Emerging Plant Viruses

C H A P T E R

46 Emerging Plant Viruses Filiz Ertunc Department of Plant Protection, Faculty of Agriculture, Ankara University, Ankara, Turkey

INTRODUCTION A plant disease is the result of interaction between susceptible host plant, virulent pathogen, and the environment. The environment, directly and indirectly, influences plants and their pathogens and changes in the environment, which are often implicated by the emergence of new diseases. The changes associated with global warming may affect the incidence and severity of plant diseases (Elad and Pertot, 2014). One of the major groups that cause infections in plants is plant viruses. All of the viruses are obligate parasites, replicate using the host cell components, amino acids, nucleotides, and ribosomes. They have a nucleic acid genome (RNA or DNA) and a protective protein coat. They enter the cells of healthy plants through the tiny wounds and transmission from plant to plant by insect vectors, nematode, and fungal vectors. It can also be transmitted by sowing-infected seeds and almost 10% of plant viruses are seed-borne. They can survive in infected pollen and can disseminate by vegetative propagation from infected planting material (root stocks, cuttings, tubers, bulb, and corms). They can occur in infected volunteer crop plants and weeds persist in the growing season. Weeds, as fast-growing plants, are well adapted to survive the climate changes. Great attention has been paid to realize what climatic alterations and changes are likely to cause the prevalence of diseases of cultivated and wild plants and the damage they have caused in different parts of the world recently. It is because of the high increase in the world’s population and the need for secure food. It causes a threat to plant biodiversity and also the population of plant diseases. A number of relationships

Emerging and Reemerging Viral Pathogens DOI: https://doi.org/10.1016/B978-0-12-819400-3.00046-6

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influence the interplay between pathogen, host, vector, and environment. As a consequence of those factors, we are facing emerging plant virus infections in different parts of the world. They are generally insecttransmitted plant virus infections and are generated because of the increase in insect populations due to global warming. Recent emerging plant viruses are mostly whitefly transmitted and are followed by aphid and thrips-transmitted infections. Recent emerging viruses are the members of Potyvirus, Ipomovirus (Potyviridae), Tospovirus (Bunyaviridae), Begomovirus (Geminiviridae), Crinivirus (Closteroviridae), Carlavirus (Betaflexiviridae), and Torradovirus (Secoviridae) genuses. Within this chapter, recently emerging plant viruses and their symptomatology, etiology, and transmission will be discussed in details.

What Is the Consequences of Climate Change on Plant Diseases? Climate change threatens crop yield both directly and indirectly; directly changes in plant growth and production, indirectly through the impact of plant disease. It has been estimated that changes in climate have already been reducing global agriculture production by 1% 5% per decade over the last 30 years. The greatest reduction has been observed in maize and rice in tropical regions (Newberry et al., 2016). Increase in internal trade and international travel of plant materials enhances the risk of new viruses and their vectors into new areas (Krishnareddy, 2013). Ecological factors, besides climate and agricultural practices, play an important role in the emergence of new plant virus diseases in different locations on the earth. Changes in the climatic conditions can contribute to the spread of newly introduced viruses and their vectors. Climate change can influence the environment on microto-macroscale and ranging from microclimate to continental and global scale. The most important virus vectors are aphids, thrips, and whiteflies.

What Is Emerging Plant Virus? Emerging virus is a causative agent of a new or previously unknown infection. The term became popular in the 1990s but in general emerging viruses are not new. A number of RNA viruses constitute a new threat on economically important vegetable crops, grapevine, citrus, blueberry, cassava, and rice. These are new disease-causing agents, socalled emerging or re-emerging viruses or strains, responsible for serious plant diseases (Martelli and Galitelli, 2009), especially changes in host range of a virus causing a disease not previously obvious. Factors

