Cassava

Cassava
Storage root (waxed)
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Eudicots
Clade:	Rosids
Order:	Malpighiales
Family:	Euphorbiaceae
Genus:	Manihot
Species:	M. esculenta
Binomial name
Manihot esculenta Crantz[1]
Synonyms[1]
Janipha aipi (Pohl) J.Presl Janipha manihot (L.) Kunth Jatropha aipi (Pohl) Göpp. Jatropha diffusa (Pohl) Steud. Jatropha digitiformis (Pohl) Steud. Jatropha dulcis J.F.Gmel. Jatropha flabellifolia (Pohl) Steud. Jatropha loureiroi (Pohl) Steud. Jatropha manihot L. Jatropha mitis Rottb. Jatropha paniculata Ruiz & Pav. ex Pax Jatropha silvestris Vell. Jatropha stipulata Vell. Mandioca aipi (Pohl) Link Mandioca dulcis (J.F.Gmel.) D.Parodi Mandioca utilissima (Pohl) Link Manihot aipi Pohl Manihot aypi Spruce Manihot cannabina Sweet Manihot diffusa Pohl Manihot digitiformis Pohl Manihot dulcis (J.F.Gmel.) Baill. Manihot edule A.Rich. Manihot edulis A.Rich. Manihot flabellifolia Pohl Manihot flexuosa Pax & K.Hoffm. Manihot loureiroi Pohl Manihot melanobasis Müll. Arg. Manihot sprucei Pax Manihot utilissima Pohl

Manihot esculenta, commonly called cassava, manioc, or yuca (among numerous regional names), is a woody shrub of the spurge family, Euphorbiaceae, native to South America, from Brazil, Paraguay and parts of the Andes. Although a perennial plant, cassava is extensively cultivated in tropical and subtropical regions as an annual crop for its edible starchy tuberous root. Cassava is predominantly consumed in boiled form, but substantial quantities are processed to extract cassava starch, called tapioca, which is used for food, animal feed, and industrial purposes. The Brazilian farofa, and the related garri of West Africa, is an edible coarse flour obtained by grating cassava roots, pressing moisture off the obtained grated pulp, and finally drying and roasting it.

Cassava is the third-largest source of carbohydrates in food in the tropics, after rice and maize, making it an important staple; more than 500 million people depend on it. It offers the advantage of being exceptionally drought-tolerant, and able to grow productively on poor soil. The largest producer is Nigeria, while Thailand is the largest exporter of cassava starch.

Cassava is grown in sweet and bitter varieties; both contain toxins, but the bitter varieties have them in much larger amounts. Cassava has to be prepared carefully for consumption, as improperly prepared material can contain sufficient cyanide to cause poisoning. The more toxic varieties of cassava have been used in some places as famine food during times of food insecurity. Farmers may however choose bitter cultivars to minimise crop losses.

The generic name Manihot and the common name "manioc" both derive from the Guarani (Tupi) name mandioca or manioca for the plant.[2][3] The specific name esculenta is Latin for 'edible'.[2] The common name "cassava" is a 16th century word from the French or Portuguese cassave, in turn from Taíno caçabi.[4] The common name "yuca" or "yucca" is most likely also from Taíno, via Spanish yuca or juca.[5]

Cassava is a perennial species that is usually harvested within one year after planting. The harvested part is the storage root, which is long and tapered, with an easily detached rough brown rind. The white or yellowish flesh is firm and even in texture. Commercial cultivars can be 5 to 10 centimetres (2 to 4 in) wide at the top, and some 15 to 30 cm (6 to 12 in) long, with a woody vascular bundle running down the middle. The tuberous roots are largely starch, with small amounts of calcium (16 milligrams per 100 grams), phosphorus (27 mg/100 g), and vitamin C (20.6 mg/100 g).[6] Content of total pro-vitamin A carotenoids has been increased through conventional breeding from a maximum of 10.3 to 24.3 υg/g FW basis [7] Cassava roots contain little protein, whereas the leaves are rich in it,[8] except for being low in methionine, an essential amino acid.[9]

