The viruses are members of the family Geminiviridae and the genus Begomovirus. The first report of cassava mosaic disease (CMD) was from East Africa in 1894.[2] Since then, epidemics have occurred throughout the African continent, resulting in great economic loss and devastating famine.[2] In 1971, a resistant line of cassava, the predominant host of the pathogen, was established and used by the 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.[2] This highly virulent strain was later discovered to be a chimaera of two distinct Begomovirus species.[1]
CMD is primarily managed through phytosanitation practices as well as the use of conventional resistance breeding. Additionally, vector management and cross-protection help to minimize transmission and symptom development.[2] Though management practices are useful, the viruses’ high rate of recombination and co-infection capabilities have caused CMD to be one of the most detrimental diseases affecting food supply in Africa.[1]
Hosts and symptoms
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 South America and was introduced to Africa in relatively recent times.[2] 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.[2] Its production is moving toward an industrialized system in which plant material is used for a variety of products including starch, flour, and animal feed.[3]
As cassava is vegetatively propagated, it is particularly vulnerable to viruses and thus Cassava geminiviruses lead to great economic loss each year.[1] When these infect a host plant, the plant’s defense system is triggered. Plants use gene silencing to suppress viral replication, though begomoviruses have evolved a counter-acting suppressor protein against this natural host defense.[1] Because different species of Begomovirus produce different variants of this suppressor protein, co-infection by multiple species typically leads to more severe disease symptoms.[4]
Initially following infection of a cassava geminivirus in cassava, systemic symptoms develop.[1] These symptoms include chlorotic mosaic of the leaves, leaf distortion, and stunted growth.[5] Leaf stalks have a characteristic S-shape.[6] 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.[1] Besides the conventional effects of host and pathogen characteristics, soil fertility was first shown to affect symptomology by Mollard 1987.[7]
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 Nicotiana and Datura.[8]
Cassava Mosaic Disease is currently spreading across SE Asia.[9]
Causal agent and disease cycle
Cassava geminiviruses[10] are transmitted in a persistent manner by the whitefly Bemisia tabaci, by vegetative propagation using cuttings from infected plants, and occasionally by mechanical means.[11][12][10] Cassava produces its first leaves within 2–3 weeks of planting; these young leaves are then colonized by the viruliferous whiteflies.[13] This is the key infection period for CMD geminiviruses, as they cannot infect older plants.[14] 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.[10]
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.[14] There is variation in the literature on this score, however, with other sources citing a 4-hour acquisition time and 4-hour latent period.[12] Symptoms appear after a 3-5 week latent period.[13] Adult whiteflies can continue to infect healthy plants 48 hours after initial acquisition of the virus.[12] A single whitefly is sufficient to infect the host; however, successful transmission increases when multiple infected whiteflies feed on the plant.[12]
After entering the plant through the leaves, the virus remains in the leaf cells for 8 days.[12] As it is a single-stranded DNA virus, it needs to enter the nucleus of the leaf cells to replicate.[14] After this initial period, the virus enters the phloem and travels to the base of the stem and out into the branches.[12] 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.[12] Some literature has indicated that infection is limited to above-ground tissue, but it is not clear why this would be the case.[15]
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.[14] In the literature, vector-preferred temperature estimates vary from 20°C to 30°C[13] to 27°C to 32°C[12] but generally high temperatures associated with high fecundity, rapid development, and greater longevity in whitefly.[13] Increased light intensity has been shown to increase activity of the whitefly vector.[12]
Whiteflies can fly at speeds up to 0.2mph, and in high-wind conditions they can move much greater distances in a shorter time, thus increasing rate of virus spread.[14] 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.[14]
Virus incidence increases when cassava is growing vigorously.[13] Thus, plant density impacts the spread of the virus, with low-density fields encouraging faster disease propagation than high-density ones.[14] In dry areas, rainfall can be a limiting factor for cassava growth so higher rainfall will be associated with higher incidence of disease.[13] Populations of whitefly will increase with rainfall, but heavy rains may impede whitefly spread and thus decrease incidence of virus.[13]
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.[14] 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.[13] 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.[12] 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.[13] 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.[16]
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.[6]
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.[6]
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.[17]
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.[7]
Importance
Mostly grown as a food source in Africa, cassava is the third largest source of carbohydrates in the world.[13] In recent times, cassava production has turned from subsistence to commercial production.[1]
CMD was first described in 1894 and is now considered one of the most damaging crop viruses in the world.[13][1] Annual economic losses in East and Central Africa are estimated to be between US$1.9 billion and $2.7 billion.[1] 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.[1]
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.