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that favor the emerging infectious plant diseases are overpopulation, globalization, environmental changes, and deforestation. Global international trade, commercial travel of plant materials, urbanization, and climate change are the main reasons of expansion of emerging viruses and their vectors to new areas, and once they have been introduced, fitness of these pathogens to new agroclimatic conditions leads to the development of new epidemics. Ecological factors including migration, climate, and agricultural practices play an important role in the emergence of plant virus diseases. Changing climate conditions can contribute to a successful spread of newly introduced viruses and their vectors. Most important vectors are aphids, whiteflies, and thrips and they are all affected by the global climate changes positively. Different biotypes of vectors have been associated with outbreaks of new viral diseases of different agricultural products (Krishnareddy, 2013). Climate change cycle is occurring more rapidly than the earlier ones as the rate of temperature change is accelerated by human activities and greenhouse gas emissions (carbon dioxide, methane, nitrous oxide) which affects the emission of infrared radiation by gases in the atmosphere and warms the surface of the world. Evidence of global warming includes increases in global warming of air and ocean temperatures and widespread melting of snow and ice at the poles. Global warming is terribly affecting the different regions in the world and will continue in the future. China will be the worst affected nation from global warming followed by Indonesia and India than Middle-East and Mediterranean Region. In American continent, Central and Southern America and some of the nations there will suffer from the global warming more (Jones, 2009). Emergence of new diseases and reemergence of other diseases have dramatically increased in the recent years due to climatic alterations of weather conditions. Different biotypes of vectors have been associated with the outbreaks of viral diseases (Krishnareddy, 2013). It is predicted that increasing global temperature will be between 0.9 C and 3.5 C in different parts of the world by 2100 (Das et al., 2016). Altered climatic conditions alter the varieties of cultivated plants grown and alter the alternative cultivated or weed reservoir hosts and cause some changes on cultivation systems of plants. Changes in mean temperatures can alter the scale of virus epidemics by modifying selection pressure and virus evolution rates. Warmer temperatures can increase virus evolution rates leading to more virulent strains with wider host ranges, higher virus multiplication rates in hosts, and increased vector transmission efficiencies, synergism, and complementation between viruses. Climate change may also affect the geographic range of potential vectors, vector phenology, overwintering density, migration, and activity (Canto et al., 2009; Elad and Pertot, 2014).

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Genome sequencing to identify the emerging viruses, global communication network, rapid diagnosis tools, and finally new approaches to vaccine are the advances facilitating the control of emerging viruses. Changes in the climate shift the selection pressure of the host populations which could result in shifts in the diversity of resistance genes (Krishnareddy, 2013). Virus diseases cause losses in the yield-cultivated plants and the quality of the yield produced. The damage is severe in tropics and subtropics and in protected and organic cropping systems. They also damage the wild plants and threaten the native plant biodiversity. Virusinfected plants are often stunted and deformed with chlorotic areas, mosaic and ringspot symptoms developed on leaves. They also reduced the size of fruits and cause diverse symptoms on the surface of the fruits, sometimes on the seed coat of the fruits, such as in plum pox virus infections. Devastating pandemics of cassava mosaic virus in East and Central Africa and Zucchini yellow mosaic virus epidemics on cucurbits in Australia forced the growers to stop producing those plants in those continents (Coutts et al., 2011). It has been reported by plant virus epidemiology group that many plant viruses are threatening the global food security such as Rice yellow mottle virus in rice, Maize streak virus in maize, Cacao swollen shoot virus in cacao, Groundnut rosette virus in peanut, Banana bunchy top virus in banana, Rice tungro bacilliform virus in South and SouthEast Asia, Tomato yellow leaf curl virus of tomato, potyviruses of cucurbits, and soil-borne viruses of cereals (Jones and Barbetti, 2012). Potyviruses, Ipomoviruses (Potyviridae), Tospoviruses (Bunyaviridae), Begomoviruses (Geminiviridae), Criniviruses (Closteroviridae), Carlaviruses (Betaflexiviridae), and Torradoviruses (Secoviridae) are seven large and economically important emerging group of viruses, although they differ in many aspects, including particle size, shape, types of vector, and infection modes (Jones, 2009; Rodoni, 2009). Viruses appear to cause a greater proportion of emerging infectious diseases of plants than the other pathogens. Insects are the most important vectors of plant viruses and among them aphids, whiteflies, and thrips are the most important ones and the others are plant hopper, leaf hopper, and mealybugs. Aphid borne viruses are the most widespread and damaging viruses of cultivated plants in temperate zones, but whitefly borne and thrips borne (Begomoviruses and Tospoviruses, respectively) are important in tropical and subtropical regions, in open fields and also in protected cultivations. Some of the plant viruses multiply within their insect vectors while multiplying in their host plants. Infection of viruses alters the host metabolism and especially increases the protein contents, making the infected plants more attractive to insect vectors. Some of the economically important viruses are transmitted by mites, and some by fungi surviving in soil. Soil-borne viruses are transmitted by