Cassava is monoecious, with separate male and female flower on the same plant, arranged in inflorescences. These develop at the apex of the growing stem; buds can sprout to form new branches below the inflorescence, enabling continued vegetative growth. Farmers prefer a non-branching (erect) plant because it is easier to collect, transport, and store. However, the erect form complicates plant breeding as flowers are scarce or absent; breeders make use of photoperiod extension, pruning, and plant growth regulators to encourage flowering.[10][11][12][13]

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  Cassava plant
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  Unprocessed tuberous roots
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  Tuberous root in cross-section
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  Leaf
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  Leaf detail
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  Flower buds
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  Seeds and opened fruit

The complete and haplotype-resolved African cassava (TME204) genome has been reconstructed and made available using the Hi-C technology.[14] The genome shows abundant novel gene loci with enriched functionality related to chromatin organization, meristem development, and cell responses.[14] Differentially expressed transcripts of different haplotype origins were enriched for different functionality during tissue development. In each tissue, 20–30% of transcripts showed allele-specific expression differences with <2% of direction-shifting. Despite high gene synteny, the HiFi genome assembly revealed extensive chromosome rearrangements and abundant intra-genomic and inter-genomic divergent sequences, with significant structural variations mostly related to long terminal repeat retrotransposons.[14]

Although smallholders are otherwise economically inefficient producers, they are vital to productivity at particular times.[15] Small cassava farmers are no exception.[15] Genetic diversity is vital when productivity has declined due to pests and diseases, and smallholders tend to retain less productive but more diverse gene pools.[15]

The molecular genetics of starchy root development in cassava have been analyzed and compared to other root and tuber crops, including possible (unproven) roles for Flowering Locus T (FT) orthologs.[16]

Wild populations of M. esculenta subspecies flabellifolia, shown to be the progenitor of domesticated cassava, are centered in west-central Brazil, where it was likely first domesticated no more than 10,000 years ago.[17] Forms of the modern domesticated species can also be found growing in the wild in the south of Brazil. By 4600 BC, cassava pollen appears in the Gulf of Mexico lowlands, at the San Andrés archaeological site.[18] The oldest direct evidence of cassava cultivation comes from a 1,400-year-old Maya site, Joya de Cerén, in El Salvador.[19] It became a staple food of the native populations of northern South America, southern Mesoamerica, and the Taino people in the Caribbean islands, who grew it using a high-yielding form of shifting agriculture by the time of European contact in 1492.[20] Cassava was a staple food of pre-Columbian peoples in the Americas and is often portrayed in indigenous art. The Moche people often depicted cassava in their ceramics.[21]

Spaniards in their early occupation of Caribbean islands did not want to eat cassava or maize, which they considered insubstantial, dangerous, and not nutritious. They much preferred foods from Spain, specifically wheat bread, olive oil, red wine, and meat, and considered maize and cassava damaging to Europeans.[22] The cultivation and consumption of cassava were nonetheless continued in both Portuguese and Spanish America. Mass production of cassava bread became the first Cuban industry established by the Spanish.[23] Ships departing to Europe from Cuban ports such as Havana, Santiago, Bayamo, and Baracoa carried goods to Spain, but sailors needed to be provisioned for the voyage. The Spanish also needed to replenish their boats with dried meat, water, fruit, and large amounts of cassava bread.[24] Sailors complained that it caused them digestive problems.[25]

Portuguese traders introduced cassava to Africa from Brazil in the 16th century. Around the same period, it was introduced to Asia through Columbian Exchange by Portuguese and Spanish traders, who planted it in their colonies in Goa, Malacca, Eastern Indonesia, Timor and the Philippines.[26] Cassava has also become an important crop in Asia. While it is a valued food staple in parts of eastern Indonesia, it is primarily cultivated for starch extraction and bio-fuel production in Thailand, Cambodia and Vietnam.[27] Cassava is sometimes described as the "bread of the tropics"[28] but should not be confused with the tropical and equatorial bread tree (Encephalartos), the breadfruit (Artocarpus altilis) or the African breadfruit (Treculia africana). This description definitely holds in Africa and parts of South America; in Asian countries such as Vietnam fresh cassava barely features in human diets.[29] Cassava was introduced to East Africa around 1850 by Arab and European settlers, who promoted its cultivation as a reliable crop to mitigate the effects of drought and famine.[30]

