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Cassava, Manihot esculenta, is a perennial shrub in the family Euphorbiaceae grown primarily for its storage roots which are eaten as a vegetable. The cassava plant is a woody plant with erect stems and spirally arranged simple lobed leaves with petioles (leaf stems) up to 30 cm in length. The plant produces petal-less flowers on a raceme. The edible roots of the plant are usually cylindrical and tapered and are white, brown or reddish in color. Cassava plants can reach 4 m in height and is usually harvested 9-12 months after planting. Cassava may also be referred to as Brazilian arrowroot, manioc, yuca or tapioca and the origins of the plant are unknown. The plant is not known to occur wild but may have first been cultivated in Brazil. Cassava is the third-largest source of food carbohydrates in the tropics, after rice and maize. It 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.
Cassava root is eaten as a vegetable, and as its toxic raw it is eaten after cooking. It is also used as a source of starch or to make flour. The root may also be used in the production of ethanol. Cassava leaves are also a good source of protein and vitamin which can be consumed after cooking.Cassava hay is used as an animal feed or in the production of adhesives, textiles and cosmetics.
Cassava thrives in tropical and subtropical regions of the world as it requires warm temperatures for optimal growth. The plants require at least 8 months of warm weather, thriving in regions with warm, moist climates with regular rainfall. Cassava can be grown in many types of soil, producing even in poor soil but but will be optimally productive in well-draining, sandy clay loam with a pH between 5.5 and 6.5. Cassava is drought resistant but will not tolerate water-logging. Root production is maximized when temperatures are between 25 and 32°C (77–90°F). Cassava should be planted in full sun and is very sensitive to shading, which leads to low yields.
Cassava is propagated from stem cuttings as the tubers do not produce buds. Stem cuttings should only be taken from plants which are free from disease, are at least 10 months old and havee borne tubers. The cuttings should be taken from hardened stems leaving at least 30 cm (11.8 in) of stem intact in the ground. The stem can be severed using a sharp knife, secateurs or saw and each cutting should have 1-2 nodes and be approximately 20 cm (7.9 in) long.
It is a good idea to dip the stem cuttings in an appropriate fungicide prior to planting to help prevent the development of diseases. The cuttings can then either be planted directly into a nursery bed or presprouted in trays or polyethylene bags. To presprout the stems, plant in a cell tray or bag which is filled with good quality soil. Plant one stem in each cell or bag by pushing it into the soil in the direction in which it was growing on the mother plant (oldest part of stem first). The trays should be kept in partial shade until the stems begin to sprout. If planting stem cuttings in a nursery bed (best for cuttings taken from higher up the stems where the wood is not mature), select a site with good quality soil in partial shade and prepare a bed at least 1 m (3.3 ft) wide. The stems can be planted horizontally in a nursery bed and this encourages the growth of multiple stems. Space the cuttings 10 x 10 cm (4 x 4 in) grid. Stem cutting should be watered immediately after planting and on a regular basis thereafter. Aim to keep the soil moist but not wet. Stems should begin to sprout 7-10 days after planting.
Plants propagated from stem cuttings are ready to be transplanted after approximately 4-6 weeks. Prepare the field for planting by cultivating the soil and removing weeds. Space transplants 75–100 cm (2.5–3.2 ft) apart in rows spaced 1–-5 m (3.2–16.4 ft) apart. Fertilize the plants as appropriate. Manure or poultry droppings can be used. Cultivate the soil to remove weeds and break up the soil around the plants.
Cassava is ready to harvest about a year after planting depending on the variety being grown. Some early maturing varieties may be ready to harvest in around nine months. In colder regions, cassava tubers can remain in the ground for up to 2 years before harvesting but can become fibrous so this is not recommended where cassava is being grown for consumption. Cassava tubers are harvested by digging. The roots should be dug up carefully to prevent damage.
Discolored pale green, yellow or white mottled leaves which may be distorted with a reduced size; in highly susceptible cassava cultivars plant growth may be stunted, resulting in poor root yield and low quality stem cuttings. Note that infected plants can express a range of symptoms and the exact symptoms depend on the species of virus and the strain as well as the environmental conditions and and the sensitivity of the cassava host.
Disease is spread by infected cuttings and by whiteflies. The leaves are yellow, mottled and distorted. If leaves are yellow all over but are a normal size or there brown leaves that that does not indicate disease. Wild cassava (kisamvu cha mpira in Kiswahili) also hosts the disease.
The disease was first observed in the late 19th Century in what is now Tanzania. It was not until work in 1938 that the disease transmission was confirmed to occur via grafting as well as vectored by the White fly.
Varieties of cassava resistant to the virus are available in many countries, most traditional varieties of cassava grown in Africa are susceptible to the virus, seek advice from an agricultural extension on suitable varieties for your region (see below). Do not plant cuttings from plants with symptoms of disease; inspect plants regularly for symptoms of disease and remove and destroy any showing symptoms. Infected plants should be uprooted ('rouged'). Replace with disease resistant varieties such as 'Rwizihiza', 'Ndamirabana', 'Cyizere', 'Seruruseke', 'Mavoka', 'Garukunsubire' and 'Mbakungahaze'. There is no agrochemical agent or organic treatment for this disease. There are both control strategies for the whitefly vector.