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roots of infected plants to healthy ones or by ectoparasitic nematodes and oomycetes and lower fungi. Soils infested by nematodes or resting spores of vector fungi can be dispersal by flooding and human activity (Jones and Barbetti, 2012). Mechanically transmitted viruses are the ones that multiply in the host cell at very high concentrations but they have no vector. They disseminate by contact of infected leaves to healthy ones, by root contacts, by animals, and humans. Usually, plants are infected with more than one virus such as in the case of potato viruses and tomato-infecting viruses. Potato virus X (PVX) together with Potato virus Y (PVY) causes rugose mosaic in potatoes and Tobacco mosaic virus together with PVX causes severe streak disease in tomatoes (Brunt et al., 2005). Virus infections also cause considerable losses in fiber production, Cotton leaf curl virus cause devastating losses of cotton in India. Losses from virus epidemics in annual and perennial grass and legume forage plants result in decreased nitrogen fixation, herbage, and seed production (Jones and Barbetti, 2012). All of the viruses are obligate parasites replicate using the host cell components amino acids, nucleotides, and ribosomes. They have a nucleic acid genome mostly RNA and single stranded and are covered with a protective protein coat. They enter to cells of healthy plants through the tiny wounds and transmission from plants to plants by insect vectors (aphids, whiteflies) mites, nematode, and fungal vectors. Spread of one generation to the next does not require wounding. It can occur through the sowing-infected seeds (almost 10% of plant viruses are seed-borne) by pollen, by vegetative propagation material originated from infected plant material (cuttings, root stocks, tubers, bulbs, and corms). It can occur in infected volunteer crop plants and in weeds persist. Plant viruses have broad host ranges but usually poorly adapted to individual hosts, whereas the ones that have the narrow host ranges and are well adapted to the hosts infect. Weeds are fast-growing plants that are well adapted to survive the climate change. In most of the cases, weeds play the role of reservoir of viruses and also aphid vectors. Recently, there has been encouraging progress in understanding some of the effects of climate change parameters especially adverse effects of elevated carbon dioxide, temperature, and rainfall-related parameters with a small number of plant viruses and with aphid vectors. But much work has to be done in order to mitigate their detrimental effects on future global food security and plant biodiversity (Jones, 2016).

Effects of Climatic Changes and Global Warming on Reservoir Wild Plants Plant pathogenic viruses are obligate parasites, simply composed of nucleic acid and a protein. Plant-to-plant spread from infected to

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healthy ones usually occurs by the activity of special insects, mites, nematodes, fungal vectors, and mechanical damage. Viruses have different temperature optima for multiplication within their host plants, some are adapted to warmer regions (Jones and Barbetti, 2012). Although the weeds are fast growing and well-adapted plants in adverse environmental conditions, they contain some viruses and act as a reservoir of such pathogens. Avena fatua (wild oat) is a major reservoir of Barley yellow dwarf virus in the wheat and barley fields which can be spread by aphid vectors easily to the healthy plants. Cucumber mosaic virus (CMV) (major virus infection of Cucurbitaceae) is transmitted by aphid vectors feeding on weed reservoirs such as Chenopodium album, Chenopodium hybridum, Datura stramonium, Senecio vulgaris, Urtica urens, and Capsella bursa-pastoris to the healthy cucurbit plants, although it is also seed transmitted at less extent (Kazinczi et al., 2004; Tomlinson et al., 1970).

Effects of Climatic Changes and Global Warming on Vector Populations Changes in climatic parameters alter vector distribution, vector abundance and vectoral activity on host plants, consequently weakens the chemical control or biological control measures against them. Great capacity of reproduction and short development times of whiteflies and aphids make them especially suited to respond to climate changes (Canto et al., 2009). It has been proved that aphids are capable of moving long distances after encountering warmer weather conditions. The main order of insects involved in the transmission of plant viruses is the sap feeding Hemiptera. Aphids (Aphididae), whiteflies (Aleyrodidae), and leaf hoppers (Cicadellidae) are the most important vectors of plant virus infections.

On Aphids Aphids are the main group of vectors, transmitting 275 virus species within 19 virus genera, and are very important vectors in temperate zones of the world (Canto et al., 2009). Aphids account for the transmission of 50% of the insect-vectored viruses. They have piercing-sucking mouthparts that facilitate the delivery of virions into plant cells without causing any damage. With the option of parthenogenetic reproduction, aphid populations can increase at high rates, thereby potentiating disease epidemics and long-distance spread of viruses (Krishnareddy, 2013). Aphids are globally distributed and more than 200 species have been identified as virus vectors (Nault, 1997).

