From Wikipedia, the free encyclopedia - View original article
|Leaves of the cassava plant|
|A manioc tuber|
|Leaves of the cassava plant|
|A manioc tuber|
Cassava (Manihot esculenta), also called manioc, yuca, balinghoy, mogo, mandioca, kamoteng kahoy, tapioca-root (predominantly in India) and manioc root, a woody shrub of the Euphorbiaceae (spurge) family native to South America, is extensively cultivated as an annual crop in tropical and subtropical regions for its edible starchy tuberous root, a major source of carbohydrates. It differs from the similarly spelled yucca, an unrelated fruit-bearing shrub in the Asparagaceae family. Cassava, when dried to a powdery (or pearly) extract, is called tapioca; its fermented, flaky version is named garri.
Cassava is the third largest source of food carbohydrates in the tropics, after rice and maize. Cassava is a major staple food in the developing world, providing a basic diet for over half a billion people. It is one of the most drought tolerant crops, capable of growing on marginal soils. Nigeria is the world's largest producer of cassava, while Thailand is the largest exporting country of dried cassava.
Cassava is classified as sweet or bitter. Like other roots and tubers, cassava contains antinutritional factors and toxins. It must be properly prepared before consumption. Improper preparation of cassava can leave enough residual cyanide to cause acute cyanide intoxication and goiters, and may even cause ataxia or partial paralysis. Nevertheless, farmers often prefer the bitter varieties because they deter pests, animals, and thieves. The more toxic varieties of cassava are a fall-back resource (a "food security crop") in times of famine in some places.
The cassava root is long and tapered, with a firm, homogeneous flesh encased in a detachable rind, about 1mm thick, rough and brown on the outside. Commercial varieties can be 5 to 10 cm in diameter at the top, and around 15 cm to 30 cm long. A woody cordon runs along the root's axis. The flesh can be chalk-white or yellowish. Cassava roots are very rich in starch and contain significant amounts of calcium (50 mg/100g), phosphorus (40 mg/100g) and vitamin C (25 mg/100g). However, they are poor in protein and other nutrients. In contrast, cassava leaves are a good source of protein (rich in lysine) but deficient in the amino acid methionine and possibly tryptophan.
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 more than 10,000 years BP. By 6,600 BC, manioc pollen appears in the Gulf of Mexico lowlands, at the San Andrés archaeological site. The oldest direct evidence of cassava cultivation comes from a 1,400-year-old Maya site, Joya de Cerén, in El Salvador, and the species Manihot esculenta likely originated further south in Brazil, Paraguay and Argentina. With its high food potential, it had become a staple food of the native populations of northern South America, southern Mesoamerica, and the Caribbean by the time of the Spanish conquest. Its cultivation was continued by the colonial Portuguese and Spanish. Forms of the modern domesticated species can be found growing in the wild in the south of Brazil. While several Manihot species are wild, all varieties of M. esculenta are cultigens.
Since being introduced by Portuguese traders from Brazil in the 16th century, maize and cassava have replaced traditional African crops as the continent's most important staple food crops. Cassava is sometimes described as the 'bread of the tropics' but should not be confused with the tropical and equatorial bread tree (Encephalartos), the breadfruit (Artocarpus altilis) or the African breadfruit (Treculia africana).
World production of cassava root was estimated to be 184 million tonnes in 2002, rising to 230 million tonnes in 2008. The majority of production in 2002 was in Africa, where 99.1 million tonnes were grown; 51.5 million tonnes were grown in Asia; and 33.2 million tonnes in Latin America and the Caribbean. Nigeria is the world's largest producer of cassava. However, based on the statistics from the FAO of the United Nations, Thailand is the largest exporting country of dried cassava, with a total of 77% of world export in 2005. The second largest exporting country is Vietnam, with 13.6%, followed by Indonesia (5.8%) and Costa Rica (2.1%). Worldwide cassava production increased by 12.5% between 1988 and 1990.