There is a legend that cassava was introduced in 1880–1885 to the South Indian state of Kerala by the King of Travancore, Vishakham Thirunal Maharaja, after a great famine hit the kingdom, as a substitute for rice.[31] However, cassava was cultivated in the state before that time.[32] Cassava is called kappa or maricheeni in Malayalam, and tapioca in Indian English usage.[33]

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  Taíno women preparing cassava bread in 1565: grating tuberous roots into paste, shaping the bread, and cooking it on a fire-heated burén
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  17th-century painting by Albert Eckhout in Dutch Brazil

Optimal conditions for cassava cultivation are mean annual temperatures between 20 and 29 °C (68 and 84 °F), annual precipitation between 1,000 and 2,500 mm (39 and 98 in), and an annual growth period of no less than 240 days.[34] Cassava is propagated by cutting the stem into sections of approximately 15 cm (5.9 in), these being planted prior to the wet season.[35] Cassava growth is favorable under temperatures ranging from 25 to 29 °C (77 to 84 °F), but it can tolerate temperatures as low as 12 °C (54 °F) and as high as 40 °C (104 °F).[36] These conditions are found, among other places, in the northern part of the Gulf Coastal Plain in Mexico.[34] In this part of Mexico the following soil types have been shown to be good for cassava cultivation: phaeozem, regosol, arenosol, andosol and luvisol.[34]

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  Stakes
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  Grafting

Before harvest, the leafy stems are removed. The harvest is gathered by pulling up the base of the stem and cutting off the tuberous roots.[35]

Cassava deteriorates after harvest, when the tuberous roots are first cut. The healing mechanism produces coumaric acid, which oxidizes and blackens the roots, making them inedible after a few days. This deterioration is related to the accumulation of reactive oxygen species initiated by cyanide release during mechanical harvesting. Cassava shelf life may be increased up to three weeks by overexpressing a cyanide-insensitive alternative oxidase, which suppressed ROS by 10-fold.[37] Post-harvest deterioration is a major obstacle to the export of cassava. Fresh cassava can be preserved like potato, using thiabendazole or bleach as a fungicide, then wrapping in plastic, freezing, or applying a wax coating.[38]

While alternative methods for controlling post-harvest deterioration have been proposed, such as preventing reactive oxygen species effects by using plastic bags during storage and transport, coating the roots with wax, or freezing roots, such strategies have proved to be economically or technically impractical, leading to breeding of cassava varieties with improved durability after harvest, achieved by different mechanisms.[39][40] One approach used gamma rays to try to silence a gene involved in triggering deterioration; another strategy selected for plentiful carotenoids, antioxidants which may help to reduce oxidization after harvest.[40]

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  Starch processing
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  Starch flour
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  Starch wet-processing
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  Spreading Casabe burrero (cassava bread) to dry, Venezuela
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  Starch being prepared for packaging
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  Starch noodles packaged for shipping
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  Frozen leaves in a Los Angeles market
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  Picked buds

An agronomist examines a diseased cassava crop in Thailand.

Cassava is subject to pests from multiple taxonomic groups, including nematodes, and insects, as well as diseases caused by viruses, bacteria, and fungi. All cause reductions in yield, and some cause serious losses of crops.[41]