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Experiments on the elevated ambient CO2 levels showed mixed effects on aphid populations, results obtained were highly species specific (Bezemer and Jones, 1998). Increases in CO2 concentrations stimulate the plant growth but decrease the nutritional value of the plants for insects feeding on them (Krishnareddy, 2013). Responses of aphids to high concentrations of CO2 and O2 are highly variable depending on the aphid species, development and fertility increase or remain unaffected or decrease by such atmospheric changes (Awmack et al., 2004). More than 275 plant viruses are transmitted by aphids. They cause huge losses in the quality and quantity of most of the agricultural products by producing unmarketable fruits of vegetable and fruit crops. Most important Potyvirus infections transmitted by aphids in temperate climate are CMV, Turnip mosaic virus, PVY, Lettuce mosaic virus, and Papaya ringspoıt virus (Tomlinson, 1987).

On Whiteflies Whiteflies are confined to warmer areas and among them Trialeurodes vaporariorum and Bemisia tabaci are well-known virus vectors, although 1140 species are present in the world. B. tabaci transmits more than 200 plant viruses that mostly belong to Geminiviridae, ssDNA viruses (Begomoviruses) in persistent manner, and also RNA viruses of Crini, Carla, and Ipomoviruses as in semipersistent manner. Whiteflies correspond a complex of genetic variants usually referred to as biotypes. Those viruses cause economic losses in many commercial crops in tropics and also warmer temperate regions, such as Mediterranean region, in the world both in open fields and protected cultivations. They can fly over to long distances and distribute plant viruses. Elevated CO2 levels adversely affect whitefly colonies but flight activity has increased at higher temperatures. Tomato yellow leaf curl virus is well-known Begomovirus, transmitted by whitefly B. tabaci adults in a persistent manner in tropic and subtropic regions (Navas Castillo et al., 2011). They can occur as in different biotypes, such as B or Q type, according to the plant they invade. Criniviruses also have been transmitted by T. vaporariorum in protected cultivations and also in open-field cultivations recently. Recent surveys have indicated generally the presence of new emerging viruses, vectored by whiteflies in temperate regions of the world, which will be discussed later on.

On Thrips Tospoviruses are vectored by thrips and became a major problem in recent years (Damsteegt, 1999). This group has a long history and is

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widespread in vegetables, ornamentals, and floral crops. They are major pathogens of Bunyaviridae family. Type virus is Tomato spotted wilt virus (TSWV) and causes infections on a wide spectrum of plants especially Solanaceous plants and ornamentals. Tospoviruses are known to be transmitted by the thrips belonging to family Thripidae, 14 thrips varieties had been found for the transmission of TSWV, Thrips tabaci and Frankliniella schultzei are the major ones. Species in Thrips, Frankliniella, Scirtothrips, Microcephalothrips, Dictyothrips, and Ceratothripoides transmit plant viruses and TSWV multiples within the vectors and only adults can transmit the virus to new hosts. The viruses are transmitted in a circulative propagative manner. The Tospoviruses are acquired by insects at the stage of larval instar and transmit at the stage of an adult after a latent period, if they acquire at the stage of adult, transmission does not occur. Three varieties of Tospoviruses are well known in the world: TSWV, Impatiens necrotic spot virus (INSV), and Iris yellow spot virus. TSWV causes necrotic leaf spots on leaves of Solanaceous plants and their major damage occurs on the fruit crops. Yellowish rings and lines occur on the surface of the fruits which makes them completely unmarketable. Thrips lay eggs on the plants and life cycle usually takes 20 days from egg to adult (Krishnareddy, 2013). Thrips development is known as dependent on temperature. Adult females can survive for 5 weeks at warm conditions and oviposit about 50 eggs (Reitz, 2008). INSV is another emerging Tospovirus infection basically on ornamental plants and is distributed worldwide (Damsteegt, 1999).

Effects of Climatic Changes and Global Warming on Plant Viruses Viruses are obligate parasites that depend on their hosts and vectors for their spread and multiplication. Increased temperatures influence the virus symptomatology and multiplication within the hosts. Plants maintained in warmer conditions become more susceptible to the virus infections. It is well known that RNA viruses such as tobacco mosaic virus (TMV) and CMV accumulate better at higher temperatures than lower temperatures in their hosts. Severe fruit symptoms have been observed on the tomato fruits infected with TSWV at 36 C, although virus concentration was lower. Increased temperature can influence the behavior of the mixed virus infections. In tobacco plants, PVX, together present with CMV, was able to move systematically at 31 C and not in lower degrees (Close, 1964). Changes in mean temperatures can alter the virus epidemics by modifying virus evolution rates. Warmer temperatures can increase virus evolution rates, leading to more virulent strains with broader natural host ranges, higher multiplication rates in

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hosts, and increased vector transmission efficiencies (Pico et al., 1996). It has been detected that when some virus-infected plants hold in warmer conditions, higher temperature enhanced the spontaneous mutations on coat protein gene.