In 2010, the average yield of cassava crops worldwide was 12.5 tonnes per hectare. The most productive cassava farms in the world were in India, with a nationwide average yield of 34.8 tonnes per hectare in 2010.
Cassava, yams (Dioscorea spp.) and sweet potatoes (Ipomoea batatas) are important sources of food in the tropics. The cassava plant gives the third highest yield of carbohydrates per cultivated area among crop plants, after sugarcane and sugar beets. Cassava plays a particularly important role in agriculture in developing countries, especially in sub-Saharan Africa, because it does well on poor soils and with low rainfall, and because it is a perennial that can be harvested as required. Its wide harvesting window allows it to act as a famine reserve and is invaluable in managing labor schedules. It offers flexibility to resource-poor farmers because it serves as either a subsistence or a cash crop.
No continent depends as much on root and tuber crops in feeding its population as does Africa. In the humid and subhumid areas of tropical Africa, it is either a primary staple food or a secondary costaple. In Ghana, for example, cassava and yams occupy an important position the agricultural economy and contribute about 46% of the agricultural gross domestic product. Cassava accounts for a daily caloric intake of 30% in Ghana and is grown by nearly every farming family. The importance of cassava to many Africans is epitomised in the Ewe (a language spoken in Ghana, Togo and Benin) name for the plant, agbeli, meaning "there is life". The price of cassava has risen significantly in the last half decade, and lower-income people have turned to other carbohydrate-rich foods, such as rice.
In Tamil Nadu, India, the National Highway 68 between Thalaivasal and Attur has many cassava processing factories alongside it—indicating a local abundance. Cassava is widely cultivated and eaten as a staple food in Andhra Pradesh and in Kerala.
In the subtropical region of southern China, cassava is the fifth-largest crop in term of production, after rice, sweet potato, sugar cane and maize. China is also the largest export market for cassava produced in Vietnam and Thailand. Over 60% of cassava production in China is concentrated in a single province, Guangxi, averaging over 7 million tonnes annually.
Alcoholic beverages made from cassava include Cauim and tiquira (Brazil), kasiri (Sub-Saharan Africa), masato (Peruvian Amazonia chicha), parakari (Guyana), nihamanchi (South America) aka nijimanche (Ecuador and Peru), sakurá (Brazil, Surinam).
Cassava can be cooked in many ways. The soft-boiled root has a delicate flavor and can replace boiled potatoes in many uses: as an accompaniment for meat dishes or made into purées, dumplings, soups, stews, gravies, etc. This plant is used in cholent in some households, as well. Deep fried (after boiling or steaming), it can replace fried potatoes, bringing a distinctive flavor. In Brazil, detoxified manioc is ground and cooked to a dry, often hard or crunchy meal which is used as a condiment, toasted in butter, or eaten alone as a side dish.
Fufu is made from the starchy cassava-root flour. Tapioca (or fecula), essentially a flavorless, starchy ingredient produced from treated and dried cassava (manioc) root, is used in cooking. It is similar to sago and is commonly used to make milky pudding similar to rice pudding. Boba tapioca pearls are made from cassava root. It is used in cereals; several tribes in South America have used it extensively. It is also used in making cassava cake, a popular pastry.
Cassava is used in making eba, a popular food in Nigeria.
Gari is a creamy-white, granular flour with a slightly sour, fermented flavor from fermented, gelatinized fresh cassava tubers. Gari soakings is a delicacy that cost less than US$1 in Nigeria, Cameroon, Congo, Ghana and other parts of Africa, where cassava is cultivated. One can simply soak gari in cold water, add a bit of sugar and roasted groundnut (peanut) to taste, and add whatever quantity of evaporated milk one desires. Gari soakings prepared with coconut water are also popular.
