R. J. Hillocks

Cassava: biology, production and utilization.

Origin, distribution and economic importance botany, crop physiology and agronomy genetics and crop improvement crop protection crop utilization.

Cassava botany and physiology.

Cassava is a perennial shrub of the family Euphorbiaceae, cultivated mainly for its starchy roots. It is one of the most important food staples in the tropics, where it is the fourth most important source of energy. On a worldwide basis it is ranked as the sixth most important source of calories in the human diet (FAO, 1999). Given the crop’s tolerance to poor soil and harsh climatic conditions, it is generally cultivated by small farmers as a subsistence crop in a diverse range of agricultural and food systems. Although cassava is a perennial crop, the storage roots can be harvested from 6 to 24 months after planting (MAP), depending on cultivar and the growing conditions (El-Sharkawy, 1993). In the humid lowland tropics the roots can be harvested after 6–7 months. In regions with prolonged periods of drought or cold, the farmers usually harvest after 18–24 months (Cock, 1984). Moreover, the roots can be left in the ground without harvesting for a long period of time, making it a very useful crop as a security against famine (Cardoso and Souza, 1999). Cassava can be propagated from either stem cuttings or sexual seed, but the former is the commonest practice. Propagation from true seed occurs under natural conditions and is widely used in breeding programmes. Plants from true seed take longer to become established, and they are smaller and less vigorous than plants from cuttings. The seedlings are genetically segregated into different types due to their reproduction by cross-pollination. If propagated by cuttings under favourable conditions, sprouting and adventitious rooting occur after 1 week.

Cassava mineral nutrition and fertilization.

Cassava is generally grown by poor farmers living in marginal areas with adverse climatic and soil conditions. The crop is very suitable for these conditions because of its exceptional tolerance to drought and to acid, infertile soils. It is often grown on sloping land because of its minimal requirement for land preparation, and its ability to produce reasonably good yields on eroded and degraded soils, where other crops would fail. It has been shown (Quintiliano et al., 1961; Margolis and Campos Filho, 1981; Putthacharoen et al., 1998), however, that growing cassava on slopes can result in severe erosion, with high soil and nutrient losses. Thus cassava cultivation on slopes requires adequate cultural and soil conservation practices that minimize erosion (Howeler, 1994). Cassava is well adapted to poor or degraded soils because of its tolerance to low pH, high levels of exchangeable aluminium (Al) and low concentrations of phosphorus (P) in the soil solution. Studying the effect of pH on the growth of several crops grown in flowing nutrient solution, Islam et al. (1980) reported that cassava and ginger (Zingiber officinale) were more tolerant of low pH (< 4) than tomatoes (Lycopersicon esculentum), wheat (Triticum aestivum L.) or maize (Zea mays L.). Centro Internacional de Agricultura Tropical (CIAT; 1978) and Howeler (1991a) also reported that cassava and cowpeas (Vigna unguiculata) were more tolerant of acid soils with high levels of exchangeable Al, and were much less responsive to lime applications than common beans (Phaseolus vulgaris), rice (Oryza sativa), maize and sorghum (Sorghum vulgaris).

Cassava brown streak disease: A review of present knowledge and research needs

Cassava (Manihot esculenta) is an important subsistence food crop in Africa where it is affected by two main virus diseases, cassava mosaic disease (CMD) and cassava brown streak disease (CBSD). CMD occurs in all the cassava-growing countries on the continent and it has been much researched. CBSD occurs mainly on the East African coast, and although the disease was first reported in 1936, it has received much less attention than CMD. The aetiology and epidemiology of CBSD are still not fully elucidated, and no insect vector has been identified. There is renewed interest in CBSD owing to the discovery of its widespread occurrence in southern Tanzania and Mozambique, where the associated symptom of root necrosis is a major cause of economic loss in the cassava crop. In view of the present importance of the disease in eastern Africa, it is timely to produce a review of work carried out on the disease since it was first described.

Cassava in Africa.

South America, probably the Amazon region, may have been the centre of origin for species that gave rise to Manihot esculenta. While there is some controversy regarding the exact botanical origins of the progenitors of modern cultivated cassava, the archaeological evidence points to the Amazon region as the centre of domestication (see Chapter 1). In the 16th century, Portuguese navigators took cassava from Brazil to the west coast of Africa (Jones, 1959) and later to East Africa through Madagascar and Zanzibar (Jennings, 1976). Although cassava appears to have been grown in Fernando Po in the Gulf of Benin and around the mouth of the Congo River by the end of the 16th century, it did not spread much in West Africa until the 20th century. Cassava was unknown north of the river Niger before 1914 (Purseglove, 1968). Some local spread seems to have taken place along rivers by Africa traders and travellers in the 17th century. Cassava was taken from Brazil to Reunion off the East African coast in 1736 and was recorded in Zanzibar in 1799 (Purseglove, 1968). With the exception of the coastal region, cassava was not widely grown in East Africa until the late part of the 18th or early 19th century. The explorer Speke found no cassava on the western shore of Lake Victoria when he went there in 1862, but Stanley recorded it in Uganda in 1878. Cassava may have reached Lake Victoria along trade routes from the east (Jameson and Thomas, 1970) or from the west (Purseglove, 1968). The crop became established in Uganda during the 19th century and its value as a food security crop was soon realized. Records show that in 1963/64, around 175,000 ha were grown in Uganda, where the drier regions to the east and north were the largest producers (Jameson and Thomas, 1970). Most of the spread of cassava in Africa away from the coast and riverside trading posts took place during the 20th century due to the colonial powers encouraging its cultivation as a reserve against famine and the ability of the crop to survive locust attack. Cassava is now grown in all African countries south of the Sahara and north of the Limpopo River. In 1972, the International Institute of Tropical Agriculture (IITA) was inaugurated with its headquarters in Ibadan, Nigeria, under the auspices of the Consultative Group on International Agricultural Research (CGIAR). IITA shares the global mandate for cassava with the Centro Internacional de Agricultura Tropical (CIAT) in Colombia and is responsible for developing the crop in Africa.

