-temperature in- stemmed yams may thus play a TROPICAL YAMS AND THEIR POTENTIAL, PART 3 37 progressively narrower role in these, the most important is flour, subsistence agrictulture. On the but neither the present processing other hand, the size of the role may techniques nor the varieties of D. increase. Better varieties, now alata now available can be recom- being selected from a worldwide mended. With respect to the vari- collection, will be distributed eties, the chief problems are ir- throughout the Tropics and will re- regular tuber shape, a difficulty in place many inferior types now in preparation; excessive poly pheno- use. In some areas, these may in- lic oxidation; and drying. The first crease the popularity of yams. two problems can be resolved, per- A successful future for Z). alata haps, through the use of better depends on extending the yam sea- varieties. The problem of drying son or avoiding waste of the too can be attacked through the use of abundant, too short harvest. The inexpensive solar dryers. To com- possibility of planting and harvest- plement the dryer, inexpensive ing all year around does not appear household devices are needed to good because seasonal response is peel and chip the tuber. These de- difficult to change and a broad vices will have to cope with the enough spectrum of varieties is not large quantities of gums released available for staggered year-long in the operation. planting. Year-round production Finally, new cooking techniques in some locations near the Equator and recipes are needed for new with well-distributed rainfall and stored yams and for yam flour. might be possible, but in most In particular, bread recipes using cases, yams will mature during the yam flour in place of imported or dry season and will be overabun- purchased flour should be devel- dant. oped. Improved storage of tubers is Overall, then, the future of the probably the best solution for ex- wing-stemmed yam in subsistence tending availability of D, alata in agriculture appears good. The subsistence agriculture. Simple distribution of varieties and techniques of storage in pits and knowledge, the development of yam barns are already known, but new techniques for preparing information about their relative flour, and new recipes will help merits is not readily available. It assure that future. However, the is hardly likely that research will greatest future for D. alata should improve techniques for the small lie not in subsistence but in modern farmer, but the economies of com- agriculture, since progress for mon storage techniques do need to large numbers of people requires be compared and evaluated. that more food be produced more Extension of yam availability economically. This has heretofore can also come about through wider been accomplished by the use of use of processed products. Of machinery powered by fossil fuel. 38 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE The energy relationships of mech- There is no evidence that the anized agriculture and the long- march of this crop will be detained, term consequences need to be but its victory will be unfortunate, restudied. Nevertheless, it is because the food value of cassava tempting to believe that long-term does not reach that of the yam. energy problems will be solved, al- In other areas aroids and sweet- lowing the continuance and exten- potatoes do and will continue to sion of mechanization. compete with the yams. To com- As previously mentioned, most pete better, select varieties of the operations in the production of wing-stemmed yam are needed, yams can be mechanized. In order and based on these varieties, mod- to benefit from mechanization, the ern agronomical techniques must scale of farming must be increased. be developed. When yams are grown on a large enough scale, two problems not LITERATURE CITED now meriting attention can be studied: the production of seed (1) ALEXANDER, J., and COURSEY, yams in special nurseries and the D.G. 1969. The origins of yam cultiva- mechanization of staking systems. tion. In Veko, P. J., and Or, as an alternative, it may be Dimebedy, G. W., (eds.), feasible to grow unstaked yams The Domestication and Ex- economically when all operations ploitation of Plants and Animals, pp. 405-425. are mechanized. Duckworth, London. In considering the future of the (2) BERWICK, J., CHAND, D., and wing-stemmed yam, its competi- RALIBOKALA, J. tors must be taken into account. 1972. Yam {Dioscorea alata) planting rate, staking, va- The African yams, with their more riety and palatability stringent growth requirements trials. Fiji Agrie. J. 34: and adaptation to certain climates, 44-50. will probably not compete success- (3) BURKILL, I. H. fully with D. alata except in West 1917. A report on races of the greater or ten month yam, Africa and a few other locations. Dioscorea alata, cultivated On the other hand, new varieties of in the botanic gardens, D. esculenta recently collected Singapore. Straits Settle- show great promise and may re- ments Garden's Bull. 9 (11- place D. alata in some places. 12) : 371-396. (4) The real competition for the 1951. The rise and decline of the wing-stemmed yam comes not greater yam in the service from other yam species but from of man. Adv. Sei. 7(28) : other roots and tubers. Chief of 443-448. these is cassava, Manihot esculenta (5) 1951. Dioscoreacea. Flora Males- Cranz, which has already displaced iana. Ser. I, vol. 43, pp. yams in many parts of the Tropics. 293-335. TROPICAL YAMS AND THEIR POTENTIAL, PART 3 39