In the Democratic Republic of the Congo, the leaves are washed with hot water, pounded in a mortar, and boiled. In the boiling process, the pot is not covered, to enable release of HCN. It is covered when ingredients such as oil, onion, fish, etc. are added, and opened as needed for stirring. The sauce eaten with rice, plantain or cassava paste is called mpondu in Lingala, sombe in Swahili or sakasaka in Kikongo. The cassava root flour is also used to make a cassava bread by boiling flour until it is a thick, rubbery ball (bukari in Swahili or luku in Kikongo. The flour is also made into a paste and fermented before boiling after wrapping in banana or other forest leaves. This fermented state is called chikwangue in French or kwanga or nkwanga in Lingala and Kikongo. This last form has a long shelf life and is a preferred food to take on long trips where refrigeration is not possible.
In Arua, Northwestern Uganda, the Lugbara grow cassava as a staple food. After harvest, they peel its skin, cut into smaller pieces, and leave to ferment and dry. When ready, it is ground into flour which is mingled to prepare enya asa bread. This cassava flour can be mixed with sorghum, millet and other cereals depending on preference. Cassava leaves are also a delicacy called pandu. In rural settings, cassava is used to brew a strong colourless drink called okaritua which men enjoy drinking while sitted in the sun on market days.
In the Philippines cassava cake is one of the most popular and enjoyed home made delicacies or Kakanin. Made from grated cassava (Kamoteng Kahoy), the root crop is mixed with coconut milk, eggs, butter and topped with a creamy milk mixture. It is also called cassava bibingka.
In Indonesia, Singkong or Ketela (cassava) is an important food. It can be cooked by frying or boiling, or processed by fermentation to make tapai and getuk cake, while the starch is made into krupuk crackers. In time of famine or food shortage, cassava is used to replace rice. In 2011, modified cassava flour became common, and some instant noodle producers have used it silently, especially for low-end instant noodles as a part substitute of pricy flour. The flour is often added to pastry flour although the result is a pastry that is a little stiff. Getuk cakes can be difficult to digest for diners not used to it and can result in severe cramps and discomfort.
In Sarawak cassava leaves (long leaf variety) is boiled and eaten with sambal (shrimp paste) or tempoyak (fermented durian). The long leaf variety is also cooked with pork, chicken, fish, or snake in a large bamboo stick. This traditional dish is called manok pansoh.
Brazilian tapioca is a crepe-like food made with cassava powder. In biju (or beiju), the tapioca is moistened, strained through a sieve to become a coarse flour, then sprinkled onto a hot griddle or pan, where the heat makes the starchy grains fuse into a tortilla, which is often sprinkled with coconut. Then it may be buttered and eaten as a toast (its most common use as a breakfast dish), or it may be filled or topped with either doces (sweet) or salgados (savory) ingredients, which define the kind of meal the tapioca is used for: breakfast, afternoon tea or dessert. Choices range from butter, cheese, chocolate, bananas with condensed milk, chocolate with bananas, to various forms of meats and served warm.
The juice of the bitter cassava, boiled to the consistency of thick syrup and flavored with spices, is called cassareep. It is used as a basis for sauces and as a culinary flavoring, principally in tropical countries. It is exported chiefly from Guyana, where it started as a traditional recipe with its origins in Amerindian practices.
In South America, a variety of cheese buns make for a popular breakfast dish and snack. Made of cassava starch and cheese, the cheese buns are distinctive because the inside is chewy and moist. Their size may range from 2 cm to 15 cm (1 to 6 inches) in diameter and approximately 5 cm (2 inches) in height.
The carimañola is a Colombian typical dish. It is a type of meat-pie in a torpedo-shaped yuca fritter.
"Brazilian Tapioca" is a crêpe like food made with cassava powder. It may be served/filled with shredded coconut, chocolate or fruit jelly.
"Sagu" is a dessert typical of south Brazil. Tapioca pearls are cooked with cinnamon and cloves in red wine and served with a milk/cassava powder cream.
The native Carib people of Dominica rely on cassava as a staple crop. Traditional "bread" is made by grinding cassava root, blending it with water (and sometimes coconut shavings) and frying it on large solid metal grills. Cassava bread cooked in this fashion is tough and chewy and can be used for wraps or sandwiches. It is most often eaten by itself. The Caribs have multiple small stations along Dominica's main highway where locals cook cassava bread in open-air kitchens for onlookers and tourists.