Breeding for Crop Improvement

Cassava has been evolving as a food crop ever since it became important in the second and third millennium BC (Reichel-Dolmatoff, 1965; Lathrap, 1973), but its adaptation to African and Asian conditions did not begin until postColumbian times. In the Americas, Africa and Asia, progress towards improved adaptation and quality was first through subconscious selection by farmers. A wide range of genetic diversity was generated through centuries of such farmer selection (Bonierbale et al., 1995). It was not until the present century that serious attempts began by national organizations to improve the crop by plant breeding. Much of this was instigated by the colonial powers and was very successful, but progress slowed considerably when countries became independent. This trend was arrested in the 1960s, when the increasing world population and the limited supply of energy foods prompted a surge of interest in the crop. High priority was given to cassava breeding and related research when the International Institute of Tropical Agriculture (IITA) was opened in Nigeria and the Centro Internacional de Agricultura Tropical (CIAT) was opened in Colombia. For the first time breeders and associated scientists were given resources to study the crop in depth and to assess the extensive variation available. The two International Centres collaborated with existing national programmes and instigated the initiation of new ones. In India the Central Tuber Crops Research Institute (CTCRI) took on a similar role. The objectives were to increase both the yield per unit area and the area under cultivation, and also to improve root quality.

The association between root necrosis and above‐ground symptoms of brown streak virus infection of cassava in southern Tanzania

Abstract A survey was conducted in southern Tanzania to determine the relationship between above‐ground symptoms of cassava brown streak virus (CBSV) infection and root necrosis which is often associated with the disease. CBSV symptoms were found in 62 of the 64 fields sampled. Seventy‐nine per cent of plants with above‐ground symptoms of CBS also exhibited root necrosis, compared with only 18% of plants with no visible symptoms of CBSV. No fungi were isolated on acidified water agar from more than half the root samples taken from plants with root necrosis. Where fungi were isolated they represented a range of common soil‐borne fungi, and no single fungus predominated. No evidence could therefore be found to indicate a cause of root necrosis other than brown streak virus. Some varietal differences in response to CBSV were apparent from the survey and at least one variety developed above‐ground symptoms but not root necrosis.

The viruses and virus diseases of cassava.

Crops that are propagated vegetatively are particularly prone to damage by viruses as infection tends to build up in successive cycles of propagation. Cassava is no exception to this generalization and at least 16 different viruses have been isolated from the crop. Moreover, other as yet undescribed viruses are likely to occur and may even be prevalent in some areas. This is because cassava has received far less attention from virologists than it merits as one of the world’s most important and widely grown food crops. A full list of the viruses that have been isolated from cassava is presented in Table 12.1 and key references appear in the bibliography. The viruses asterisked in the table have been detected somewhat fortuitously in studies undertaken for other reasons. There is only limited information on the properties, distribution, effects and importance of these viruses. They require further attention, but meanwhile they should be considered in operating quarantine controls on the movement of vegetative propagules between different cassava-growing areas. These viruses are not considered further here and the main emphasis is on those known to cause diseases of economic importance. A feature of cassava viruses is that they are of diverse taxonomic groups (Table 12.1). Another is that their known distribution is largely or entirely restricted to only one of the continents in which cassava is grown, or to an even more localized geographic area. For this reason the viruses and virus diseases of Africa, South/Central America and the Indian subcontinent are considered separately.

Optimization of diagnostic RT-PCR protocols and sampling procedures for the reliable and cost-effective detection of Cassava brown streak virus

Sampling procedures and diagnostic protocols were optimized for accurate diagnosis of Cassava brown streak virus (CBSV) (genus Ipomovirus, family Potyviridae). A cetyl trimethyl ammonium bromide (CTAB) method was optimized for sample preparation from infected cassava plants and compared with the RNeasy plant mini kit (Qiagen) for sensitivity, reproducibility and costs. CBSV was detectable readily in total RNAs extracted using either method. The major difference between the two methods was in the cost of consumables, with the CTAB 10x cheaper (0.53 pounds sterling=US

Transcriptional response of virus-infected cassava and identification of putative sources of resistance for cassava brown streak disease.

0.80 per sample) than the RNeasy method (5.91 pounds sterling=US