Recently Emerging Plant Viruses on Different Plant Hosts We are facing many different emerging viruses on various host plants. Most of them are whitefly-transmitted viruses belonging to Torradovirus genus of Secoviridae, Crinivirus genus of Closteroviridae, and Begomovirus genus of Geminiviridae families. The other genuses are Ipomovirus of Potyviridae and Carlavirus of Betaflexiviridae families (Table 46.1).

Some Features of Important Emerging Viruses Pepino Mosaic Potexvirus The virus is originally described in Peru, on originally infected pepino plants (Solanum muricatum) and found to infect tomato crops. It is prevalent in Canada, United States, and many European countries. It also presents in quarantine list of many nations and European Plant Protection Organization (EPPO). PepMV causes yellow mosaic on young leaves of pepino plants but the infection results with severe stunting of whole plants on infected tomato plants. Foliar symptoms resemble herbicide damage as interveinal chlorosis, lower leaves show necrotic spots. The infection causes blotchy ripening of fruits; therefore, yellow speckles and spots on ripe fruits make them unmarketable. Crop reduction in glasshouse-grown tomatoes is estimated to be 40% (Martelli and Galitelli, 2009). PepMV is one of the most important virus infections of tomato because of the adverse effects on quality and yield, in the whole world. PepMV is a member of Potexvirus genus of Flexiviridae family. The particles are flexible rods, 508 nm long 11 nm wide, and contain five open reading frames (ORFs) and 6410 6425 nucleotides. Coat protein sequences are located on the fifth ORF. PepMV has no vector and distributed by contacts, contaminated hands, and seeds. Hygienic precautions are very important for the prevention of contamination. Torradovirus Infections Torradovirus genus is a genus of Secoviridae family.

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TABLE 46.1 List of the New Emerging Plant Viruses. Disease

Genus

Pepino mosaic virus (PepMV)

Potexvirus

ToTV

Major host

Potential vectors

Distribution

Literature cited

Pepino, tomato

Not present

Europe, Southern American continent

Martelli and Galitelli (2009)

Torradovirus

Tomato

Whiteflies

Europe

Navas Castillo et al. (2011)

ToMarV

Torradovirus

Tomato

Whiteflies

Middle East

Van der Vlugt et al. (2015)

CaTV

Torradovirus

Carrot

United Kingdom

Van der Vlugt et al. (2015); RozadoAguirre et al. (2016, 2017)

CsTLV

Torradovirus

Cassava

Africa

Van der Vlugt et al. (2015)

SCLSV

Torradovirus

Cucurbits

Whiteflies

Sudan

Van der Vlugt et al. (2015)

Tomato chocolate spot virus (ToChV)

Torradovirus

Tomato

Not present

Central and Southern American continents

Verbeek et al. (2010)

LNLCV

Torradovirus

Lettuce

Nasonovia ribisnigri

The Netherlands

Verbeek et al. (2014, 2017)

MLNV

Unknown

Corn

Unknown, Seed transmitted

Africa, United States, China

De Groote et al. (2016)

GRBaV

Geminivirus

Grapevine

Unknown

United States

Bahter et al. (2016); Sudarshana et al. (2015)

Citrus leprosis virus (CLV)

Cilevirus

Citrus

Brevipalpus phoenicis (Acari: Tenuipalpidae)

Central and Southern American continent

Maia, O.M. de A and Oliveira, C.A.L. de (2006)

PYVV

Crinivirus

Potato

Not present

South America

Rodriquez et al. (2015)

Tomato infectious chlorosis virus (TICV)

Crinivirus

Tomato

Whiteflies, esp. Trialeurodes vaporariorum

United States, Europe, Middle East

Navas Castillo et al. (2011)

Tomato chlorosis virus (ToCV)

Crinivirus

Tomato

T. vaporariorum, Bemisia tabaci

United States, Europe, Middle-East, China

Navas Castillo et al. (2011)

Lettuce infectious yellows virus (LIYV)