Oval shaped scales on stems, roots and/or tubers; infections which occur at an early age kill plants and prevent the production of tubers; plant becomes shriveled and discolored at feeding sites
Insect is indigenous to Africa and is particularly prevalent in forest areas of Ghana and Congo
Improve soil by adding organic matter to make soil more fertile; remove and destroy infested stems; do not plant cuttings with scale
Cankers on stems and leaf petioles; leaves drooping downwards; wilting leaves which die and fall from plant leading to plant defoliation; death of shoots; soft parts of plant become twisted and distorted
Disease emerges at the beginning of wet season (Africa) and worsens; spores spread by wind
Anthracnose usually does not cause large-scale economic damage to cassava and control is usually not necessary; avoid planting cuttings with cankers; if disease does occur crop debris should be removed and destroyed after harvest
Circular or irregular brown spots with darker margin between leaf veins on older leaves; centers of lesions may drop out givinf leaves a shothole appearance; if infection is severe, leaves may turn yellow, dry out and drop from the plant
Disease emergence favored by high temperature and humidity
Remove leaves and crop debris from around plants to prevent disease spreading; remove weeds from around crop
Patches of dark brown or gray fungal growth on stems; necrotic areas covering buds on the stem
Disease most commonly found on cassava grown in humid regions (e.g humid forest zones) and is less common in drier savanna areas; fungal spores are carried by wind to new plants and farms
Only plant cassava cuttings taken from healthy plants which are free from necrotic lesions; space plants widely to allow good air circulation around plants and reduce disease incidence; remove weeds around plants; id disease is present, burn all necrotic stems and crop debris immediately after harvest to prevent spread
Small, angular, brown, water-soaked lesions between leaf veins on lower surfaces of leaves; leaf blades turning brown as lesion expands; lesions may have a yello halo; lesions coalesce to form large necrotic patches; defoliation occurs with leaf petioles remaining in horizontal position as leaves drop; dieback of shoots; brown gum may be present on stems, leaves and petioles
Most important bacterial disease of cassava; spread by water splash and infected tools; disease more severe in wet conditions; particularly destructive in South America and Africa; most important method of spread is probably through exchange of infected plant cuttings
Rotate cassava crop with non-host; plow crop debris into soil after harvest or remove and burn it; prune infected parts from plant; propagate cuttings only from healthy plants; intercrop cassava with corn (maize) and melon
Brown elongated necrotic lesions on young stems; chlorotic or necrotic vein banding in mature leaves which may merge later to form large yellow patch; necrosis of tubers; roots develop knots; internal tissues of roots and tubers stained brown and may rot due to secondary fungus infection.
Histroy and origin:
Disease is prevalent in East Africa (Tanzania, Uganda and Kenya). It was first reported in 1930's in Tanzania and become endemic in later years. It is currently travelling from East Africa westwards and has been reported in the DRC. There is considerable concern that it will appear in the major West African growing regions, notably Nigeria.
The virus is transmitted through whiteflies and stem cuttings. The origin of CBSD is suspected to have arisen from the viruses that are already present on the indigenous African flora.
Virus structure and properties:
The microscopic studies revealed that the virus is 650 nm long and earlier it was believe to be carlavirus. But further studies associate the virus to Ipomovirus. Cassva Brown Streak Disease is caused by two distinct species of single-stranded RNA (ssRNA) viruses, Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), belonging to the genus Ipomovirus of the family Potyviridae.
Cassava is an important stable crop in Africa and the continent produces an estimated 54% of the world's cassava production. As such the threat from CBSD seems inevitable due to its presence in many eastern African countries and its rate of transmission westwards to the major growing regions in Nigeria (which produced 30.8% of the world's supply). Though the economic loss from brown streak virus depends on region, cultivars and environment conditions, in general it is estimated up to 70% yield loss in susceptible variety is common with losses as high as 100% being observed in some regions. If the disease is unchecked it may cause 2 billion dollar in loss in Nigeria alone and lead to increase widespread poverty and malnutrition in West and Central Africa.
The first and foremost important aspect is to identify the disease correctly. Cassava brown streak disease varies in symptoms which made it difficult to identify in the field. It makes further complicated if both cassava brown streak and cassava mosaic diseases occur together. There are few techniques like serological and molecular methods are used to identify the virus in laboratory but have their limitations.
Use only healthy and disease free cuttings for planting.
Plant cassava varieties that are more tolerant of brown streak virus such as Garukunsubire and Seruruseke.
Roguing and sanitation:
Remove and destroy any plants which are symptomatic of the disease including alternative hosts.
Early Harvesting of tubers:
Harvest crop early to avoid severe losses due to necrosis of tubers.
Follow proper plant quarantine practices to avoid spread of virus to new region.
Control insect vector:
Whiteflies can be controlled by encouraging beneficial insects in the field like spiders, ladybird beetles etc. Use yellow sticky traps to monitor infestation of whiteflies. Spraying insecticidal soaps under leaf surface to kill flies.
