Cassava brown streak viruses (cassava brown streak disease)
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- Risk of Introduction
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Growth Stages
- List of Symptoms/Signs
- Biology and Ecology
- Seedborne Aspects
- Vectors and Intermediate Hosts
- Similarities to Other Species/Conditions
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Cassava brown streak viruses
Preferred Common Name
- cassava brown streak disease
Other Scientific Names
- cassava brown streak carlavirus
- Cassava brown streak potyvirus
- Cassava brown streak virus
- cassava brown streak-associated (?) carlavirus
- Ugandan cassava brown streak virus
International Common Names
- English: cassava brown streak
- CBSAV0 (Cassava brown streak-associated carlavirus
Taxonomic TreeTop of page
- Domain: Virus
- Unknown: "Positive sense ssRNA viruses"
- Unknown: "RNA viruses"
- Family: Potyviridae
- Genus: Ipomovirus
- Species: Cassava brown streak viruses
Notes on Taxonomy and NomenclatureTop of page
Cassava brown streak disease (CBSD) was previously thought to be caused by either one or a complex of two viruses (then designated Cassava brown streak-associated virus and Cassava brown streak virus) (Lennon et al., 1985). It was then shown by molecular characterization that Cassava brown streak virus (CBSV) alone causes the disease (Monger et al., 2001a, b). Genome analysis of virus isolates collected in cassava-growing regions of East Africa provided evidence that CBSD is caused by two distinct virus species for which Koch’s postulates were fulfilled (Winteret al., 2010). These viruses are now named: Cassava brown streak virus and Ugandan cassava brown streak virus (UCBSV). The genome properties of CBSV and UCBSV are similar to those of Cucumber vein yellowing virus (Lecoq et al., 2000) and Squash vein yellowing virus (Adkinset al., 2008) and thus to be a member of the Potyviridae family and species of the genus Ipomovirus although the status of Ugandan brown streak virus as a species is pending ICTV approval.
DescriptionTop of page
CBSVs have slightly flexuous particles with a modal length of ca 650 nm. Particles contain single-stranded genomic RNA of approximately 9008-9070 nt which is encapsidated in a coat protein of ca 43 kDa for UCBSV and ca 45 kDa for CBSV (Winteret al., 2010). The genome structure of the cassava brown streak viruses is unique because of a MAf/Ham1-like sequence inserted upstream the coat protein gene and a P1 gene to which gene silencing suppression function was assigned.
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Burundi||Present||Bigirimana et al., 2011; CABI/EPPO, 2013; EPPO, 2014|
|Congo Democratic Republic||Present||Mulimbi et al., 2012; CABI/EPPO, 2013|
|Equatorial Guinea||Present||Monger et al., 2001b|
|Kenya||Widespread||Nichols, 1950; Bock, 1994; Munga and Thresh, 2002; CABI/EPPO, 2013; EPPO, 2014||CBSV, UCBSV|
|Malawi||Widespread||Nichols, 1950; Thresh et al., 1994; CABI/EPPO, 2013; EPPO, 2014|
|Mayotte||Present||Roux-Cuvelier et al., 2014|
|Mozambique||Present||Monger et al., 2001b; Hillocks et al., 2002; Thresh and Hillocks, 2003; CABI/EPPO, 2013; EPPO, 2014|
|Rwanda||Present||FAO, 2011; CABI/EPPO, 2013|
|Tanzania||Widespread||STOREY, 1936; Legg and Raya, 1998; CABI/EPPO, 2013; EPPO, 2014|
|Uganda||Present||Alicai et al., 2007; CABI/EPPO, 2013; EPPO, 2014|
|Zambia||Present||Mulenga et al., 2018|
|Zimbabwe||Absent, unreliable record||CABI/EPPO, 2013; EPPO, 2014|
Risk of IntroductionTop of page
There is no known risk from the movement of seed but movement of vegetative propagation materials is a serious hazard as shown from recent records (Bigirimana et al., 2012; Mulimbi et al., 2012). Measures are required to restrict the movement of infected cuttings within and from countries where CBSD occurs. It is particularly important to prevent entry of infected material from affected to unaffected countries of Africa and to other cassava-growing regions of Asia and the Americas where CBSD has not yet been reported.