Several viruses cause enough damage to cassava crops to be of economic importance. The African cassava mosaic virus causes the leaves of the cassava plant to wither, limiting the growth of the root.[42] An outbreak of the virus in Africa in the 1920s led to a major famine.[43] The virus is spread by the whitefly and by the transplanting of diseased plants into new fields. Sometime in the late-1980s, a mutation occurred in Uganda that made the virus even more harmful, causing the complete loss of leaves. This mutated virus spread at a rate of 80 kilometres (50 miles) per year, and as of 2005 was found throughout Uganda, Rwanda, Burundi, the Democratic Republic of the Congo and the Republic of the Congo.[44] Viruses are a severe production limitation in the tropics. They are the primary reason for the complete lack of yield increases in the 25 years up to 2021[update].[45] Cassava brown streak virus disease is a major threat to cultivation worldwide.[43] Cassava mosaic virus (CMV) is widespread in Africa, causing cassava mosaic disease (CMD).[46] Bredeson et al. 2016 find the M. esculenta cultivars most widely used on that continent have M. carthaginensis subsp. glaziovii genes of which some appear to be CMD resistance genes.[46] Although the ongoing CMD pandemic affects both East and Central Africa, Legg et al. found that these two areas have two distinct subpopulations of the vector, Bemisia tabaci whiteflies.[47][48] Genetically engineered cassava offers opportunities for the improvement of virus resistance, including CMV and CBSD resistance.[49]

Among the most serious bacterial pests is Xanthomonas axonopodis pv. manihotis, which causes bacterial blight of cassava. This disease originated in South America and has followed cassava around the world.[50] Bacterial blight has been responsible for near catastrophic losses and famine in past decades, and its mitigation requires active management practices.[50] Several other bacteria attack cassava, including the related Xanthomonas campestris pv. cassavae, which causes bacterial angular leaf spot.[51]

Several fungi and oomycetes bring about significant crop losses, one of the most serious being cassava root rot; the pathogens involved are species of Phytophthora, the genus which causes potato blight. Cassava root rot can result in losses of as much as 80 percent of the crop.[41] A major pest is a rust caused by Uromyces manihotis.[52] Superelongation disease, caused by Elsinoë brasiliensis, can cause losses of over 80 percent of young cassava in Latin America and the Caribbean when temperature and rainfall are high.[41][53][54]

Nematode pests of cassava are thought to cause harms ranging from negligible to seriously damaging,[55][56][57] making the choice of management methods difficult.[58] A wide range of plant parasitic nematodes have been reported associated with cassava worldwide. These include Pratylenchus brachyurus, Rotylenchulus reniformis, Helicotylenchus spp., Scutellonema spp. and Meloidogyne spp., of which Meloidogyne incognita and Meloidogyne javanica are the most widely reported and economically important.[59] Meloidogyne spp. feeding produces physically damaging galls with eggs inside them. Galls later merge as the females grow and enlarge, and they interfere with water and nutrient supply.[57] Cassava roots become tough with age and restrict the movement of the juveniles and the egg release. It is therefore possible that extensive galling can be observed even at low densities following infection.[58] Other pests and diseases can gain entry through the physical damage caused by gall formation, leading to rots. They have not been shown to cause direct damage to the enlarged tuberous roots, but plant height can be reduced if the root system is reduced.[60] Nematicides reduce the numbers of galls per feeder root, along with fewer rots in the tuberous roots.[61] The organophosphorus nematicide fenamiphos does not reduce crop growth or harvest yield. Nematicide use in cassava does not increase harvested yield significantly, but lower infestation at harvest and lower subsequent storage loss provide a higher effective yield. The use of tolerant and resistant cultivars is the most practical management method in most locales.[62][58][63]

Grasshoppers, here on cassava in Nigeria, are secondary pests of cassava.[41]

Insects such as stem borers and other beetles, moths including Chilomima clarkei, scale insects, fruit flies, shootflies, burrower bugs, grasshoppers, leafhoppers, gall midges, leafcutter ants, and termites contribute to losses of cassava in the field,[41] while others contribute to serious losses, between 19% and 30%, of dried cassava in storage.[64] In Africa, a previous issue was the cassava mealybug (Phenacoccus manihoti) and cassava green mite (Mononychellus tanajoa). These pests can cause up to 80 percent crop loss, which is extremely detrimental to the production of subsistence farmers. These pests were rampant in the 1970s and 1980s but were brought under control following the establishment of the Biological Control Centre for Africa of the International Institute of Tropical Agriculture (IITA) under the leadership of Hans Rudolf Herren.[65] The Centre investigated biological control for cassava pests; two South American natural enemies Anagyrus lopezi (a parasitoid wasp) and Typhlodromalus aripo (a predatory mite) were found to effectively control the cassava mealybug and the cassava green mite, respectively.[66]