Cassava brown streak virus disease

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Cassava brown streak virus
Virus classification OOjs UI icon edit-ltr.svg
(unranked): Virus
Realm: Riboviria
Kingdom: Orthornavirae
Phylum: Pisuviricota
Class: Stelpaviricetes
Order: Patatavirales
Family: Potyviridae
Genus: Ipomovirus
Species:
Cassava brown streak virus
Cassava plant with the typical symptoms of CBSD Distribution of cassava brown streak disease (CBSD) symptoms on cassava.JPG
Cassava plant with the typical symptoms of CBSD

Cassava brown streak virus disease (CBSD) is a damaging disease of cassava plants, and is especially troublesome in East Africa. [1] It was first identified in 1936 in Tanzania, and has spread to other coastal areas of East Africa, from Kenya to Mozambique. Recently, it was found that two distinct viruses are responsible for the disease: cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV). Both have (+)ss RNA genomes, belong to the genus Ipomovirus in the family Potyviridae, and produce generally similar symptoms in infected plants. [2] [3] [4] Root rot renders the cassava tuber inedible, resulting in severe loss of economic value; therefore, current research focuses on achieving cultivars that do not develop the necrotic rot. [4] This disease is considered to be the biggest threat to food security in coastal East Africa and around the eastern lakes. [5]

Contents

Symptoms

CBSD is characterized by severe chlorosis and necrosis on infected leaves, giving them a yellowish, mottled appearance. [3] Chlorosis may be associated with the veins, spanning from the mid vein, secondary and tertiary veins, or rather in blotches unconnected to veins. Leaf symptoms vary greatly depending on a variety of factors. The growing conditions (i.e. altitude, rainfall quantity), plant age, and the virus species account for these differences. [5] Brown streaks may appear on the stems of the cassava plant. Also, a dry brown-black necrotic rot of the cassava tuber exists, which may progress from a small lesion to the whole root. Finally, the roots can become constricted due to the tuber rot, stunting growth. [6] Typically the affected plants do not possess all of these characteristics, but those that are severely affected may. Farmers may be unaware of their infected cassava crops until they are harvested and see the tuber lesions because leaves are asymptomatic. [1] The cassava mosaic virus (CMV) is another cassava virus that exhibits foliar symptoms similar to CBSD, but they are more obvious.

UCBSV has milder symptoms than CBSV, and lower pathogenicity. [2] [7]

Vector and disease cycle

After a period of ambiguity among researchers, the consensus is that the most likely candidate of CBSD vector is Bemisia tabaci biotype B, the silverleaf whitefly. [1] [8] [9] It is also sometimes referred to as B. argentifolii . [10] There is a close association between surging whitefly populations and CBSD incidence. [11] This whitefly species is also considered to be the vector of CMV. It is suggested, however, that B. tabaci whiteflies transmit CBSVs less effectively than CMVs. [4] Also, the CBSD retention period in B. tabaci may not exceed more than 24 h, but more research is needed to confirm this. [4]

The adult B. tabaci lives an average of sixteen days, and the maturation process from egg to adult is thirty days. [12] Eggs may be deposited haphazardly or in a spiral fashion on the leaf undersides. Both juvenile and adult whiteflies feed on the phloem of the leaves by inserting a sucker mouth part into the leaf, thereby transmitting the virus to the plant. Saliva containing toxins is also injected into the cassava plant while whiteflies feed, disturbing plant growth and ultimately reducing yield. Seedlings are particularly affected. [13]

Disease spread

After its first identification in 1936, CBSD was almost totally eliminated in Uganda due to program efforts, and there were relatively small yield losses in affected areas. The disease was restricted to < 1000 m above sea level along coastal Kenya to Mozambique, and the shores of Lake Malawi. However, as of the year 2000, CBSD has spread rapidly throughout Eastern Africa. [14] Midaltitude levels (1200–1500 m above sea level) now accommodates CBSD, as it has been reported in Uganda, Democratic Republic Congo, and around Tanzanian lake zones. [5] As of 2009, CBSD outbreaks were most prevalent in south - central Uganda and in Mara Region. There have also been reports of CBSD in Rwanda and Burundi. [4]

The incidence of CBSD is greatest in Uganda where there is resistance to CMV in cassava and other locations in general. [1] [2] Recent surveys demonstrated that of the 23 districts in Uganda surveyed, 70% had CMD-resistant cassava varieties, and all are vulnerable to CBSD – causing viruses. These varieties also hosted as many as 200 adult whiteflies on the top five cassava leaves. [5]

Predictions cannot be made about the spreading pattern. Because the disease does not fan out from only one source point, but rather appears in remote areas, or “hot spots,” models are difficult to devise. This challenge may arise from the movement of cuttings from infected regions and/or an abundance of whiteflies in a particular area. [4]

Management of CBSD

Management tools are still being explored for the control of CBSD, and progress has been slow. [11] The development of cassava with resistance to both CMD and CBSD is needed. [5]

Germplasm resistance screen

In a few cassava varieties, natural resistance has been found against UBCSV. [2] Widespread distribution of germplasm of these varieties can reduce disease incidence on a large scale. Furthermore, screening for resistance in farmer-preferred cassava genotypes in Africa is crucial for effective CBSD control and management. [7]

Ugandan cassava brown streak virus is a causing agent of CBSD and was the first member of the genus Ipomovirus to be cloned and rescued using a plasmid cDNA vector system. [15] Pasin et al., 2017 develops and presents the pLX transformation vector and uses it to produce a UCBSV clone. [15] This plasmid-only system greatly simplifies plant inoculation and screen for resistance against UCBSV.

