Cassava brown streak virus | |
---|---|
Virus classification ![]() | |
(unranked): | Virus |
Realm: | Riboviria |
Kingdom: | Orthornavirae |
Phylum: | Pisuviricota |
Class: | Stelpaviricetes |
Order: | Patatavirales |
Family: | Potyviridae |
Genus: | Ipomovirus |
Species: | Cassava brown streak virus |
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]
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]
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]
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 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]
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 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]
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]
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.
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]
Manihot esculenta, commonly called cassava, manioc, or yuca, is a woody shrub of the spurge family, Euphorbiaceae, native to South America, from Brazil and parts of the Andes. Although a perennial plant, cassava is extensively cultivated as an annual crop in tropical and subtropical regions for its edible starchy root tuber, a major source of carbohydrates. Cassava is predominantly consumed in boiled form, but substantial quantities are used 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.
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.
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.
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:
Groundnut rosette virus (GRV) is a peanut pathogenic virus found in Sub-Saharan Africa. It is transmitted between plants by insect vectors such as the groundnut aphid.
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.
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.
Rice yellow mottle virus (RYMV) is a plant pathogenic virus, belonging to the genus Sobemovirus. The genome is a positive-sense single strand RNA of 4450 nucleotides in length and is not polyadenylated. It was first reported in Kenya in 1966 in one of Africa's first cultivation intensification schemes, due to RYMV's association with intensification, but DNA analysis of its evolutionary history shows it to have evolved in East Africa in the 19th century. Since its identification in Kenya it has been detected in many countries in sub-Saharan Africa. It has also been detected in Central Africa, but has yet to be seen outside the continent. The genomic organization of RYMV is most similar to that of Cocksfoot mottle sobemovirus. RYMV is one of the better-studied plant-virus pathosystems.
Banana Xanthomonas Wilt (BXW), or banana bacterial wilt (BBW) or enset wilt is a bacterial disease caused by Xanthomonas campestris pv. musacearum. After being originally identified on a close relative of banana, Ensete ventricosum, in Ethiopia in the 1960s, BXW emanated in Uganda in 2001 affecting all types of banana cultivars. Since then BXW has been diagnosed in Central and East Africa including banana growing regions of: Rwanda, Democratic Republic of the Congo, Tanzania, Kenya, Burundi, and Uganda.
Ipomovirus is a genus of positive-strand RNA viruses in the family Potyviridae. Member viruses infect plants and are transmitted by whiteflies. The name of the genus is derived from Ipomoea – the generic name of sweet potato. There are seven species in this genus.
Beet vascular necrosis and rot is a soft rot disease caused by the bacterium Pectobacterium carotovorum subsp. betavasculorum, which has also been known as Pectobacterium betavasculorum and Erwinia carotovora subsp. betavasculorum. It was classified in the genus Erwinia until genetic evidence suggested that it belongs to its own group; however, the name Erwinia is still in use. As such, the disease is sometimes called Erwinia rot today. It is a very destructive disease that has been reported across the United States as well as in Egypt. Symptoms include wilting and black streaks on the leaves and petioles. It is usually not fatal to the plant, but in severe cases the beets will become hollowed and unmarketable. The bacteria is a generalist species which rots beets and other plants by secreting digestive enzymes that break down the cell wall and parenchyma tissues. The bacteria thrive in warm and wet conditions, but cannot survive long in fallow soil. However, it is able to persist for long periods of time in the rhizosphere of weeds and non-host crops. While it is difficult to eradicate, there are cultural practices that can be used to control the spread of the disease, such as avoiding injury to the plants and reducing or eliminating application of nitrogen fertilizer.
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.
Papaya leaf curl virus(PaLCuV) is a DNA virus from the genus Begomovirus and the family Geminiviridae. PaLCuV causes severe disease in papaya (Carica papaya), but can sometimes infect other crops such as tobacco or tomato. It can be found in tropical and subtropical regions primarily in India, but closely related species have also been detected in countries such as China, Malaysia, Nigeria and South Korea. This virus is transmitted by an insect vector from the family Aleyrodidae and order Hemiptera, the whitefly Bemisia tabaci. PaLCuV has been responsible for several epidemics and causes severe economic losses. Because of the broad diversity of these viruses, their characterization and control remains difficult.
Sweet potato leaf curl virus is commonly abbreviated SPLCV. Select isolates are referred to as SPLCV followed by an abbreviation of where they were isolated. For example, the Brazilian isolate is referred to as SPLCV-Br.
Laura Boykin is an American computational biologist who uses supercomputing and genomics to help farmers in Sub-Saharan Africa. She has studied the evolution of the agricultural pest whitefly and identified genetic differences between various species. She works with African scientists to develop computing and genomics skills across the continent, and is a Senior TED fellow.
Viral diseases of potato are a group of diseases caused by different types of Viruses that affect potato crops worldwide and, although they do not affect human or animal health since they are viruses that only infect vegetables, they are a source of great economic losses annually. About 28 viruses have been reported infecting potato crops. However, potato virus X (PVX), potato virus Y (PVY), and potato leafroll virus (PLRV) are the most important viruses worldwide. Some others are of economic importance only in some regions. Such is the case of potato virus M (PVM) in some Asian and European countries.
{{cite journal}}
: CS1 maint: multiple names: authors list (link){{cite web}}
: CS1 maint: multiple names: authors list (link){{cite web}}
: CS1 maint: multiple names: authors list (link){{cite journal}}
: CS1 maint: multiple names: authors list (link)[[Category:Cassava diseases]