In Brazil, a crunchy meal called farinha de mandioca (Portuguese pronunciation: [faˈɾĩ ȷ̃ɐ dʒi mɐ̃diˈɔkɐ], "manioc flour") of varying coarseness is produced for use as a condiment, a base for farofa, or a stand-alone side dish. Detoxified manioc roots are ground to a pulp called a massa and squeezed with a device called a tipiti to dry it out (the liquid produced by this may be collected and settled to produce tapioca, locally known as polvilho). The dried massa is then dried over a large copper stove to produce the dried meal. This process varies regionally and by manioc species, and may include additional steps of re-soaking, re-drying and toasting the flour. Manioc agriculture and refinement to farinha is a major economic activity in the Western Amazon.
Farinha de mandioca and tapioca are the most important caloric staples of the Indigenous peoples of Brazil which already practiced agriculture when Europeans colonized the country, so for Brazilians manioc would be included in its equivalent of the North American three sister crops or the Mesoamerican milpa.
Fried cassava is a typical substitute for French fries in Brazil, Venezuela, Colombia Ecuador, and several Central American countries including Panama, also in Malaysia.
Cassava root is essentially a carbohydrate source. Its composition shows 60–65 percent moisture, 20–31 percent carbohydrate, 1–2 percent crude protein and a comparatively low content of vitamins and minerals. However, the roots are rich in calcium and vitamin C and contain a nutritionally significant quantity of thiamine, riboflavin and nicotinic acid. Cassava starch contains 70 percent amylopectin and 20 percent amylose. Cooked cassava starch has a digestibility of over 75 percent.
Cassava root is a poor source of protein. Despite the very low quantity, the quality of cassava root protein is fairly good in terms of essential amino acids. Methionine, cysteine and cystine are, however, limiting amino acids in cassava root.
Cassava is attractive as nutrition source in certain ecosystems because cassava is one of the most drought-tolerant crops, can be successfully grown on marginal soils, and gives reasonable yields where many other crops do not grow well. Cassava is well adapted within latitudes 30° north and south of the equator, at elevations between sea level and 2000 meters above sea level, in equatorial temperatures, with rainfalls of 50 millimeters to five meters annually, and to poor soils with a pH ranging from acidic to alkaline. These conditions are common in certain parts of Africa and South America.
Cassava is a highly productive crop in terms of food calories produced per unit land area per unit of time, significantly higher than other staple crops. Cassava can produce food calories at rates exceeding 250,000 cal/hectare/day compared with 176,000 for rice, 110,000 for wheat, and 200,000 for maize (corn).
Cassava, like other foods, also has antinutritional and toxic factors. Of particular concern are the cyanogenic glucosides of cassava (linamarin and lotaustralin). These, on hydrolysis, release hydrocyanic acid (HCN). The presence of cyanide in cassava is of concern for human and for animal consumption. The concentration of these antinutritional and unsafe glycosides varies considerably between varieties and also with climatic and cultural conditions. Selection of cassava species to be grown, therefore, is quite important. Once harvested, bitter cassava must be treated and prepared properly prior to human or animal consumption, while sweet cassava can be used after simple boiling.
The following table shows the nutrient content of cassava and compares it with major staple foods in a raw form. Raw forms of these staples, however, are not edible and cannot be digested. These must be sprouted, or prepared and cooked as appropriate for human consumption. In sprouted or cooked form, the relative nutritional and antinutritional contents of each of these grains is remarkably different from that of raw form of these grains reported in this table. The nutrition value for each staple food in cooked form depends on the cooking method (boiling, baking, steaming, frying, etc.).