Genetic engineering

Genetic engineering specific to the RNA genome is used to encourage resistance in cassava cultivars. A recent study demonstrated that inducing the expression of hairpin RNA homologous to viral sequences is a potentially effective lab technique because it imitates the behavior of the plant immune system encountering foreign bodies. Specifically, they were able to use hairpin RNA homologous to the 3’ end of CBSV coat protein sequences in the cassava cultivar 60444 to develop resistance to both CBSV and UCBSV. The resulting construct was transferred to a cultivar that farmers prefer (Nigerian landrace TME 7). This particular cultivar exhibited CMV resistance originally in its natural state, the motive being to foster resistance to both CMV and CBSV post – grafting, which was successful. Therefore, the suggestion is that exploiting the immune system of plants that already have natural resistance to CMV is a potentially viable method to combat both viruses. [2]

Gomez et al. 2019 demonstrate that CBSV and UCBSV viral genome-linked proteins (VPg) interact with cassava novel cap-binding proteins (nCBP, specifically nCBP-1 and nCBP-2). [16] They then generated CRISPR mutagenesis cassava mutants and demonstrated tolerance to both viruses. [16] This indicates these interactions are necessary for pathogenesis. [16]

Education

Farmers need to be better educated on the subject of CBSD, including cause, diagnosis and disease spread. The most obvious symptom of the disease is the cassava root rot, and farmers are inclined to believe that too much water causes the rotting rather than the virus. The identification of the foliar symptoms is important, because farmers can get a more accurate yield expectation without waiting for the harvest period. Also, awareness of tolerant varieties can be promoted. [11] It is suggested that workshops be held for researchers, so that they are aware of new diagnostics. [6]

Need for more data

More surveys need to be conducted so that the disease spread and variant affinity can be better understood. Discouragement of affected varieties as crops can be more quickly done with tracking. [1] There is emphasis on the need for more research pertaining to the viral pathogenesis.

Other suggestions

Economic importance

Cassava is a very important staple crop for many in Africa, and the demand for it increases with high population growth rates. [4] CBSD poses a serious threat to farmers in East Africa, because crop yields can be reduced as drastically as 70%. [14] Upon harvesting, farmers will cut out the necrotic lesions of affected tubers or they will discard tubers that are severely affected. 10 – 30% of root rot constitutes moderate infection, decreasing the market value of tubers by 90%. It is estimated that African farmers collectively lose revenue of up to $100 million annually due to the devastating disease. [5]

Related Research Articles

<span class="mw-page-title-main">Cassava</span> Staple crop

Manihot esculenta, commonly called cassava, manioc, or yuca, is a woody shrub of the spurge family, Euphorbiaceae, native to South America, from Brazil, Paraguay and parts of the Andes. Although a perennial plant, cassava is extensively cultivated in tropical and subtropical regions as an annual crop for its edible starchy root tuber. Cassava is predominantly consumed in boiled form, but substantial quantities are processed to extract cassava starch, called tapioca, which is used for food, animal feed, and industrial purposes. The Brazilian farinha, and the related garri of West Africa, is an edible coarse flour obtained by grating cassava roots, pressing moisture off the obtained grated pulp, and finally drying it.

<span class="mw-page-title-main">Silverleaf whitefly</span> Species of true bug

The silverleaf whitefly is one of several species of whitefly that are currently important agricultural pests. A review in 2011 concluded that the silverleaf whitefly is actually a species complex containing at least 40 morphologically indistinguishable species.

<i>Begomovirus</i> Genus of viruses

Begomovirus is a genus of viruses, in the family Geminiviridae. They are plant viruses that as a group have a very wide host range, infecting dicotyledonous plants. Worldwide they are responsible for a considerable amount of economic damage to many important crops such as tomatoes, beans, squash, cassava and cotton. There are 445 species in this genus.

African cassava mosaic virus is a plant pathogenic virus of the family Geminiviridae that may cause either a mosaic appearance to plant leaves, or chlorosis. In Manihot esculenta (cassava), the most produced food crop in Africa, the virus causes severe mosaic. Cassava is a staple food crop in many places throughout the tropics and subtropics as a source of carbohydrates, but the transmission and severity of disease for cassava in Africa is greatest with ACMV.