Crinivirus

Lettuce

B. tabaci

United States

Damsteegt (1999)

(Continued)

TABLE 46.1 (Continued) Disease

Genus

Major host

Potential vectors

Distribution

Literature cited

LCV

Crinivirus

Lettuce

Bemisia argentifolii

United States

Damsteegt (1999)

Cucurbit yellow stunting disorder virus (CYSDV)

Crinivirus

Cucurbits

B. argentifolii

Europe, Middle East

Damsteegt (1999)

Cucurbit chlorotic yellows virus (CCYV)

Crinivirus

Cucurbits

B. tabaci

China Europe, Middle East

Tang et al. (2017)

Sweet potato chlorotic stunt virus (SPCSV)

Crinivirus

Sweet potato

B. tabaci

Argentina, Peru

Muller et al. (2002); Nome et al. (2007)

TYLCSV

Begomovirus

Tomato

B. tabaci

South East, Spain

Navas Castillo et al. (2011)

Latent Indian cassava mosaic virus (LICsMV)

Begomovirus

Cassava

B. tabaci

India

Karthikeyan et al. (2016)

African cassava mosaic disease (ACsMV)

Begomovirus

Cassava

Local whiteflies

Africa

Navas Castillo et al. (2011)

SRBSDV

Fijivirus

Rice

Sogatella furcifera

China, Vietnam

Zhou et al. (2013)

RBDV

Idaeovirus

Raspberry

Pollen

United States

Strik and Martin (2003)

ToNSV

Ilarvirus

Tomato

Not present

United States

Batuman et al. (2011)

BlShV

Ilarvirus

Blueberry

Pollen

United States

Damsteegt (1999); Bristow and Martin (1999)

BlScV

Carlavirus

Blueberry

Aphids

United States

Martin et al. (2009)

Blueberry red ringspot virus (BlRRV)

Soymovirus

Blueberry

United States

Martin et al. (2009)

BlScV, Blueberry scorch virus; BlShV, Blueberry shock virus; CaTV, Carrot torrado virus; CsTLV, Cassava torrado-like virus; GRBaV, Grapevine red blotch-associated virus; LCV, Lettuce chlorosis virus; LNLCV, Lettuce necrotic leaf curl virus; MLNV, Maize lethal necrosis disease; PYVV, Potato yellow vein virus; RBDV, Raspberry bushy dwarf virus; SCLSV, Squash chlorotic leaf spot virus; SRBSDV, Southern rice black-streaked dwarf virus; ToMarV, Tomato marchitez virus; ToNSV, Tomato necrotic spot virus; ToTV, Tomato torrado virus; TYLCSV, Tomato yellow leaf curl Sardinia virus.

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Tomato Torrado Virus Tomato torrado virus (ToTV) was first seen in Murcia (Spain) in 2004 on tomato plants. Torrado is a Spanish word, meaning burnt and the infections cause necrotic lesions on the base of the leaves, surrounded by yellowish areas (Verbeek et al., 2007). These later develop into severe necrosis on foliage and fruits and plants show an overall severe reduction on growth. It has been reported from many European countries, Australia, Panama, and Colombia. Molecular Structure

Virus particles of torradoviruses are spherical, approximately 30 nm in diameter, and consist of two single stranded positive sense RNAs, RNA1, and RNA2. Coat proteins are 23, 26, and 35 kDa and coat protein genes are located on RNA2. Proteinase cofactor, helicase, RNA-dependent RNA polymerases are coated on RNA1. RNA1 is longer than RNA2 and is 6900 8100 nt long whereas RNA2 is only 5600 4700 nt long. Molecular structure of all torradoviruses is the same. ToTV is transmitted by whiteflies T. vaporariorum (Westwood), B. tabaci (Gennadius), Trialeurodes abutilonea (Haldeman) in semipersistent manner (Van der Vlugt et al., 2015). Those whitefly species are also the vectors of other Torradovirus infections of tomato crop. Tomato Marchitez Virus Symptoms of Tomato marchitez virus resembles ToTV and prevalent only in Mexico. It causes necrosis on fruits so all of the crops are unmarketable. Its genomic conformation also resembles ToTV, as particle size and sizes of coat protein and RNAs. Tomato Chocolate Spot Virus This infection is only observed in Guatemala. It causes dark spots on leaves and fruits and generally is mixed infected by tomato chocolate virus. Particle morphology is similar to other Torradovirus infections. Tomato Necrotic Dwarf Virus It is presented in California, United States, and infects Solanaceous crops and some weeds. It causes necroses on stems and leaves of tomato plants and the whole plant is severely stunted. It is transmitted with all three whitefly species: T. vaporariorum (Westwood), B. tabaci (Gennadius), and T. abutilonea (Haldeman). Tomato necrotic dwarf virus (ToNDV) is also transmitted efficiently by grafting, and it can be transmitted by mechanical inoculation. Complete genomic sequences are obtained and proved that ToNDV is a member of Torradovirus genome.

