Cassava

Uses

Approximately 65% of cassava production is used for human consumption, fresh or processed. The short postharvest period requires rapid use, whether for direct consumption, marketing, or processing.

In Africa, roots are prepared for human consumption in various ways: dried parts are ground into flour, grated, fermented, and roasted to produce gari and a sticky dough or porridge called fufu. <span lang="pt">Farinha de mandioca</span>, roasted flour, is a traditional and popular food in Brazil. Additional processed products are tapioca, fried chips, sun-dried flakes, and beer. A significant amount of cassava is used for animal feed and starch production. Bitter cultivars are most often used for starch and fuel alcohol.

The main value of cassava lies in its high calorie content; fresh roots contain about 35-40% dry matter, of which 90% is carbohydrates. Where edaphoclimatic conditions are limited, for example in sub-Saharan Africa and northeastern Brazil, cassava is often the main source of carbohydrates. On a fresh-weight basis, cassava provides about 150 kcal/100 g versus 115 kcal/100 g for sweet potato, and per area it competes with cereal crops in calories and labor efficiency. Cassava is a good source of vitamin C, contained at 30-35 mg/100 g fresh weight, and usually has low fiber (1.4%) and fat (0.3%).

The presence of cyanogenic glucosides in all tissues of the plant is a serious concern. The roots contain the enzyme linamarase, which hydrolyzes glucosides to prussic acid. The glucosides are water-soluble, and the enzyme is inactivated at temperatures above 50 °C; the main glucoside is linamarin. There is a general but not absolute correlation between glucoside content and root bitterness. A glucoside content above 50 mg/100 g tissue is poisonous; above 100 mg/100 g it is dangerous and can lead to death. Roots of bitter cassava cultivars must be processed before consumption to reduce the content of toxic substances. Detoxification is achieved by soaking the roots and discarding the soak water; glucosides are readily soluble in water. Grinding or milling the tissues and discarding the resulting liquid, as well as fermentation and drying, are other detoxification methods. Glucosides decompose at temperatures above 150 °C. Glucoside content strongly depends on cultivar and growing conditions.

The inedible peel, accounting for about 20% of root weight, is also an undesirable feature, as is the relatively low protein content, only 2-3% on a dry-weight basis. In addition, protein quality leaves much to be desired; the content of sulfur-containing amino acids, particularly methionine, is low. Protein content in young roots is somewhat higher than in mature roots, but this trait is not realized because of the impracticality of harvesting small roots. Cases of kwashiorkor caused by acute protein deficiency are sometimes observed in some population groups for which cassava is the staple food.

In many tropical areas, cassava leaves are used as vegetables. They are usually harvested from sweet plants with low glucoside content. Yields reach 20 t/ha where cassava is grown mainly for leaf harvest. The leaves are a good source of vitamin C, contain sufficient provitamin A, and about 30% protein on a dry-weight basis. Interestingly, in Zaire leaves infected with mosaic virus are preferred, since they are sweeter and more tender than healthy leaves. In addition, virus-infected leaves contain fewer cyanogenic glucosides than uninfected leaves. In South China, leaves are used to feed silkworms.

History and origin

The center of origin of cassava is thought to be somewhere in northern Amazonia in Brazil, from where it spread to adjacent areas thousands of years ago. Southwestern Mexico is also considered a region of origin or diversity. Considerable diversity is observed between cassava forms in southern Mexico and Central America compared with forms in northeastern Brazil.

In the post-Columbian period, cassava was introduced to Africa at the end of the 16th century and to India in the early 19th century. In these places, as well as in Southeast Asia, cassava quickly became an important food.

Cultivation areas

Cassava is an important staple food because of its significant daily calorie contribution for millions of people. Basically all cassava production occurs in developing countries, and much of this production comes from small farms, often on marginal soils. Cassava is very important for the rural poor, since it is a reliable crop even during drought. Drought tolerance and flexible harvest periods make cassava a valuable reserve crop against famine.

The large volume and rapid spoilage require local distribution and consumption, and only a limited volume of fresh product is sent to distant markets. However, because the dried processed product is less bulky and more durable, cassava has a certain importance in international trade.

