View source for Biology:Cassava mosaic virus

{{Short description|Genus of viruses}}
{{Paraphyletic group
| name = Cassava mosaic virus
| image = Forestryimages cassava.jpg
| image_alt = Artistic rendering of symptoms of a cassava mosaic virus in Guinea-Bissau
| image_caption = Artistic rendering of symptoms of a cassava mosaic virus in Guinea-Bissau
| auto = virus
| parent = Begomovirus
| includes = 
*''African cassava mosaic Burkina Faso virus''
*''[[Biology:African cassava mosaic virus|African cassava mosaic virus]]''
*''Cassava mosaic Madagascar virus''
*''East African cassava mosaic Cameroon virus''
*''East African cassava mosaic Kenya virus''
*''East African cassava mosaic Malawi virus''
*''[[Biology:East African cassava mosaic virus|East African cassava mosaic virus]]''
*''East African cassava mosaic Zanzibar virus''
*''[[Biology:Indian cassava mosaic virus|Indian cassava mosaic virus]]''
*''South African cassava mosaic virus''
*''Sri Lankan cassava mosaic virus''
| excludes = 
All the 398 other species of ''Begomovirus''
}}

'''Cassava mosaic virus''' is the common name used to refer to any of eleven different species of plant pathogenic [[Biology:Virus|virus]] in the genus ''[[Biology:Begomovirus|Begomovirus]]''.  ''[[Biology:African cassava mosaic virus|African cassava mosaic virus]]'' (ACMV), ''[[Biology:East African cassava mosaic virus|East African cassava mosaic virus]]'' (EACMV), and ''South African cassava mosaic virus'' (SACMV) are distinct species of circular single-stranded [[Biology:DNA virus|DNA virus]]es which are transmitted by [[Biology:Whitefly|whiteflies]] and primarily infect [[Biology:Cassava|cassava]] plants; these have thus far only been reported from Africa. Related species of viruses (''[[Biology:Indian cassava mosaic virus|Indian cassava mosaic virus]]'', ICMV) are found in India and neighbouring islands (''Sri Lankan cassava mosaic virus'', SLCMV), though cassava is cultivated in Latin America as well as Southeast Asia. Nine species of cassava-infecting [[Biology:Geminiviridae|geminiviruses]] have been identified between Africa and India based on genomic sequencing and [[Biology:Phylogenetics|phylogenetic]] analysis. This number is likely to grow due to a high rate of natural transformation associated with CMV.<ref name="Patil2009" />

The viruses are members of the family [[Biology:Geminiviridae|Geminiviridae]] and the genus ''[[Biology:Begomovirus|Begomovirus]]''. The first report of cassava mosaic disease (CMD) was from East Africa in 1894.<ref name="Legg2004" /> Since then, epidemics have occurred throughout the African continent, resulting in great economic loss and devastating famine.<ref name="Legg2004" /> In 1971, a resistant line of cassava, the predominant [[Biology:Host|host]] of the pathogen, was established and used by the [[Organization:International Institute of Tropical Agriculture|International Institute of Tropical Agriculture]] in Nigeria. This resistance worked as an effective control for many years. However, in the late 20th century, a more virulent virus broke out in Uganda and quickly spread to East and Central Africa.<ref name="Legg2004" /> This highly virulent strain was later discovered to be a [[Biology:Chimaera|chimaera]] of two distinct ''Begomovirus'' species.<ref name="Patil2009" /> 

CMD is primarily managed through [[Biology:Sanitary and phytosanitary measures and agreements|phytosanitation]] practices as well as the use of conventional resistance breeding. Additionally, vector management and cross-protection help to minimize transmission and symptom development.<ref name="Legg2004" /> Though management practices are useful, the viruses’ high rate of [[Biology:Genetic recombination|recombination]] and co-infection capabilities have caused CMD to be one of the most detrimental diseases affecting food supply in Africa.<ref name="Patil2009" />

==Hosts and symptoms==
[[File:Manihot esculenta flowers.jpg|thumb|upright=1|The inflorescence of cassava (''Manihot esculenta'', Family Euphorbiaceae), a tropical tuber crop. Muruwere, Manica Province of Mozambique. The leaves show symptoms of cassava mosaic disease, caused by a virus.]]