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Lettuce Necrotic Leaf Curl Virus It is first described in the Netherlands on open field grown lettuces and is the first Torradovirus infection, infecting crops other than tomatoes (Verbeek et al., 2017). It causes leaf curling and necrosis on leaf lettuces and according to the full genome sequences, it is a proposed for to be included in Torradovirus genus. Carrot Torrado Virus It was first described in the United Kingdom, on open field grown carrots. Its main symptom is root necrosis and can be confused with fungal infections of carrot (Rozado-Aguirre et al., 2016). Next generation sequencing (NGS) analysis yielded large fragments (6900 and 4700 nt) of a bipartite viral genome. Further analysis suggested that the 6900-nt fragment was the genomic RNA1 of a novel Torradovirus. The smaller fragment contained two ORFs. ORF1 encoded a putative 202 aa (22 kDa) protein with 43% homology to the RNA2 ORF1 of Lettuce necrotic leaf curl virus. The second ORF encoded a putative 130-kDa polyprotein that appeared to contain movement-protein and coat protein domains and had 35% homology to the RNA2-ORF2 of ToTV (RozadoAguirre et al., 2017). Cassava Torrado-Like Virus Genomic investigations of Cassava plants showing frog skin disease symptoms in Colombia revealed the presence of mixed virus infections. One of them is described as Cassava torrado-like virus (CsTLV) and this virus, which showed only limited sequence similarity (50% 60%) to other torradoviruses, was tentatively named CsTLV. Partial sequences of CsLTV RNA1 and RNA2 have been published and present in genbank. After Cassava frogskin-associated virus, CsTLV is the second most frequently found virus infection found in virus infecting cassava plants that show leaf and root symptoms in Colombia. Maize Lethal Necrosis Disease It is recently reported in maize plantations in Kenya in 2011 and quickly spreading in Kenya, Uganda, and Tanzania. It is a mixed infection of two viruses: Maize chlorotic mosaic virus together with either with Maize dwarf mosaic virus or wheat streak mosaic virus. Leaves of the infected plants are of pale color and usually chlorosis develops and dies at the flowering stage. Ears are small and infected plants set little or no seeds (De Groote et al., 2016). In 2012 the disease was estimated in 18,500 ha in Kenya.

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Grapevine Red Blotch Virus This virus is the main virus problem of the vineyards in the United States and Canada. Its symptomatology is quite similar to Grapevine leaf roll associated virus infections and differs according to the specifications of the cultivars. It causes reddening on the old leaves of red berried cultivars but severe vein clearing and chlorosis are usually observed on the white berried cultivars (Sudarshana et al., 2015). Symptoms are severe and easily predicted in late summer and fall. It is a geminivirus infection and contains ssDNA (Bahter et al., 2016). Citrus Leprosis Rhabdovirus It is widespread in Central and Southern American continent and infects mainly orange (Citrus sinensis) and grapefruit (Citrus paradisi). The virus is a member of Cilevirus genus of Rhabdoviridae family. It causes necrotic lesions on leaves, fruits, and the branches of infected trees. Lesions are concentric and contain 2 3 haloes, starting from 2 3 mm up to 10 20 mm in diameter according to the location on the tree. Lesions are chlorotic at the initial stages of infection than later all became necrotic. The virus also causes symptoms on the bark tissues of branches, as necrotic lesions, then the die-back occurs. Rhabdovirus particles were observed on the paranchimatic cells of the leaves. Its particles are 120 130 nm long (maximum 300 nm) and 50 55 nm wide. It is transmitted by Acari species Brevicoryne phoenicis (Acarina). Potato Yellow Vein Virus Potato yellow vein virus (PYVV) (family Closteroviridae, genus Crinivirus) is the etiologic agent of the Potato yellow vein disease (PYVD), a quarantine disease in Europe and the United States. PYVV is a reemergent virus in Andean countries, infecting potato plants in Colombia, Venezuela, Peru, and Ecuador. Symptoms of PYVD in potato begin with vein clearing of secondary and tertiary leaf veins, followed by a yellowing that covers the leaf lamina, resulting in the leaflet becoming bright yellow. PYVD affects the weight and number of tubers produced by different varieties and Solanum species. It has been estimated that, for Solanum tuberosum subsp. Andigena (STA) and Solanum phureja (SPH), losses between 30% and 50% can occur depending upon the host, weather conditions, vector presence, and other variables (Salazar et al., 2000; Rodriquez et al., 2015). PYVV is transmitted by the insect vector T. vaporariorum (West) in a semipersistent manner by planting infected potato tubers (Salazar et al., 2000). PYVV has a tripartite RNA (ss 1 ) genome consisting of RNA1 (8035 nt), RNA2 (5399 nt), and RNA3 (3892 nt). RNA2 codes for the coat protein which covers most of the virion.