Hosts/Species AffectedTop of page
The following plant species have been used as experimental hosts for CBSV: Nicotiana tabacum, N. rustica, N. glutinosa, N. debneyi, N. benthamiana, Datura stramonium, Petunia hybrida, Chenopodium quinoa and C. amaranticolor.N. benthamiana shows a differential reaction to the virus species. Necrotic lesions are a response to infection with CBSV isolates which eventually results in plant deterioration and death whereas UCBSV isolates cause chlorotic mottle and leaf curl symptoms (Winter et al., 2010).
Host Plants and Other Plants AffectedTop of page
Growth StagesTop of page Flowering stage, Fruiting stage, Post-harvest, Vegetative growing stage
SymptomsTop of page
The name brown streak was given to the disease from the brown lesions which sometimes appear on young green stems. These were the first symptoms of the disease to be recognised; however stem lesions are not the most characteristic symptom of infection and occur only infrequently.
Unlike symptoms induced by the majority of plant viruses, those of CBSD in cassava normally affect mature or nearly mature leaves but not expanding, immature leaves. They consist of a characteristic yellow or necrotic vein banding which may enlarge and coalesce to form comparatively large, yellow patches. Tuberous root symptoms may also be present: these consist of dark-brown necrotic areas within the tuber and reduction in root size; lesions in roots can result in post-harvest spoilage of the crop. Leaf and/or stem symptoms can occur without the development of tuber symptoms; thus, of plants with above-ground symptoms surveyed in southern Tanzania, 21% failed to develop root necrosis (Hillocks et al., 1996).
The symptoms of the disease vary greatly with variety and environmental conditions, making diagnosis difficult, particularly when plants are infected both with CBSD and cassava mosaic disease.
The virus species UCBSV and CBSV cannot be differentiated according to symptoms induced in cassava. However several cassava cultivars are not susceptible to UCBSV (Winter et al., 2010).
List of Symptoms/SignsTop of page
|Leaves / abnormal colours|
|Leaves / abnormal leaf fall|
|Leaves / abnormal patterns|
|Stems / discoloration of bark|
|Vegetative organs / internal rotting or discoloration|
Biology and EcologyTop of page
CBSVs are disseminated widely in infected cuttings. Natural spread of CBSD also occurs. Thus, seedling progenies sometimes become infected in circumstances which preclude infection by seed transmission, and West African varieties and other introductions from areas where CBSD does not occur have also become infected when grown in infected localities in Malawi, Tanzania and Kenya. There is also published evidence on rates of spread which are variable from site to site and between seasons.
The whitefly Bemisia tabaci is confirmed as the vector of the virus (Maruthi et al., 2005; Mware et al., 2009).
There is little epidemiological information on the rates and pattern of spread of CBSD or on the role of wild or weed hosts of infection.
Seedborne AspectsTop of page There is no evidence of transmission in or on true seed.
Vectors and Intermediate HostsTop of page
ImpactTop of page
CBSD was formerly of limited importance in Africa as a whole because of its restricted distribution along lowland coastal areas of Kenya and Tanzania and in parts of Malawi (Legg and Raya, 1998; Hillocks et al., 2001; Gondwe et al., 2003; Shaba et al., 2003). However, the disease was later found to be prevalent in northern Mozambique (Hillocks et al., 2002). Since 2004 it has been causing increasingly severe problems in Uganda and the Lake Victoria zone of Tanzania. It has also been detected recently in Rwanda, Burundi and parts of DRC.
There is only limited evidence as to the effects of CBSD on vegetative growth and on the yield of tuberous roots (Bock, 1994; Hillocks et al., 2001). Varieties differ greatly in their sensitivity and response to infection. The growth and yield of sensitive varieties are severely affected as there is extensive dieback of the stems, and the tuberous roots develop extensive necrosis and rot to such an extent that they are virtually worthless. Thus, field trials have shown that CBSV can decrease root yields of the most sensitive varieties by 70% and induce necrosis of roots which renders them unsalable (Hillocks et al., 2001).Tolerant varieties are much less severely affected and there is little effect on root yield or quality; cv. Nachinyaya, a local cultivar in coastal areas of southern Tanzania has a form of tolerance in which leaf symptoms are produced but the development of root necrosis is so delayed that the full potential yield is obtainable (Hillocks et al., 2001).