Although Brazil is the leader in world production (more than 15%), African countries collectively produce more than 47% of world production. African production is mainly used as food, while proportionally more cassava produced in South America and some parts of Asia, namely Thailand, is used for animal feed and industrial products, a significant volume of which is exported.

Average world yield is low, since much production occurs under marginal agricultural conditions in developing countries, where cultivar development, agronomic methods, and pest control methods are limited.

Taxonomy

The genus Manihot (Manihot) is endemic only to the Western Hemisphere and occurs mainly in dry frost-free tropics. Manihot esculenta, a possible allotetraploid, (2n = 36) is not found in the wild.

Botanical description

Cassava is a monoecious dicotyledonous plant, grown mainly for the easily digestible starch contained in its storage roots, which are incorrectly called tubers.

A perennial shrub 1-4 m tall with large deeply palmate leaves, the blade of which has from five to nine lobes. The leaves, located on long petioles, are deciduous and persist for only a few months or less.

The stems show characteristic branching that varies depending on the cultivar. Preference is given to initial erect growth until branches appear because of ease of cultivation; excessive and especially basal branching is undesirable. On old parts of the stems, noticeable leaf scars are visible; wide internodes indicate a rapid rate of growth.

Seed-propagated plants form a prominent taproot. Vegetatively propagated plants form fibrous roots from the base of stem cuttings. Storage roots develop from secondary thickening of previously fibrous adventitious roots. Enlargement occurs first at the proximal end, then progresses to the distal end. Storage roots differ in shape, and although most are cylindrical and tapering, some branch. A short narrowed part with a woody texture connects the storage root to the stem. Storage roots vary from 15 to 100 cm in length and from 3 to 15 cm in width. Depending on age, root weight ranges from several hundred grams to 15 kg. Storage root growth depends on day length and may begin as early as 8 weeks after planting. Usually plants form about 5-10 storage roots. The outer cork-like layer of the periderm is often reddish, varying from dark brown to whitish. The periderm surrounds a thin cortical layer and thick fleshy parenchyma tissues, and cracks often appear on it because of constant expansion. The inner flesh is usually white, although yellow and red shades are observed. In the center of the storage root there is a prominent strand of hard vascular tissues.

Latex is present in the storage root, as well as in all parts of the plant.

Cultivars are often determined by the degree of bitterness, which is related to the toxic cyanogenic glucosides contained in the roots. "Sweet" cultivars contain few glucosides and do not require detoxification before consumption. The amount of glucosides formed in the roots depends on growing conditions, so even sweet cultivars have some bitterness.

Flowering is characteristic of most cultivars and may occur as early as 6 weeks after planting, but some cultivars flower rarely. Flowers at formation reach about 1 cm in diameter and are arranged in loose clusters on terminal panicles at the ends of branches. Flower color varies from greenish-purple to pale greenish-yellow. Female flowers become receptive long before male flowers open, which limits self-pollination. Being outcrossers, Manihot esculenta and wild cassava species show inbreeding depression upon self-pollination. The flowers secrete nectar, and insects carry out cross-pollination. Seed capsules are small and strongly inclined with narrow wings; they dehisce explosively and usually contain three flat hard seeds, which require scarification to improve germination.

Environmental requirements

The best growth of cassava is observed in regions located 15° latitude above and below the equator, where the average temperature is from 25 to 27 °C. However, adequate growth occurs in the temperature range of 16-30 °C and at latitudes up to 30 °C. Growth stops at temperatures below 10 °C, and at temperatures above 35 °C growth decreases; the plant dies from frost. Low elevations are preferred, but if temperature permits, cassava can be grown at elevations up to 2000 m. The plant converts solar energy into carbohydrates very efficiently, so high light intensity is important for high productivity. Cassava is capable of accumulating and storing more carbohydrates per unit area than any of the major cereals and most root or tuber crops.

Moisture requirements are usually met by rainfall, and because of the relatively low monetary value, additional irrigation is infrequent. Plantings are made at the beginning of the rainy season, since sufficient soil moisture is crucial for crop rooting. Cassava has broad adaptation to soil moisture and grows in areas with 500 to 5000 mm of rainfall, but in most major production regions the average is 1000 to 2000 mm. Plants tolerate long periods of drought and lack of crop care, limiting growth by shedding leaves and entering a state of dormancy. After rain, plants quickly produce new leaves.