(6) CAMPBELL, J. S., CHUKWUEKE, V. (15) GOODING, H. J. 0., TERIBA, F. A., and HO-A-SHU, 1960. West India "Dioscorea H. V. S. alata'' cultivars. Trop. 1962. Some physiological investi- Agrie. (Trinidad) 37(1): gations into the White Lis- 11-30. bon yam {Dioscorea alata (16) HAYNES, P. H., and COURSEY, L.). I. The breakage of the D. G. rest period in tubers by 1969. Gigantism in the yam. chemical means. Emp. J. Trop. Sei. 11(2) : 93-96. Exp. Agrie. 30(118): 108- (17) 114. 1968. Yams. The Texaco Food (7) Crops Demonstration Farm 1962. Some physiological experi- Bull. 3: 1-9. ments with the White Lis- (18) LAWTON, J. R. S., and LAWTON, bon yam (Dioscorea alata JUNE R. L.) in Trinidad. III. The ef- 1967. The morphology of the fect of chemicals on stor- dormant embryo and young age. Emp. J. Exp. Agrie. seedling of five species of 30(120): 335-344. Dioscorea from Nigeria. (8) CANDOLLE, A. DE. Proc. Linn. Soc. Lond. 1886. Origin of cultivated plants. 178(2): 153-159. 2d ed. Reprinted by Hafner, (19) LEA, D. A. M. New York, 1967. 1966. Yam growing in the Map- (9) COURSEY, D. G. rik area. Papua New 1967. Yams. An account of the Guinea Agrie. J. 18(1) : 5- nature, origins, cultivation 15. and utilisation of the use- (20) MARTIN, F. W. ful members of the Dio- 1966. Sex ratio and sex determi- scoreaceae. 230 pp. Long- nation in Dioscorea. J. mans, London. Hered. 57: 95-99. (10) (21) and ORTíZ, SONIA. 1967. Internal brown spot—a 1963. Chromosome numbers and condition in yams in Bar- behavior in some species of bados. J. Agrie. Sbc. Trini- Dioscorea. Cytologia 28 dad Tobago 67: 473-482. (1) : 96-101. (11) (22) and RHODES, A. M. 1968. Low temperature injury in 1973. Correlations among great- yams. J. Food Technol. 3: er yam (Dioscorea alata 143-150. L.) cultivars. Trop. Agrie. (12) FOOD AND AGRICULTURE ORGANI- 50(3): 183-192. ZATION. (23) and RUBERTÉ, RUTH. 1970. FAO Nutr. Stud. 24. 1972. Yam (Dioscorea spp.) for (13) GOODING, E. G. B. production of chips and 1971. Effects of fertilizing and french fries. J. Agrie. other factors on yams in Univ. P.R. 56(3) : 228-234. Barbados. Exp. Agrie. 7: (24) and THOMPSON, A. E. 315-319. 1971. Crude protein content of (14) yams. Hortic. Sei. 6(6): 1973. The production of instant 545-546. yam in Barbados. Part 1. (25) OCHSE, J. J., and BAKHUIZEN VAN Process development. Trop. DEN BRINK, R. C. Sei. 14(4) : 323-333. 1931. Vegetables of the Dutch 40 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE

East Indies. Department 1956. Polyploidy in Dioscorea, of Agriculture, Industry, Genética 28: 112-120. and Commerce of the (29) THOMPSON, A. K., BEEN, B. 0., Netherlands East Indies, and PERKINS, CYNTHIA. Buitenzorg. 1973. Nematodes in stored yams. (26) RHODES, A. M., and MARTIN, Exp. Agrie. 9: 281-286. F. W. (30) WHOLEY, D. W., and HAYNES, 1972. Multivariate studies of va- P.H. riations in yams (Diosco- 1971. A yam staking system for rea alata L.). J. Am. Soc. Trinidad. World Crops, Hortic. Sei. 97(5): 685- May/June, pp. 123-126. 688. (31) WiLDEMAN, E. DE. 1938. Dioscorea alimentares et (27) ROXBURGH, WILLIAM. toxiques (morphologie et 1832. Dioscorea. Flora Indica, or biologie). Espèces et vari- Descriptions of Indian étés congolaises. Inst. R. Plants, vol. 3, pp. 797-806. Colon. Belge, Sect. Sei. Nat. Ed. by W. Carey. Seram- Med., Collect. Octavo, pore. Mem., vol. 7, pt. 2, 262 pp. (28) SHARMA, A. K., and DEEPESH, (especially pp. 81-109,169- N. DE. 188).

lir U.S. GOVERNMENT PRINTING OFFICE : 1976 O-209-183