The table shows that cassava is a good energy source, but like potato, cassava's protein and essential nutrients density is lower than other staple foods.
|STAPLE:||Maize / Corn[A]||Rice[B]||Wheat[C]||Potato[D]||Cassava[E]||Soybean (Green)[F]||Sweet potato[G]||Sorghum[H]||Yam[Y]||Plantain[Z]|
|Component (per 100g portion)||Amount||Amount||Amount||Amount||Amount||Amount||Amount||Amount||Amount||Amount|
|Vitamin C (mg)||0||0||0||19.7||20.6||29||2.4||0||17.1||18.4|
|Pantothenic acid (mg)||0.424||1.01||0.954||0.30||0.11||0.15||0.80||-||0.31||0.26|
|Vitamin B6 (mg)||0.622||0.16||0.3||0.30||0.09||0.07||0.21||-||0.29||0.30|
|Folate Total (μg)||19||231||38||16||27||165||11||0||23||22|
|Vitamin A (IU)||214||0||9||2||13||180||14187||0||138||1127|
|Vitamin E, alpha-tocopherol (mg)||0.49||0.11||1.01||0.01||0.19||0||0.26||0||0.39||0.14|
|Vitamin K (μg)||0.3||0.1||1.9||1.9||1.9||0||1.8||0||2.6||0.7|
|Saturated fatty acids (g)||0.667||0.18||0.269||0.03||0.07||0.79||0.02||0.46||0.04||0.14|
|Monounsaturated fatty acids (g)||1.251||0.21||0.2||0.00||0.08||1.28||0.00||0.99||0.01||0.03|
|Polyunsaturated fatty acids (g)||2.163||0.18||0.627||0.04||0.05||3.20||0.01||1.37||0.08||0.07|
|A corn, yellow||B rice, white, long-grain, regular, raw|
|C wheat, hard red winter||D potato, flesh and skin, raw|
|E cassava, raw||F soybeans, green, raw|
|G sweet potato, raw, unprepared||H sorghum, raw|
|Y yam, raw||Z plantains, raw|
In many countries, significant research has begun to evaluate the use of cassava as an ethanol biofuel feedstock. Under the Development Plan for Renewable Energy in the Eleventh Five-Year Plan in the People's Republic of China, the target is to increase the application of ethanol fuel by nongrain feedstock to 2 million tonnes, and that of biodiesel to 200 thousand tonnes by 2010. This will be equivalent to a substitute of 10 million tonnes of petroleum. As a result, cassava (tapioca) chips have gradually become a major source for ethanol production. On December 22, 2007, the largest cassava ethanol fuel production facility was completed in Beihai, with annual output of 200 thousand tons, which would need an average of 1.5 million tons of cassava. In November 2008, China-based Hainan Yedao Group reportedly invested $51.5m (£31.8m) in a new biofuel facility that is expected to produce 33 million US gallons (120,000 m3) a year of bioethanol from cassava plants.
Cassava is used worldwide for animal feed, as well. Cassava hay is produced at a young growth stage at three to four months, harvested about 30–45 cm above ground, and sun-dried for one to two days until it has final dry matter of less than 85%. The cassava hay contains high protein (20–27% crude protein) and condensed tannins (1.5–4% CP). It is used as a good roughage source for dairy or beef cattle, buffalo, goats, and sheep by either direct feeding or as a protein source in the concentrate mixtures
Manioc is also used in a number of commercially-available laundry products, especially as starch for shirts and other garments. Using manioc starch diluted in water and spraying it over fabrics before ironing helps harden collars.
Cassava root has been promoted as a treatment for bladder and prostate cancer. However, according to the American Cancer Society, "there is no convincing scientific evidence that cassava or tapioca is effective in preventing or treating cancer".