Cassava originated in [[Earth:South America|South America]] and was introduced to Africa in relatively recent times.<ref name="Legg2004" /> It is known to be a very drought-tolerant crop with the ability to yield even when planted in poor soils. When cassava was first grown in Africa, it was used for subsidiary purposes though it is now considered to be one of the most important food staple crops on the continent.<ref name="Legg2004" /> Its production is moving toward an industrialized system in which plant material is used for a variety of products including [[Chemistry:Starch|starch]], flour, and animal feed.<ref name="Thresh2006" />

As cassava is vegetatively propagated, it is particularly vulnerable to viruses and thus Cassava geminiviruses lead to great economic loss each year.<ref name="Patil2009" /> When these infect a host plant, the plant’s defense system is triggered. Plants use [[Biology:Gene silencing|gene silencing]] to suppress viral replication, though begomoviruses have evolved a counter-acting suppressor protein against this natural host defense.<ref name="Patil2009" /> Because different species of ''Begomovirus'' produce different variants of this suppressor protein, co-infection by multiple species typically leads to more severe disease symptoms.<ref name="Harrison1999" />

Initially following infection of a cassava geminivirus in cassava, systemic symptoms develop.<ref name="Patil2009" /> These symptoms include chlorotic mosaic of the leaves, leaf distortion, and stunted growth.<ref name="Legg2000" /> Leaf stalks have a characteristic S-shape.<ref name=":0">{{Cite web|last=|first=|date=|title=Plantwise Knowledge Bank {{!}} Cassava mosaic disease|url=https://www.plantwise.org/KnowledgeBank/pmdg/20167801149|url-status=live|archive-url=https://web.archive.org/web/20200604104405/https://www.plantwise.org/KnowledgeBank/pmdg/20167801149 |archive-date=2020-06-04 |access-date=2020-06-04|website=www.plantwise.org}}</ref> Infection can be overcome by the plant especially when a rapid onset of symptoms occurs. A slow onset of disease development usually correlates with death of the plant.<ref name=Patil2009/> Besides the conventional effects of host and pathogen characteristics, [[Chemistry:Soil fertility|soil fertility]] was first shown to affect symptomology by Mollard 1987.<ref name="Sseruwagi-et-al-2004">{{cite journal | last1=Sseruwagi | first1=P. | last2=Sserubombwe | first2=W.S. | last3=Legg | first3=J.P. | last4=Ndunguru | first4=J. | last5=Thresh | first5=J.M. | title=Methods of surveying the incidence and severity of cassava mosaic disease and whitefly vector populations on cassava in Africa: a review | journal=Virus Research | publisher=[[Company:Elsevier|Elsevier]] | volume=100 | issue=1 | year=2004 | issn=0168-1702 | doi=10.1016/j.virusres.2003.12.021 | pages=129–142| pmid=15036844 }}</ref>

Though the cassava-infecting geminiviruses causes most of their economic damage in cassava, they are able to infect other plants. The host range depends on the species of virus and most are able to be transmitted and to cause disease on plants of the genera ''[[Biology:Nicotiana|Nicotiana]]'' and ''[[Biology:Datura|Datura]]''.<ref name="Bock1983" />

Cassava Mosaic Disease is currently spreading across SE Asia.<ref>{{Cite web | url=https://ciat.cgiar.org/event/cmd-sea/ |title = Cassava Mosaic Disease (CMD) in Southeast Asia}}</ref>

== Causal agent and disease cycle == 
Cassava geminiviruses<ref name="Timmermans1994" /> are transmitted in a persistent manner by the whitefly ''Bemisia tabaci'', by vegetative propagation using cuttings from infected plants, and occasionally by mechanical means.<ref name="Fargette1994-1" /><ref name="Thurston1998" /><ref name="Timmermans1994" /> Cassava produces its first leaves within 2–3 weeks of planting; these young leaves are then colonized by the viruliferous whiteflies.<ref name="Fargette1994-2" /> This is the key infection period for CMD geminiviruses, as they cannot infect older plants.<ref name="Fauquet1990" /> As the genome of the viruses has two components, DNA A and B, that are encapsidated in separate geminate particles, it requires a double inoculation to cause infection.<ref name="Timmermans1994" />