Geminiviridae is a family of plant viruses that encode their genetic information on a circular genome of single-stranded (ss) DNA. There are 520 species in this family, assigned to 14 genera. Diseases associated with this family include: bright yellow mosaic, yellow mosaic, yellow mottle, leaf curling, stunting, streaks, reduced yields. They have single-stranded circular DNA genomes encoding genes that diverge in both directions from a virion strand origin of replication. According to the Baltimore classification they are considered class II viruses. It is the largest known family of single stranded DNA 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.
Cacao swollen shoot virus (CSSV) is a plant pathogenic virus of the family Caulimoviridae that primarily infects cacao trees. It decreases cacao yield within the first year of infection, and usually kills the tree within a few years. Symptoms vary by strain, but leaf discoloration, stem/root swelling, and die-back generally occur. The virus is transmitted from tree to tree by mealybug vectors. It was first discovered in Ghana in 1936, and is currently endemic in Togo, Ghana and Nigeria. Over 200 million trees have already been claimed by this disease, which has prompted Ghana to launch the most ambitious and costly eradication effort of any country in the world against a viral plant disease.
Cassava green mottle virus (CGMV) is a plant pathogenic virus of the family Secoviridae. Pathogens:
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).
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.
Tobacco rattle virus (TRV) is a pathogenic plant virus. Over 400 species of plants from 50 families are susceptible to infection.
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.
Soybean mosaic virus (SMV) is a member of the plant virus genus Potyvirus. It infects mainly plants belonging to the family Fabaceae but has also been found infecting other economically important crops. SMV is the cause of soybean mosaic disease that occurs in all the soybean production areas of the world. Soybean is one of the most important sources of edible oil and proteins and pathogenic infections are responsible for annual yield losses of about $4 billion in the United States. Among these pathogens, SMV is the most important and prevalent viral pathogen in soybean production worldwide. It causes yield reductions of about 8% to 35%, but losses as high as 94% have been reported.
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.
Orthotospovirus is a genus of negative-strand RNA viruses, in the family Tospoviridae of the order Bunyavirales, which infects plants. Tospoviruses take their name from the species Tomato spotted wilt orthotospovirus (TSWV) which was discovered in Australia in 1919. TSWV remained the only known member of the family until the early 1990s when genetic characterisation of plant viruses became more common. There are now at least twenty species in the genus with more being discovered on a regular basis. Member viruses infect over eight hundred plant species from 82 different families.
Cassava brown streak virus disease (CBSD) is a damaging disease of cassava plants, and is especially troublesome in East Africa. 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. 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. This disease is considered to be the biggest threat to food security in coastal East Africa and around the eastern lakes.
Abutilon mosaic virus (AbMV) is a virus of the genus Begomovirus. It infects Abutilon species, notably the flowering maple, Abutilon striatum. The mottled or variegated effect on the leaves of Abutilon striatum is sought after.
Maize lethal necrosis disease is a viral disease affecting maize (corn) predominantly in East Africa, Southeast Asia and South America, which was recognised in 2010. It is caused by simultaneous infection with two viruses, MCMoV and any of several Potyviridae.
Cocoa necrosis virus (CoNV) is a plant pathogenic virus of the genus nepovirus that infects Theobroma cacao en natura causing cacao necrosis disease. CoNV is considered synonymous with Strain S of cacao swollen shoot virus. Unlike Cacao swollen shoot virus, it is not transmitted by mealybugs nor vectored by aphids, beetles, or leafhoppers that also commonly infest cacao. It is serologically, distantly related to Tomato black ring virus and very distantly related to Grapevine chrome mosaic virus.
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.
Pepper leaf curl virus(PepLCV) is a DNA virus from the genus Begomovirus and the family Geminiviridae. PepLCV causes severe disease especially in pepper (Capsicum spp.). It can be found in tropical and subtropical regions such as Thailand and India, but has also been detected in countries such as the United States and Nigeria. This virus is transmitted by an insect vector from the family Aleyrodidae and order Hemiptera, the whitefly Bemisia tabaci. The primary host for PepLCV are several Capsicum spp.. PepLCV 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 source of resistance to the virus has been identified in the Bhut Jolokia pepper.
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.
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.
References
1 2 3 4 5 6 7 8 9 10 11 Patil B & Fauquet C (2009). Cassava mosaic geminiviruses: actual knowledge and perspectives. Molecular Plant Pathology. 10: 685–701.
↑ Thresh J (2006). Control of tropical plant virus diseases. Virus Research. 67:245–295.
↑ Harrison B & Robinson D (1999). Natural genomic and antigenic variation in whitefly-transmitted geminiviruses (begomoviruses). Annual Review of Phytopathology. 37: 369–398.
↑ Legg J & Thresh J (2000). Cassava mosaic virus disease in East Africa: a dynamic disease in a changing environment. Virus Research. 71: 135–149.
1 2 3 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.
↑ 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.
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