The results of three surveys conducted in Malawi during 2001-2002 showed that CBSD was widely distributed and the severity was high in low-lying areas, particularly along the shores of Lake Malawi, with farmers estimating losses as high as 60%. In a study of the economic impact of CBSD on small-scale farmers in Malawi, Gondwe et al. (2003) reported yield losses of 18-25%.
DiagnosisTop of page
Antisera are available for the serological diagnosis of UCBSV and CBSV (www.dsmz.de) and can be used for virus detection in cassava. Reverse transcription-PCR protocols have been developed that can detect virus in young symptomless leaves of infected cassava (Abarshiet al., 2010; Adams et al., 2012). However, sequence variability of the viruses can interfere with virus detection and caution on the use of molecular tests also has to be exercised.
Similarities to Other Species/ConditionsTop of page
CBSV and UCBSV are distinct virus species of the genus Ipomovirus family Potyviridae. The genome organisation is similar to the Bemisia tabaci-transmitted Cucumber vein yellowing virus (Lecoq et al., 2000; Janssenet al., 2007) and Squash vein yellowing virus (Adkinset al., 2008). The diverging genome organisation and genetic relatedness shows only a limited relationship with Sweet potato mild mottle virus.
Prevention and ControlTop of page
CBSD has until recently received such limited research attention that current control recommendations are largely based on general pathological principles and experience rather than on extensive research (Hillocks, 1997).
Cassava is usually propagated vegetatively from hardwood stem cuttings, and CBSD is perpetuated and disseminated in this way. There is a considerable traffic in plant material within and sometimes between countries and inevitably CBSVs are disseminated in this way. Until recently, selected 'virus-free' planting material was seldom available, until the Gates-funded Great Lakes Cassava Initiative in the six countries of Kenya, Uganda, Tanzania, Burundi, Rwanda and eastern parts of DRC, except when produced specially for official introductions using meristem-tip therapy and/or heat therapy in order to meet quarantine requirements. It is important that such requirements are strictly enforced because of the present limited distribution of CBSD in parts of eastern, central and southern Africa.
A basic approach to control should be the use of virus-free planting material (Thresh et al., 1994). With CBSD there are difficulties in selecting virus-free material because the symptoms of infection can be vague and indistinct. However, effective means of detection are now available and being utilized. Furthermore, it is likely that considerable improvement could be made in the present unsatisfactory situation by selecting cuttings for propagation only from plants that have been inspected during growth and also at harvest and found to be free of leaf, stem and root symptoms. Foundation stocks could be established in this way for subsequent multiplication and distribution to farmers. There are also advantages in farmers selecting propagating material only from unaffected plants at the time they collect cuttings. Unfortunately, however, farmers are not familiar with the whole range of CBSD symptoms, and cuttings may be collected at times when the source plants are almost leafless or severely affected by the cassava green mite (Mononychellus tanajoa), cassava mealybug (Phenacoccus manihoti), leaf spot or bacterial blight. Further studies are required on the most effective means of selection and on the ways in which simple procedures can be introduced to farmers.
The differences between varieties in their sensitivity to infection have long been exploited by farmers in coastal Tanzania and elsewhere who tend to discard those severely affected by CBSD and retain those that grow and yield satisfactorily even when infected. There is certainly a high turnover in cassava varieties as recorded in many parts of Africa and influenced by various pests and diseases (Nweke et al., 1994).
Early experience in Tanzania showed the scope for breeding resistant or tolerant varieties. Moreover, in crop improvement programmes there is a need to evaluate the response of introduced genotypes before they are released for use by farmers in areas where CBSD is prevalent.
Integrated Pest Management
There is a general acceptance of the need to develop overall IPM programmes for the whole range of cassava pests and diseases (Thresh et al., 1994). However, little progress has been made in developing such programmes, even though there are likely to be important interactions between CBSD and other diseases, and also with cassava mealybug and green mite.
ReferencesTop of page
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ContributorsTop of page
22/01/13 Review by:
Stephan Winter, Plant Virus Department, DSMZ, Messeweg 11/12, 38104 Braunschweig, Germany
21/12/11 Review by:
Mike Thresh, Consultant, UK
Distribution MapsTop of page
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