Generally, whitefly requires 3 hours feeding time to acquire the virus, a latent period of 8 hours, after which it needs 10 minutes to infect the young leaves.<ref name="Fauquet1990" /> There is variation in the literature on this score, however, with other sources citing a 4-hour acquisition time and 4-hour latent period.<ref name="Thurston1998" /> Symptoms appear after a 3-5 week latent period.<ref name="Fargette1994-2" /> Adult whiteflies can continue to infect healthy plants 48 hours after initial acquisition of the virus.<ref name=Thurston1998/> A single whitefly is sufficient to infect the host; however, successful transmission increases when multiple infected whiteflies feed on the plant.<ref name="Thurston1998" />

After entering the plant through the leaves, the virus remains in the leaf cells for 8 days.<ref name="Thurston1998" /> As it is a single-stranded DNA virus, it needs to enter the [[Biology:Cell nucleus|nucleus]] of the leaf cells to replicate.<ref name="Fauquet1990" /> After this initial period, the virus enters the [[Biology:Phloem|phloem]] and travels to the base of the stem and out into the branches.<ref name="Thurston1998" /> Travel to the branches of the plant is much slower than travel through the stem, so cuttings of branches from infected stems may be free of disease.<ref name="Thurston1998" /> Some literature has indicated that infection is limited to above-ground tissue, but it is not clear why this would be the case.<ref name="oisat" />

== Environment == 
The severity of cassava mosaic disease is impacted by environmental factors such as light intensity, wind, rainfall, plant density and temperature. Given that the viruses are transmitted by whitefly, the spread of the virus is going to depend largely on the vector. Temperature is the most important environmental factor controlling the size of the vector population.<ref name="Fauquet1990" /> In the literature, vector-preferred temperature estimates vary from 20&nbsp;°C to 30&nbsp;°C<ref name="Fargette1994-2" /> to 27&nbsp;°C to 32&nbsp;°C<ref name="Thurston1998" /> but generally high temperatures associated with high [[Philosophy:Fecundity|fecundity]], rapid development, and greater longevity in whitefly.<ref name="Fargette1994-2" /> Increased light intensity has been shown to increase activity of the whitefly vector.<ref name="Thurston1998" />

Whiteflies can fly at speeds up to 0.2&nbsp;mph, and in high-wind conditions they can move much greater distances in a shorter time, thus increasing rate of virus spread.<ref name="Fauquet1990" /> This wind-dependent spread is reflected in the location of the whitefly in cassava fields, with populations greatest in upwind borders and lowest within the field.<ref name="Fauquet1990" />

Virus incidence increases when cassava is growing vigorously.<ref name="Fargette1994-2" /> Thus, plant density impacts the spread of the virus, with low-density fields encouraging faster disease propagation than high-density ones.<ref name="Fauquet1990" /> In dry areas, rainfall can be a limiting factor for cassava growth so higher rainfall will be associated with higher incidence of disease.<ref name="Fargette1994-2" /> Populations of whitefly will increase with rainfall, but heavy rains may impede whitefly spread and thus decrease incidence of virus.<ref name="Fargette1994-2" />

Timing of planting can play an important role in the severity of disease, with cassava planted in March showing a 74% incidence rate of CMV, compared with 4% in August.<ref name="Fauquet1990" /> Seasonal distribution of the virus will vary with the climate. In tropical rain forest type climates, where it is wet and humid most of the year, rapid virus distribution occurred from November to June, and slow progress occurred from July to September.<ref name="Fargette1994-2" /> This timing correlated with higher and lower temperatures. In a study of the disease in the Ivory Coast of Africa, maximum rate of disease spread was reached two months after planting.<ref name="Thurston1998" /> Little to no infection occurs after three months, and variation in spread was due to change in temperature, radiation and population levels of whitefly.