Cassava roots and leaves should not be consumed raw because they contain two cyanogenic glucosides, linamarin and lotaustralin. These are decomposed by linamarase, a naturally occurring enzyme in cassava, liberating hydrogen cyanide (HCN). Cassava varieties are often categorized as either sweet or bitter, signifying the absence or presence of toxic levels of cyanogenic glucosides, respectively. The so-called sweet (actually not bitter) cultivars can produce as little as 20 milligrams of cyanide (CN) per kilogram of fresh roots, whereas bitter ones may produce more than 50 times as much (1 g/kg). Cassavas grown during drought are especially high in these toxins. A dose of 25 mg of pure cassava cyanogenic glucoside, which contains 2.5 mg of cyanide, is sufficient to kill a rat. Excess cyanide residue from improper preparation is known to cause acute cyanide intoxication, and goiters, and has been linked to ataxia (a neurological disorder affecting the ability to walk, also known as konzo). It has also been linked to tropical calcific pancreatitis in humans, leading to chronic pancreatitis.
Societies that traditionally eat cassava generally understand some processing (soaking, cooking, fermentation, etc.) is necessary to avoid getting sick.
Symptoms of acute cyanide intoxication appear four or more hours after ingesting raw or poorly processed cassava: vertigo, vomiting, and collapse. In some cases, death may result within one or two hours. It can be treated easily with an injection of thiosulfate (which makes sulfur available for the patient's body to detoxify by converting the poisonous cyanide into thiocyanate).
"Chronic, low-level cyanide exposure is associated with the development of goiter and with tropical ataxic neuropathy, a nerve-damaging disorder that renders a person unsteady and uncoordinated. Severe cyanide poisoning, particularly during famines, is associated with outbreaks of a debilitating, irreversible paralytic disorder called konzo and, in some cases, death. The incidence of konzo and tropical ataxic neuropathy can be as high as 3% in some areas."
Brief soaking (four hours) of cassava is not sufficient, but soaking for 18–24 hours can remove up to half the level of cyanide. Drying may not be sufficient, either.
For some smaller-rooted, sweet varieties, cooking is sufficient to eliminate all toxicity. The cyanide is carried away in the processing water and the amounts produced in domestic consumption are too small to have environmental impact. The larger-rooted, bitter varieties used for production of flour or starch must be processed to remove the cyanogenic glucosides. and then ground into flour, which is then soaked in water, squeezed dry several times, and toasted. The starch grains that float to the surface during the soaking process are also used in cooking. The flour is used throughout South America and the Caribbean. Industrial production of cassava flour, even at the cottage level, may generate enough cyanide and cyanogenic glycosides in the effluents to have a severe environmental impact.
A safe processing method used by the pre-Columbian indigenous people of the Americas is to mix the cassava flour with water into a thick paste and then let it stand in the shade for five hours in a thin layer spread over a basket. In that time, about 83% of the cyanogenic glycosides are broken down by the linamarase; the resulting hydrogen cyanide escapes to the atmosphere, making the flour safe for consumption the same evening.
The traditional method used in West Africa is to peel the roots and put them into water for three days to ferment. The roots then are dried or cooked. In Nigeria and several other west African countries, including Ghana, Benin, Togo, Ivory Coast, and Burkina Faso, they are usually grated and lightly fried in palm oil to preserve them. The result is a foodstuff called gari. Fermentation is also used in other places such as Indonesia (see Tapai). The fermentation process also reduces the level of antinutrients, making the cassava a more nutritious food.
The reliance on cassava as a food source and the resulting exposure to the goitrogenic effects of thiocyanate has been responsible for the endemic goiters seen in the Akoko area of southwestern Nigeria.
A project called "BioCassava Plus" is developing a cassava with lower cyanogen glucosides and fortified with vitamin A, iron and protein to help the nutrition of people in sub-Saharan Africa. In 2011, the director of the program said he hoped to obtain regulatory approvals by 2017.
Cassava is harvested by hand by raising the lower part of the stem and pulling the roots out of the ground, then removing them from the base of the plant. The upper parts of the stems with the leaves are plucked off before harvest. Cassava is propagated by cutting the stem into sections of approximately 15 cm, these being planted prior to the wet season.
Cassava undergoes postharvest physiological deterioration, or PPD, once the tubers are separated from the main plant. The tubers, when damaged, normally respond with a healing mechanism. However, the same mechanism, which involves coumaric acids, initiates about 15 minutes after damage, and fails to switch off in harvested tubers. It continues until the entire tuber is oxidized and blackened within two to three days after harvest, rendering it unpalatable and useless.
