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The Viruses and Virus Diseases of Cassava Color profile: Disabled Composite Default screen Chapter 12 The Viruses and Virus Diseases of Cassava L.A. Calvert1 and J.M. Thresh2 1Centro Internacional de Agricultura Tropical (CIAT), A.A. 6713, Cali, Colombia; 2Natural Resources Institute, University of Greenwich, Chatham Maritime, Kent ME4 4TB, UK Introduction A feature of cassava viruses is that they are of diverse taxonomic groups (Table 12.1). Crops that are propagated vegetatively are Another is that their known distribution is particularly prone to damage by viruses as largely or entirely restricted to only one of the infection tends to build up in successive cycles continents in which cassava is grown, or to an of propagation. Cassava is no exception to this even more localized geographic area. For this generalization and at least 16 different viruses reason the viruses and virus diseases of Africa, have been isolated from the crop. Moreover, South/Central America and the Indian sub- other as yet undescribed viruses are likely to continent are considered separately. 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 The Viruses and Virus Diseases food crops. of Cassava in South and A full list of the viruses that have been Central America isolated from cassava is presented in Table 12.1 and key references appear in the bibliography. Cassava originated in the Neotropics and was The viruses asterisked in the table have been not introduced to other regions until relatively detected somewhat fortuitously in studies under- recently. This may explain why only one of the taken for other reasons. There is only limited viruses of cassava occuring in South and Central information on the properties, distribution, effects America has been found elsewhere. Moreover, and importance of these viruses. They require several of the Neotropical viruses of cassava do further attention, but meanwhile they should be not cause symptoms and have no obvious considered in operating quarantine controls on deleterious effects, which may reflect a long the movement of vegetative propagules between period of co-evolution between the host and its different cassava-growing areas. These viruses pathogens. are not considered further here and the main Three virus diseases justify detailed atten- emphasis is on those known to cause diseases of tion here. Three other viruses are listed in Table economic importance. 12.1 and are considered briefly, but they do not ©CAB International 2002. Cassava: Biology, Production and Utilization (eds R.J. Hillocks, J.M. Thresh and A.C. Bellotti) 237 249 Z:\Customer\CABI\A4101 - Hillocks - Cassava\A4212 - Hillocks - Cassava #R.vp Monday, February 04, 2002 11:23:06 AM Color profile: Disabled Composite Default screen 238 L.A. Calvert and J.M. Thresh Table 12.1. The viruses of cassava. Africa African cassava mosaic virus (Geminiviridae: Begomovirus) East African cassava mosaic virus (Geminiviridae: Begomovirus) South African cassava mosaic virus (Geminiviridae: Begomovirus) Cassava brown streak virus (Potyviridae: Ipomovirus) Cassava Ivorian bacilliform virus* (unassigned) Cassava Kumi viruses* Cassava ‘Q’ virus* Cassava common mosaic virus* (Potexvirus) South/Central America Cassava common mosaic virus (Potexvirus) Cassava virus X (Potexvirus)* Cassava vein mosaic virus (Caulimoviridae) Cassava Colombian symptomless virus (Potexvirus)* Cassava American latent virus (Comoviridae: Nepovirus)* Cassava frogskin ‘virus’ Asia/Pacific Cassava common mosaic virus* (Potexvirus) Indian cassava mosaic virus (Geminiviridae: Begomovirus) Cassava green mottle virus* (Comoviridae: Nepovirus) Officially recognized viruses are given in italics, together with family and genus in parentheses. Viruses that are unimportant or for which little information is available are asterisked and not considered in detail in the accompanying text. (Source: Thresh et al., 1994b; Thresh et al., 1998c.) cause symptoms, appear to be unimportant and relatively cool periods and cassava grown in are not discussed further. the semitropical areas of South America is most affected by the disease. In these relatively cool conditions, the affected plants are sometimes Cassava common mosaic disease stunted and yield losses can be up to 60% (Costa and Kitajima, 1972b). History Distribution and prevalence Cassava common mosaic disease (CsCMD) was first reported in southern Brazil (Silberschmid, CsCMD has been reported from many South 1938; Costa, 1940). The disease has since been American countries, but was not recorded in recorded in other South American countries a survey of cassava-growing areas of Colombia and there is one report from Africa (Aiton et al., (Nolt et al., 1992). The disease is most prevalent 1988) and another from Asia (Chen et al., in southern Brazil and Paraguay. In these 1981). CsCMD has no known vector and spread regions the disease is important and phyto- in the field is attributed to mechanical trans- sanitary control measures are recommended mission. The disease is generally of only minor to reduce losses. More than 