<span class="mw-page-title-main">Cassava mosaic virus</span> Genus of viruses

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

Cassava green mottle virus (CGMV) is a plant pathogenic virus of the family Secoviridae. Pathogens:

<i>Cucumber mosaic virus</i> Species of virus

Cucumber mosaic virus (CMV) is a plant pathogenic virus in the family Bromoviridae. This virus has a worldwide distribution and a very wide host range, having the reputation of the widest host range of any known plant virus. It can be transmitted from plant to plant both mechanically by sap and by aphids in a stylet-borne fashion. It can also be transmitted in seeds and by the parasitic weeds, Cuscuta sp. (dodder).

Indian cassava mosaic virus(ICMV) is a plant pathogenic virus of the family Geminiviridae. It affects cassava (Manihot esculenta) in India and certain other countries. It is considered to be an invasive species.

Soil-borne wheat mosaic virus is a rod-shaped plant pathogen that can cause severe stunting and mosaic in susceptible wheat, barley and rye cultivars. The disease has often been misdiagnosed as a nutritional problem, but this has actually allowed in part for the fortuitous visual selection by breeding programs of resistant genotypes. Soil-borne wheat mosaic virus is part of the genus Furovirus. Members of this genus are characterized by rigid rod-shaped particles and positive sense RNA genomes consisting of two molecules that are packaged into separate particles that code for either replication, mobility, structure or defense against the host. The virus is spread by a fungal-like protist, Polymyxa graminis, whose asexual secondary and sexual primary cycles help the virus spread. The disease produces secondary symptoms from the root cell infection. The disease is a serious contributor to loss in crop yield.

Tomato yellow leaf curl virus (TYLCV) is a DNA virus from the genus Begomovirus and the family Geminiviridae. TYLCV causes the most destructive disease of tomato, and it can be found in tropical and subtropical regions causing severe economic losses. This virus is transmitted by an insect vector from the family Aleyrodidae and order Hemiptera, the whitefly Bemisia tabaci, commonly known as the silverleaf whitefly or the sweet potato whitefly. The primary host for TYLCV is the tomato plant, and other plant hosts where TYLCV infection has been found include eggplants, potatoes, tobacco, beans, and peppers. Due to the rapid spread of TYLCV in the last few decades, there is an increased focus in research trying to understand and control this damaging pathogen. Some interesting findings include the virus being sexually transmitted from infected males to non-infected females, and an evidence that TYLCV is transovarially transmitted to offspring for two generations.

Sweet potato mild mottle virus (SPMMV) is a plant pathogenic virus of the family Potyviridae.

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Cassava brown streak virus is a species of positive-strand RNA viruses in the genus Ipomovirus and family Potyviridae which infects plants. Member viruses are unique in their induction of pinwheel, or scroll-shaped inclusion bodies in the cytoplasm of infected cells. Cylindrical inclusion bodies include aggregations of virus-encoded helicase proteins. These inclusion bodies are thought to be sites of viral replication and assembly, making then an important factor in the viral lifecycle. Viruses from both the species Cassava brown streak virus and Ugandan cassava brown streak virus (UCBSV), lead to the development of Cassava Brown Streak Disease (CBSD) within cassava plants.

Chilli leaf curl virus(ChiLCV) is a DNA virus from the genus Begomovirus and the family Geminiviridae. ChiLCV causes severe disease especially in pepper (Capsicum spp.), but also affects other crops such as tomato (Solanum lycopersicum). It can be found in tropical and subtropical regions primarily in India, but has also been detected in countries such as Indonesia and Sri Lanka. This virus is transmitted by an insect vector from the family Aleyrodidae and order Hemiptera, the whitefly Bemisia tabaci. The primary host for ChiLCV are several Capsicum spp., but host species also include tomato and amaranth. ChiLCV has been responsible for several epidemics and causes severe economic losses. It is the focus of research trying to understand the genetic basis of resistance. Currently, a few sources of resistance have been discovered and used to breed resistant varieties.

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<span class="mw-page-title-main">Laura Boykin</span> American computational biologist and botanist

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References

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  8. ↑ Alicai, T.; Omongo, C. A.; Maruthi, M. N.; Hillocks, R. J.; Baguma, Y.; Kawuki, R.; Bua, A.; Otim-Nape, G. W.; Colvin, J. (1 January 2007). "Re-emergence of Cassava Brown Streak Disease in Uganda". Plant Disease . 91 (1). American Phytopathological Society (APS): 24–29. doi:10.1094/PD-91-0024. PMID   30781061.
  9. ↑ Mware; Olubayo;Narla;Songa;Amata;Kyamanywa;Ateka, B.;F.; R.;J.;R.;S.;E.M (2010). "First record of spiraling whitefly in coastal Kenya: emergence, host range, distribution and association with cassava brown streak virus disease". International Journal of Agriculture . 12: 411–415.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. ↑ Belotti;Arias, A.C;B. "Host plant resistance to whiteflies with emphasis on cassava as a case study" (PDF). Retrieved 26 April 2013.{{cite web}}: CS1 maint: multiple names: authors list (link)
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  12. ↑ Hoddle, Mark. "Silverleaf whitefly, Bemisia argentifolii" . Retrieved 25 April 2013.
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[[Category:Cassava diseases]

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