Sandy or sandy loam soils are preferred. Deep loose soils provide better penetration of developing storage roots; shallow and compacted soils impede growth and root formation. Poorly aerated or waterlogged soils suppress growth and promote root rot.

Although a slightly acid soil environment is optimal, cassava is tolerant of soil pH from 4 to 8; high salinity limits growth. Plants are also tolerant of low soil calcium and high aluminum and manganese content — conditions characteristic of acid tropical soils with high rainfall that most vegetables cannot tolerate.

Cassava has a reputation as a soil-depleting crop, which is not entirely true, since its nutrient requirements are similar to those of many other crops. This impression probably arose because in slash-and-burn agriculture cassava is often the last crop grown before the land is abandoned. Such a reputation developed because of its ability to grow relatively well on nutrient-depleted soils.

Vegetative growth is favored by high soil nitrogen and irrigation, and for high yields several applications of nitrogen fertilizers not exceeding 100 kg/ha during the growth period are often recommended. However, excessive vegetative growth delays storage root development, so it should be avoided.

Propagation

Cassava plants can produce viable seeds, but they are not used for propagation because of variability. Instead, propagation occurs almost exclusively by stem cuttings, called stakes. This method makes it possible to obtain uniform plant populations.

Stakes 20-30 cm long, taken from the previous crop, are planted horizontally, at an angle, or vertically. In horizontal planting, the stakes are covered with 5-10 cm of soil. Otherwise, one end of the stake is placed in the soil to a depth of 10-15 cm. The orientation of the stake affects storage root formation; with horizontal placement, a greater number of storage roots are formed, located closer to the surface, than with vertical or slanted placement of the stake. From 10,000 to 20,000 stems are planted per hectare.

Spacing typical of traditional plantings on ridges or raised beds is 50 cm on ridges 1 m apart; flat plantings are most often equidistant at about 80-100 cm. On soils with low fertility, more stakes are planted, and although total yield does not increase, the number of storage roots increases. Cassava is planted in pure stand, as well as in combination with maize, peanut, or beans, in which case the spacing between plants is increased.

Stakes should be selected from disease-free plants that demonstrate high yield. Old, thick, and long stakes are considered more productive because of their larger carbohydrate reserves. However, old stakes are more likely to be infected with viruses. As a compromise, stakes are often taken from the middle of mature woody stems, which have a lower probability of virus infection. Harvested stakes can be used immediately or, if necessary, stored until use. With poor organization of stake collection, the quality and health of stakes often do not meet standards. Progressive gardeners use nursery techniques to quickly obtain virus-free stem cuttings. The advantage of using stem cuttings is that the edible products of the crop are not used for propagation, as is the case with other underground starchy vegetable organs.

In traditional cultivation, stakes are planted in ridges or mounds, which allows roots to develop and remain below the soil surface. Minimum tillage is common, although the soil at the stake planting site may be loosened. By providing better drainage, ridge or mound plantings facilitate manual harvesting. Large producers able to mechanize their cultivation and harvesting methods often use flat planting. Equipment is used to create shallow furrows, into which stakes are placed horizontally and then covered with soil. Light-textured soils are an advantage for mechanization.

Growth and development

Representative growth of a cassava plant in the first year.

As the buds on the stem cuttings germinate and grow, callus tissue forms at the base of the cut surface of the stake. Soon roots emerge from this tissue, as well as from the nodes of new shoot parts in the soil. Depending on temperature, germination usually occurs after 1-4 weeks, with an average of two to three shoots developing from each stake. It is important that the soil be sufficiently moist at this time. At first the leaves are small, but a full canopy and maximum leaf area index are usually reached 4-6 months after planting. Representative growth of a cassava plant during the usual production period, demonstrating changes in leaf area index and total accumulated dry weight of the plant and storage root, is shown in the figure. First-year harvest usually occurs at the end of the rainy season, although harvest may be delayed until the second growth season. At the end of the rainy season, leaf shedding occurs with a decrease in leaf area index. In the dry season there is virtually no growth, but with the renewal of rains leaf growth resumes, and leaf area index and dry weight of storage roots increase.

The formation of cassava roots is greatly influenced by leaf development and its content. Studies of various cultivars show that plants with a high leaf area index (LAI) produce high yields.