==Control strategies==
Control strategies for cassava mosaic disease include sanitation and plant resistance. In this case, sanitation means using cuttings from healthy plants to start with a healthy plot and maintaining that healthy plot by identifying unhealthy plants and immediately removing them. This strategy does not protect them from being inoculated by whiteflies, but research shows that the virus is more aggressive in plants infected from contaminated cuttings than by insect vectors. There are also specific varieties that fare better against some viruses than others, so plant resistance is possible.<ref name="Fargette1994-2" /> For example, hybrids that are a result of crossing cassava and other species, such as Manihot melanobasis and M. glaziovii, have been shown to have considerable resistance to CMV.<ref name="Thresh2005" /> 

Prevention methods of CMV spread include, avoiding planting alternative hosts of the virus, such as castor bean (''Ricinus communis''), adjacent to cassava, avoiding planting cassava if neighbouring fields have virus-infected cassava as these could be carried by Whiteflies. Prevention methods also include not planting alternative hosts of the virus vector Whitefly e.g. tomato.<ref name=":0" />

The CABI-led programme, Plantwise suggests intercropping with cereals and legumes, such as maize or cowpeas, to repel whiteflies and rotate cassava with non-host crops including sorghum.<ref name=":0" /> 

The Ministry of Agriculture Food Security and Cooperatives of Tanzania recommend uprooting diseased plants once every week by pulling them out by hand. Plants should be carried away from the field and exposed to the sunlight for drying and then burned to kill the viruses.<ref>{{Cite web|last=|first=|date=|title=Plantwise Knowledge Bank {{!}} Control of cassava mosaic disease by uprooting|url=https://www.plantwise.org/KnowledgeBank/factsheetforfarmers/20157800492|url-status=live|archive-url=https://web.archive.org/web/20200604104659/https://www.plantwise.org/KnowledgeBank/factsheetforfarmers/20157800492 |archive-date=2020-06-04 |access-date=2020-06-04|website=www.plantwise.org}}</ref>

Most research has been done under unrealistic conditions and so relates only poorly to application in the field. Rare studies under realistic conditions include Mollard 1987 and Otim-Nape et al 1994.<ref name="Sseruwagi-et-al-2004" />

==Importance==
Mostly grown as a food source in Africa, cassava is the third largest source of carbohydrates in the world.<ref name="Fargette1994-2" /> In recent times, cassava production has turned from subsistence to commercial production.<ref name="Patil2009" />

CMD was first described in 1894 and is now considered one of the most damaging crop viruses in the world.<ref name="Fargette1994-2" /><ref name="Patil2009" /> Annual economic losses in East and Central Africa are estimated to be between US$1.9 billion and $2.7 billion.<ref name="Patil2009" /> Although cassava is also cultivated in Latin America and South East Asia, the geminiviruses infecting it are only found in Africa and the Indian sub-continent. This has been mainly attributed to the inability of ''B. tabaci''  to colonize cassava effectively in this part of the world.<ref name="Patil2009" />

The Ministry of Agriculture Food Security and Cooperatives of Tanzania recommend uprooting diseased plants once every week by pulling them out by hand. Plants should be carried away from the field and exposed to the sunlight for drying and then burned to kill the viruses.

== Sources ==
{{Free-content attribution
| title = PMDG Green List: Cassava mosaic disease
| author = Plantwise
| publisher = CABI
| page numbers = 
| source = 
| documentURL = https://www.plantwise.org/KnowledgeBank/pmdg/20167801149
| license statement URL = https://www.cabi.org/terms-and-conditions/
| license = CC-BY-SA
}}

{{Free-content attribution
| title = Plantwise Factsheets for Farmers: Control of cassava mosaic disease by uprooting
| author = Jumbe A. Ahmed
| publisher = CABI
| page numbers = 
| source = 
| documentURL = https://www.plantwise.org/KnowledgeBank/factsheetforfarmers/20157800492
| license statement URL = https://www.cabi.org/terms-and-conditions/
| license = CC-BY-SA
}}

