Editing Cassava (section)

=== Viruses ===

Several viruses cause enough damage to cassava crops to be of economic importance. The [[African cassava mosaic virus]] causes the leaves of the cassava plant to wither, limiting the growth of the root.<ref>{{cite web |title=Cassava (manioc) |url=https://www.plantvillage.com/en/topics/cassava-manioc/infos/diseases_and_pests_description_uses_propagation |archive-url=https://web.archive.org/web/20150630145744/https://www.plantvillage.com/en/topics/cassava-manioc/infos/diseases_and_pests_description_uses_propagation |archive-date=30 June 2015 |access-date=29 May 2015}}</ref> An outbreak of the virus in Africa in the 1920s led to a major famine.<ref name="NYT 2010">{{cite news |date=31 May 2010 |title=Virus ravages cassava plants in Africa |work=[[The New York Times]] |url=https://www.nytimes.com/2010/06/01/science/01cassava.html |access-date=24 February 2017 |archive-date=16 March 2017 |archive-url=https://web.archive.org/web/20170316113128/http://www.nytimes.com/2010/06/01/science/01cassava.html |url-status=live }}</ref> The virus is spread by the [[whitefly]] and by the transplanting of diseased plants into new fields. Sometime in the late-1980s, a mutation occurred in Uganda that made the virus even more harmful, causing the complete loss of leaves. This mutated virus spread at a rate of {{convert|50|mi|km|sigfig=1|order=flip|abbr=off}} per year, and as of 2005 was found throughout Uganda, Rwanda, Burundi, the Democratic Republic of the Congo and the Republic of the Congo.<ref>{{cite web |date=31 August 2005 |title=Hungry African nations balk at biotech cassava |url=http://www.redorbit.com/news/science/225369/hungry_african_nations_balk_at_biotech_cassava/ |archive-url=https://web.archive.org/web/20120303183231/http://www.redorbit.com/news/science/225369/hungry_african_nations_balk_at_biotech_cassava/ |archive-date=3 March 2012 |access-date=11 August 2008 |work=[[St. Louis Post-Dispatch]]}}</ref> Viruses are a severe production limitation in the tropics. They are the primary reason for the complete lack of yield increases in the 25 years {{as of|2021|alt=up to 2021}}.<ref name="Afedraru-2019" /> [[Cassava brown streak virus disease]] is a major threat to cultivation worldwide.<ref name="NYT 2010" /> [[Cassava mosaic virus]] (CMV) is widespread in Africa, causing cassava mosaic disease (CMD).<ref name="Lebot-2020" /> Bredeson et al. 2016 find the ''M. esculenta'' cultivars most widely used on that continent have [[Manihot carthaginensis subsp. glaziovii|''M. carthaginensis'' subsp. ''glaziovii'']] genes of which some appear to be CMD [[plant disease resistance gene|resistance gene]]s.<ref name="Lebot-2020">{{cite book |last=Lebot |first=Vincent |title=Tropical Root and Tuber Crops: Cassava, Sweet Potato, Yams And Aroids |publisher=CABI ([[Centre for Agriculture and Bioscience International]]) |year=2020 |isbn=978-1-78924-336-9 |publication-place=[[Wallingford, Oxfordshire]], UK; [[Boston]], USA |page=541 |oclc=1110672215}}</ref> Although the ongoing CMD [[pandemic]] affects both East and Central Africa, Legg ''et al.'' found that these two areas have two distinct [[subpopulation]]s of the [[vector (epidemiology)|vector]], ''[[Bemisia tabaci]]'' whiteflies.<ref>{{Cite book |year=2015 |publisher=Academic Press |issn=0065-3527 |last1=Legg |first1=James P. |author2=Kumar, P. Lava |author3=Makeshkumar, T. |author4=Tripathi, Leena |author5=Ferguson, Morag |author6=Kanju, Edward |author7=Ntawuruhunga, Pheneas |author8=Cuellar, Wilmer |display-authors=3 |doi=10.1016/bs.aivir.2014.10.001 |chapter=Cassava Virus Diseases: Biology, Epidemiology, and Management |editor1=Loebenstein, Gad |editor2=Katis, Nikolaos I. |series=Control of Plant Virus Diseases: Vegetatively-Propagated Crops |title=Advances in Virus Research |volume=91 |pages=85–142 |pmid=25591878 |isbn=9780128027622}}</ref><ref>{{cite journal |last1=Legg |first1=James P. |last2=Sseruwagi |first2=Peter |last3=Boniface |first3=Simon |last4=Okao-Okuja |first4=Geoffrey |last5=Shirima |first5=Rudolph |last6=Bigirimana |first6=Simon |last7=Gashaka |first7=Gervais |last8=Herrmann |first8=Hans-Werner |last9=Jeremiah |first9=Simon |last10=Obiero |first10=Hannington |last11=Ndyetabula |first11=Innocent |last12=Tata-Hangy |first12=Willy |last13=Masembe |first13=Charles |last14=Brown |first14=Judith K. |display-authors=3 |title=Spatio-temporal patterns of genetic change amongst populations of cassava Bemisia tabaci whiteflies driving virus pandemics in East and Central Africa |journal=Virus Research |volume=186 |date=2014 |doi=10.1016/j.virusres.2013.11.018 |pages=61–75 |pmid=24291251 }}</ref> Genetically engineered cassava offers opportunities for the improvement of virus resistance, including CMV and CBSD resistance.<ref>{{cite journal |last1=Rey |first1=Chrissie |last2=Vanderschuren |first2=Hervé |title=Cassava Mosaic and Brown Streak Diseases: Current Perspectives and Beyond |issue=1 |volume=4 |year=2017 |pages=429–452 |publisher=[[Annual Reviews (publisher)|Annual Reviews]] |journal=[[Annual Review of Virology]] |issn=2327-056X |s2cid=25767024 |pmid=28645239 |doi=10.1146/annurev-virology-101416-041913 |doi-access=free}}</ref>