Branching affects plant height and the efficiency of foliage use for photosynthate production and, consequently, yield. A comparison of an erect unbranched cultivar and a branched cultivar shows the advantage of branching with respect to the final yield of dry storage roots.

Erect non-branching plants, although taller, produce fewer stems and leaves early in growth, resulting in a lower LAI than plants with a branching habit. Apical dominance of the stem tends to suppress branching and axillary bud growth, and this characteristic varies among cultivars. Apical dominance also weakens when flowers form at the apex. Cultivars with extensive branching and foliage development may produce more dry matter in leaves and stems at the expense of storage root dry weight accumulation. The degree of leaf canopy coverage is more important than canopy thickness, which may be greater in tall plants but may be disadvantageous if shading reduces the photosynthetic efficiency of lower-level leaves. Ideally, high-yielding cultivars should have a high LAI and a high harvest index.

Root enlargement occurs under short-day conditions (less than 12 hours). Roots developing from the central stem are long, cylindrical, and conical. Once thickening begins, the storage root decreases or ceases to function as an absorbing organ. Roots accumulate starch and enlarge as early as 6-7 weeks after planting.

First-year storage roots are 20-50 cm long and 5-10 cm in diameter. Size increases with continued growth; the longer the growth period and the greater the leaf area, the higher the yield. However, a prolonged growth period can reduce edible quality because of increased fiber development. Nevertheless, as insurance against hunger and crop failure, growth can continue for several years.

Harvesting

Since storage roots continually increase, they have no definite period of maturity. As a result, the harvest period can vary, which partly explains differences in yield, which can range from less than 2 t/ha to more than 20 t/ha; on average about 10 t/ha. The crop is usually harvested 12-15 months after planting, but it can be harvested as early as 6 months or even after 2-3 years. In general, sweet cultivars mature in 6-9 months compared with bitter cultivars, which are grown for 12-18 months to obtain high yields. The latter are mainly used for the production of food, animal feed, and industrial products. Under ideal experimental conditions, a yield of 80 t/ha has been achieved.

Harvesting is carried out by cutting the stems and leaves, leaving sufficient stem length to facilitate uprooting and extraction of the roots from the soil. In most countries of the world, manual harvesting is the usual practice, although in large-scale production specialized equipment cuts the stems and extracts the roots from the soil. In traditional cultivation, complete harvest in the field is not practiced because of the rapid perishability of the roots after harvest. Cassava leaves are often used as litter, and excessive defoliation and removal of tops can also reduce yield.

Postharvest handling and storage

Usually cassava has a short postharvest life, with quality deterioration beginning soon after harvest. At ambient temperature, quality loss occurs rapidly; storage roots of sweet cultivars deteriorate faster than those of bitter cultivars. Internal discoloration (streaking) of vascular tissues can occur within 2-3 days, and sometimes this is followed by rotting. The most common storage method is to leave the roots unharvested until they are needed.

In the tropics during cool periods, storage of layers of harvested roots between layers of straw, leaves, or similar dry material is practiced. All of this is covered with a layer of soil about 15 cm thick. In warm periods, a thicker, 30-40 cm, layer of soil insulation is used. Ventilation holes are placed in the pile to ensure air movement and cooling. Roots are removed as needed, but the storage period is relatively short. Alternatively, roots placed in moist sawdust and put in plastic bags are stored in boxes or containers. If refrigeration equipment is available, cassava can be stored reasonably well for 1-2 weeks at 0-5 °C and 85-95% relative humidity.

Any prolonged storage requires careful handling. If the easily bending roots are damaged, grayish discoloration of the internal tissues soon appears, leading to increased susceptibility to rot. Coating the roots with wax or wrapping them in polyethylene film helps reduce losses from drying and slow deterioration.

Diseases and pests

Phytosanitary practice makes it possible to avoid and significantly reduce the incidence of cassava disease. Production and maintenance of disease-free propagation material reduces yield losses. Although chemical pesticides are an important resource for pest and disease control, they are rarely used by subsistence farmers. Some integrated pest management programs use local predators and parasites to control insects such as mealybugs and green spider mites.