==References==
{{reflist|2|refs=
<ref name=Patil2009>Patil B & Fauquet C (2009). Cassava mosaic geminiviruses: actual knowledge and perspectives. Molecular Plant Pathology. 10: 685–701.</ref>

<ref name=Legg2004>Legg J & Fauquet C (2004). Cassava mosaic geminiviruses in Africa. Plant Molecular Biology. 56: 585–599.</ref>

<ref name=Legg2000>Legg J & Thresh J (2000). Cassava mosaic virus disease in East Africa: a dynamic disease in a changing environment. Virus Research. 71: 135–149.</ref>

<ref name=Thresh2006>Thresh J (2006). Control of tropical plant virus diseases. Virus Research. 67:245–295.</ref>

<ref name=Harrison1999>Harrison B & Robinson D (1999). Natural genomic and antigenic variation in whitefly-transmitted geminiviruses (begomoviruses). Annual Review of Phytopathology. 37: 369–398.</ref>

<ref name=Bock1983>Bock K & Woods R (1983). The etiology of African cassava mosaic disease. Plant Dis. 67: 994–995.</ref>

<ref name=Timmermans1994>Timmermans, M.C.P., Das, O.P., Messing, J. (1994). Geminivurses and Their Uses as Extrachromosomal Replicons. Annu. Rev. Plant Physiol. Plant Mol. Biol. 45:79–112.</ref>

<ref name=Fargette1994-1>Fargette, D. and Thresh, J.M. (1994). The Ecology of African Cassava Mosaic Geminivirus. In: Bakeman, J.P., Williamson, B. (Eds). Ecology of Plant Pathogens, CABI.</ref>

<ref name=Thurston1998>Thurston, H.D. Tropical Plant Diseases. St. Paul: APS press, 1998.</ref>

<ref name=Fargette1994-2>Fargette, D., Jeger, M., Fauquet, C., Fishpool, L.D. (1994). Analysis of Temporal Disease Progress of African Cassava Mosaic Virus. Phytopathology. 54; 1 91–98.</ref>

<ref name=Fauquet1990>Fauquet, C. and Fargette, D. (1990) African Cassava Mosaic Virus: Etiology, Epidemiology, and Control. Laboratoire de Phytovirologie, ORSTOM, Abidjan, Ivory Coast. Plant Disease. 74: 404-411.</ref>

<ref name=oisat>{{Cite web | url=http://www.oisat.org/pests/diseases/viral/cassava_mosaic_disease.html | title=Cassava mosaic disease}}</ref>

<ref name=Thresh2005>Thresh, JM & Cooter, T.J. (2005). Strategies for controlling Cassava Mosaic Disease in Africa. Plant Pathology. 54: 587–614.</ref>
}}

==External links==
*{{Cite web | url=http://phene.cpmc.columbia.edu/ICTVdB//00.029.0.03.004.htm | archive-url=https://web.archive.org/web/20070803183535/http://phene.cpmc.columbia.edu/ICTVdB//00.029.0.03.004.htm | url-status=dead | archive-date=2007-08-03 |title = ICTVdB Virus Description - 00.029.0.03.004. African cassava mosaic virus}}
*https://web.archive.org/web/20190223033858/https://www.apsnet.org/edcenter/advanced/topics/EcologyAndEpidemiologyInR/DiseaseProgress/Pages/CassavaMosaic.aspx
*{{Cite web | url=http://www.ictvdb.org/ICTVdB/00.029.0.03.004.htm | archive-url=https://web.archive.org/web/20110723194127/http://www.ictvdb.org/ICTVdB/00.029.0.03.004.htm | url-status=dead | archive-date=2011-07-23 |title = ICTVdB Virus Description - 00.029.0.03.004. African cassava mosaic virus}}

{{DEFAULTSORT:Cassava Mosaic Virus}}
[[Category:Begomovirus]]

{{Sourceattribution|Cassava mosaic virus}}