Yellow stipping of leaves; chlorotic spots on leaves; chlorosis of entire leaves; if infestation is very high then leaves may be stunted and deformed; terminal leaves may die and drop from plant; pest responsible is a tiny green mite
Green spider mites are very common pests in most African growing regions and become problematic during the dry season; can cause significant tuber losses
Plant tolerant cassava varieties where possible; plant at the beginning of the rainy season to encourage vigorous growth which allows plant to tolerate attack; intercropping with crops such as cowpea may reduce damage; introductions of the predatory mite Typhlodromalus aripo have been very successful at controlling the green spider mite in many regions of Africa
Leaves on affected plants turning brown and wilting and plant has a scorched appearance; leaves may remain attached to the plant or drop to the ground; plant death will occur; examination of roots reveals root dieback and swelling of tubers; tubers may have light brown, dark gray, blue or pink discoloration; rotting roots may be soft and produce a foul odor; infection by Botryodiplodia fungi may cause the appearance of white fungal structures at the base of the stem, particularly during the wet season
Root rot disease emergence is often favored by waterlogged, poorly-draining soils
Plant cassava in well-draining soils; remove and destroy all crop debris by burning; sanitize all tools after use
Stem surfaces covered with white waxy substance; leaves wilting and dropping from plant; severe infestations may result in stunted plants and poor tuber yields; cutting from infected plants do not sprout; insect is a flattened oval scale with an elongated white cover
Using pesticides to control scale insects reduces populations of beneficial insects such as natural enemies
Plant material that is completely free of scale insects; remove and destroy infested stems from existing plantations; apply organic matter to soil to improve fertility
Defoliated plants; bark removed from stems; insects are large brightly colored grasshoppers
Grasshoppers have a wide host range and mainly attack seedlings; cooperation between neighboring farms can help with grasshopper control as the insect tends to lay its eggs outwith the plantation with nymphs migrating to the crop to feed
Hand pick any grasshoppers found on plants; locate any egg pods around cassava field and destroy to reduce grasshopper populations; biopesticides such as "Green Muscle" are available in South and West Africa which are very effective at reducing the grasshopper population; products containing neem have also given good control of variegated grasshoppers
Galls on roots which can be up to 3.3 cm (1 in) in diameter but are usually smaller; reduction in plant vigor; yellowing plants which wilt in hot weather
Galls can appear as quickly as a month prior to planting; nematodes prefer sandy soils and damage in areas of field or garden with this type of soil is most likely
Plant resistant varieties if nematodes are known to be present in the soil ;check roots of plants mid-season or sooner if symptoms indicate nematodes; solarizing soil can reduce nematode populations in the soil and levels of inoculum of many other pathogens
Leaves stippled with yellow; leaves may appear bronzed; webbing covering leaves; mites may be visible as tiny moving dots on the webs or underside of leaves, best viewed using a hand lens; usually not spotted until there are visible symptoms on the plant; leaves turn yellow and may drop from plant
Spider mites thrive in dusty conditions; water-stressed plants are more susceptible to attack
Spraying plants with a strong jet of water can help reduce buildup of spider mite populations; if mites become problematic apply insecticidal soap to plants; certain chemical insecticides may actually increase mite populations by killing off natural enemies and promoting mite reproduction
Large, diffuse white spots on upper surface of leaves; spots with grey center on underside of leaves in humid weather
Prevalent in wetter, warmer regions where cassava is grown e.g. Brazil and Colombia; common in West Africa
Remove leaves and crop debris from around plants to prevent disease spreading; remove weeds from around crop
Plants are stunted with an excessive proliferation of branches; shoots have small leaves and shortened internodes; no chlorosis is present; cuttings from the shoots are weak but show no visible symptoms; few shoots successfully grow from cuttings.
Disease has been detected in Vietnam, Thailand, Cambodia, Laos, Indonesia, and the Philippines. The disease will cause yield loss from 30 to 80 percent.
Remove and destroy and plants suspected of being infected; remove all cassava debris from field after harvest; disinfect all tools and equipment between cuttings.
Adekunle, A. A., Dixon, A., Ojurongbe, J, Ilona, P, Muthada, L & Adisa, S. Growing Cassava Commercially in Nigeria. Information and Communication Support for Agricultural Growth in Nigeria (ICS-Nigeria). Available at: http://www.cassavabiz.org/agroenterpr.... [Accessed 10 November 14]. Free to access.
CABI Crop Protection Compendium. (2008). Manihot esculenta datasheet. Available at: http://www.cabi.org/cpc/datasheet/17585. [Accessed 05 November 14]. Paid subscription required.
James, B., Yaninek, J, Tumanteh, A., Maroya, N., Dixon, A., Salawu, R. & Kwarteng, J. (2000). Starting a Cassava Farm. IPM Field Guide for Extension Agents. International Institute for Tropical Agriculture. Available at: http://www.infonet-biovision.org/res/.... [Accessed 10 November 14]. Free to access.