PPD is one of the main obstacles currently preventing farmers from exporting cassavas abroad and generating income. Cassava can be preserved in various ways such as coating in wax or freezing.
The major cause of losses during cassava chip storage is infestation by insects. A wide range of species that feed directly on the dried chips have been reported as the cause of weight loss in the stored produce. Some loss assessment studies and estimations on dried cassava chips have been carried out in different countries. Hiranandan and Advani (1955) measured 12 - 14% post-harvest weight losses in India for chips stored for about five months. Killick (1966) estimated for Ghana that 19% of the harvest cassava roots are lost annually, and Nicol (1991) estimated a 15–20% loss of dried chips stored for eight months. Pattinson (1968) estimated for Tanzania a 12% weight loss of cassava chips stored for five months, and Hodges et al. (1985) assessed during a field survey postharvest losses of up to 19% after 3 months and up to 63% after four to five months due to the infestation of Prostephanus truncatus (Horn). In Togo, Stabrawa (1991) assessed postharvest weight losses of 5% after one month of storage and 15% after three months of storage due to insect infestation, and Compton (1991) assessed weight losses of about 9% for each store in the survey area in Togo. Wright et al. (1993) assessed postharvest losses of chips of about 14% after four months of storage, about 20% after seven months of storage and up to 30% when P. truncatus attacked the dried chips. In addition, Wright et al. (1993) estimated about 4% of the total national cassava production in Togo is lost during the chip storage. This was about equivalent to 0.05% of the GNP in 1989.
Plant breeding has resulted in cassava that is tolerant to PPD. Sánchez et al. identified four different sources of tolerance to PPD. One comes from Walker's Manihot (M. walkerae) of southern Texas in the United States and Tamaulipas in Mexico. A second source was induced by mutagenic levels of gamma rays, which putatively silenced one of the genes involved in PPD genesis. A third source was a group of high-carotene clones. The antioxidant properties of carotenoids are postulated to protect the roots from PPD (basically an oxidative process). Finally, tolerance was also observed in a waxy-starch (amylose-free) mutant. This tolerance to PPD was thought to be cosegregated with the starch mutation, and is not a pleiotropic effect of the latter.
In Africa, the cassava mealybug (Phenacoccus manihoti) and cassava green mite (Mononychellus tanajoa) can cause up to 80% 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 Center for Africa of the IITA under the leadership of Dr. Hans Rudolf Herren. The Centre investigated biological control for cassava pests; two South American natural enemies Apoanagyrus lopezi (a parasitoid wasp) and Typhlodromalus aripo (a predatory mite) were found to effectively control the cassava mealybug and the cassava green mite, respectively.
The cassava mosaic virus causes the leaves of the cassava plant to wither, limiting the growth of the root. The virus caused a major African famine in the 1920s. 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 has been spreading at a rate of 50 miles per year, and as of 2005 may be found throughout Uganda, Rwanda, Burundi, the Democratic Republic of the Congo and the Republic of the Congo.
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. 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. 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. Other pest 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 storage roots, but plants can have reduced height if there was loss of enlarged root weight.
Research on nematode pests of cassava is still in the early stages; results on the response of cassava is, therefore, not consistent, ranging from negligible to seriously damaging. Since nematodes have such a seemingly erratic distribution in cassava agricultural fields, it is not easy to clearly define the level of direct damage attributed to nematodes and thereafter quantify the success of a chosen management method.
The use of nematicides has been found to result in lower numbers of galls per feeder root compared to a control, coupled with a lower number of rots in the storage roots. The nematicide Femaniphos, when used, did not affect crop growth and yield parameter variables measured at harvest. Nematicide use in cassava is neither practical nor sustainable; currently the use of tolerant and resistant varieties is the most practical and sustainable management method.
|Wikimedia Commons has media related to Manihot esculenta.|
|Look up Cassava in Wiktionary, the free dictionary.|