Important bacterial diseases affecting cassava include:

• Xanthomonas campestris pv. manihotis (English: Cassava bacterial blight);
• Xanthomonas campestris pv. cassavae (English: Bacterial angular leaf spot);
• Erwinia carotovora pv. carotovora (English: Bacterial stem rot).

Fungal diseases:

• Colletotrichum manihotis, Colletotrichum gloeosporioides (English: Anthracnose);
• Cercospora henningsii (English: Brown leaf spot);
• Uromyces manihotis (English: Cassava rust);
• Diplodia manihotis (English: Diplodia dry root and stem rot);
• Fusarium spp. (English: Fusarium root and stem rot);
• Phoma spp., also Phyllosticta spp. (English: Phoma leaf spot or concentric-ring leaf spot);
• Phytophthora spp. (English: Phytophthora soft root rot);
• Sphaceloma manihoticola, also Elsinoe brasiliensis (English: Super-elongation);
• Fomes lignosus (English: White root or thread rot).

Bacterial blight of cassava is the most common disease, causing reduced growth and significant production losses. Super-elongation is an interesting disease that causes excessive elongation of the internodes of young stems. It is the result of growth of the pathogen in the intercellular spaces of the epidermis and cortex. Young shoots and petioles are deformed, and leaves are underdeveloped and die. Resistant cultivars have been developed and bred.

Viruses are a common problem for cassava production. African cassava mosaic virus is the most important. Other viruses are also important: Cassava Common Mosaic Virus (Cassava Common Mosaic Virus), Cassava Frog Skin Virus (Cassava Frog Skin Virus), Brown Streak Virus (Brown Streak Virus), and Cassava Vein Mosaic Virus (Cassava Vein Mosaic Virus).

Probably the most important insect pests are the green spider mite (Mononychellus tanajoa), mealybug (Phenacoccus spp.), and whitefly (Bemisia tabaci). Other pests include the burrowing bug (Cyrtomenus bergi), red spider mite (Tetranychus urticae), hornworm (Erinnysis ello), cassava scale (Aonidomytilus albus), and variegated grasshopper (Zonocerus variegatus), as well as thrips, larvae, and stem borers. Rodents, usually rats, and other vertebrate pests are also known to cause significant yield losses.

In the early 1970s, the mealybug accidentally entered African production zones when a new cassava cultivar from South America was introduced for evaluation. Having no natural enemies in Africa, the mealybug thrived, and within a few years this pest reportedly destroyed up to 80% of the crop in some regions of Africa. To solve this problem, a tiny wasp from South America, a natural enemy of the mealybug, was introduced into Africa, and within a few years the problem was brought under control.

Nematodes cause great damage, especially when cassava is repeatedly grown on the same field. Important nematodes are root knot (Meloidogyne incognita and Meloidogyne javanica) and species of Pratylenchus and Rotylenchulus.

Early in growth, cassava does not tolerate competition with weeds, so hoeing is usually practiced. Mulching is often used to control weeds and erosion. The cassava leaf canopy develops relatively quickly, and after it is established it suppresses weed growth effectively enough. Herbicides are available but are not widely used, since the value of the crop often does not justify the cost.

Crop improvement

Future progress in cassava production depends on genetic development to address the main problems associated with low protein content, the presence of cyanogenic glucosides, relatively rapid postharvest deterioration, resistance to diseases and pests, and mechanization. Centro Internacional de Agricultura Tropical (CIAT) in Colombia and the International Institute of Tropical Agriculture (IITA) in Nigeria have active programs dedicated to cassava improvement. The Brazilian government agency EMBRAPA is also involved in the development and application of new technologies to improve cultivars and agronomic performance. In addition, Thailand, India, and Cuba have active research programs, and these organizations also function as centers for collecting and conserving germplasm. Research on products derived from cassava starch and flour may open new markets and stimulate production growth. If one considers the ability of this crop to produce carbohydrates under unfavorable conditions, this reduces concern about low protein content, which can be balanced by other food sources.

Other species of the family Euphorbiaceae (Euphorbiaceae) grown for vegetables in the tropics are chaya (Cnidoscolus chayamansa), katuk (Sauropus androgynus), and castor bean (Ricinus communis).

Sources

1. Rubatzky, Vincent E., Yamaguchi, Mas. (1997). World vegetables: principles, production, and nutritive values. 2nd ed.