1000 cassava importance, although there are some areas accessions in the EMBRAPA/CNPMF (Empresa where it is prevalent and control efforts are Brasileira de Pesquisa Agropecuaria/Centro needed. Nacional de Pesquisa en Mandioca y Fruti- cultura) collection at Cruz das Almas in Bahia, Symptoms northeast Brazil, have been tested for the causal virus and the incidence was < 1%. Leaves of cassava plants affected by CsCMD develop mosaic and chlorotic symptoms (Plate Aetiology 1a). On some of the affected leaves there are dark and light green patches that are delimited CsCMD is caused by Cassava common mosaic virus by veins. Symptoms are most severe during (CsCMV) which can infect species belonging to 250 Z:\Customer\CABI\A4101 - Hillocks - Cassava\A4212 - Hillocks - Cassava #R.vp Monday, February 04, 2002 11:23:06 AM Color profile: Disabled Composite Default screen Viruses and Virus Diseases of Cassava 239 several families of dicotyledonous plants (Silva Cassava vein mosaic disease et al., 1963; Kitajima et al., 1965). The virus was ascribed originally to the potexvirus group that History is now referred to as the genus Potexvirus. The first report of cassava vein mosaic disease The CsCMV virion is a semi-flexuous rod (CVMD) was in 1940 (Costa, 1940). The areas that is c.15× 495 nm (Kitajima et al., 1965) and where this disease is most prevalent are remote contains RNA (Silva et al., 1963). Nuclear inclu- and the conditions are semiarid. The region is sions typical of the potexviruses can be found inhabited mostly by poor rural communities and in cassava and the herbaceous host Nicotiana the lack of economic resources has contributed benthamiana. CsCMV is known to systemically to the incomplete knowledge about this disease. infect cassava, Euphorbia spp., Cnidoscolus aconi- Probably because the symptoms are sporadic tifolius (chaya), N. benthamiana and species of and generally less apparent at the end of the several other dicotyledonous families (Costa and cassava growth cycle, this disease has received Kitajima, 1972a). inadequate attention, especially considering the The viral particles of CsCMV contain a single large area now known to be affected. coat protein having a relative molecular weight of 26,000 daltons (Nolt et al., 1991). The CsCMV genome is single-stranded RNA and the complete Symptoms sequence is known (Calvert et al., 1996). The organizational structure, proteins and their The leaf symptoms of CVMD occur in flushes. predicted weights are similar to those of other After an infected stem cutting sprouts, the potexviruses. first four to six leaves express vein chlorosis that appears as a chevron pattern or coalesces to form ringspots (Plate 1b). Leaf deformation Epidemiology and control and epinasty are common severe symptoms. Plants then appear to ‘grow out’ of the infection There are no known vectors of CsCMV and the and produce several symptomless leaves. These primary source of inoculum is infected planting are followed by another series of leaves with material. The virus is systemic in cassava and symptoms. The expression of symptoms is almost all stem cuttings are infected when influenced by the climatic conditions prevailing. obtained from an infected plant. CsCMV is very Symptoms are more pronounced in the semi- stable and can be spread by mechanical trans- arid areas as compared to those expressed by mission on machetes and other implements the same variety grown in the wetter coastal used to prepare cuttings. Although this mode of regions of northeast Brazil. Except for the period transmission is inefficient, it is the only known just after sprouting, CVMD does not seem to means of plant-to-plant spread. affect plant vigour. The affected leaves senesce Eliminating (roguing) plants that express and fall prematurely from the plants which CsCMV symptoms provides adequate control. reduces leaf area. As infected cassava matures, it The symptoms are usually obvious on the first is often difficult to see any leaves with mosaic leaves produced by infected stem cuttings. This is symptoms. the best time to identify and remove diseased plants. If the plants are not rogued early they should be marked and the stems burned later Distribution and prevalence after harvesting the tuberous roots. Only healthy plants should be selected as a source of vegetative CVMD is very common in the semiarid zone propagules. To minimize the risk of mechanical of northeastern Brazil, although there are also transmission, cutting tools should be disinfected reports from other regions of the country. The at regular intervals (Lozano and Nolt, 1989). disease is common in the Brazilian States of With care in selecting planting material, CsCMD Ceará, Pernambuco, Alagoas, Piaui and Bahia can be eradicated or reduced to a level of minor (Calvert et al., 1995) and the distribution economic significance. extends into some of the neighbouring states. 251 Z:\Customer\CABI\A4101 - Hillocks - Cassava\A4212 - Hillocks - Cassava #R.vp Monday, February 04, 2002 11:23:06 AM Color profile: Disabled Composite Default screen 240 L.A.
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