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Tomato Infectious Chlorosis Crinivirus and Tomato Chlorosis Crinivirus The infection is first described in the United States together with Tomato chlorosis virus in mid-1990s as two new whitefly-transmitted viruses emerged in tomato plantations. Both infections belong to Crinivirus genus of Closteroviridae family (from Latin crini means “hair”). Symptomatology of both infections is so similar, interveinal yellowing and thickening of leaves. Yield is reduced by the production of small fruits and less fruit set. Symptomatology is generally confused with mineral deficiency symptoms. Both are transmitted by T. vaporariorum (West) and ToCV is also transmitted by T. abutiloneus and B. tabaci. TICV has been found mainly in North America and Europe, whereas ToCV has been detected in more than 20 countries including European, African, Middle-East, and Asian countries (Navas Castillo et al., 2011). This can explain that more vector species are involved in transmission of the ToCV. B. tabaci (Gennadius) (Sweet potato whitefly) (Hemiptera, Aleyrodidae) Q Biotype has been detected in China as the vector of ToCV in semipersistent manner (Tang et al., 2017). Lettuce Infectious Yellows Crinivirus Members of the Criniviruses all induce yellowing symptoms in their plant hosts, are generally phloem-limited, nonmechanically transmissible, and have large ssRNA genomes. Lettuce infectious yellows virus (LIYV) causes interveinal chlorosis on the infected leaves of especially crispy lettuces and later became completely chlorotic. Its genome is composed of two ssRNAs of 8.1 and 7.2 kb. RNA1 encodes for proteins associated with RNA replication and RNA2 encodes the hallmark closterovirus gene array. LIYV is now recognized as the type member of the genus Crinivirus within the family Closteroviridae. It is also transmitted by B. tabaci Q biotype (Damsteegt, 1999). Lettuce Chlorosis Virus It is a recently found virus infection in lettuce plantations in the United States. Symptomatology is quite similar to LIYV but the vector differs: Bemisia argentifolii. It also belongs to Crinivirus genus. Cucurbit Yellow Stunting Disorder Crinivirus and Cucurbit Chlorotic Yellowing Virus Both are Crinivirus infections and restricted to Cucurbits species and esp. cucumber, squash, melon, and watermelon plants are susceptible. Cucurbit yellow stunting disorder crinivirus (CYSDV) causes great stunting of the infected plants whereas Cucumber chlorotic yellowing virus veinal chlorosis on the infected plants. Infected cucumber and squash plants

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show vein yellowing on apical leaves and systemic green mosaic occurs on cucumber fruits. Infected melon plants show yellow leaf veins but fruits are asymptomatic. In some cases, sudden deaths of melons are observed. Infection can be asymptomatic on watermelon leaves but fruits split when they are ripen and reach to harvestable size (Navas Castillo et al., 2011). Both cause great yield reduction in greenhouse grown cucurbits. CYSDV is transmitted by B. argentifolii and CCYV by B. tabaci as semipersistent manner. Sweet Potato Chlorotic Stunt Crinivirus It is the major virus infection of sweet potato [Ipomoea batatas L. (Lam)] plants and causes veinal chlorosis on leaves and infected plants are severely stunted. Leaf areas are greatly reduced by the virus infection. SPCSV belongs to Crinivirus genus and presents in phloem tissues. SCCSV-infected plants are usually coinfected with sweet potato feathery mottle virus (SPFMV, Potyvirus genus). Synergism occurs in dualinfected plants and stunting and leaf distortions are severe. Infection is present in South American continent: Peru and Argentina. Tomato Yellow Leaf Curl Sardinia Virus Tomato yellow leaf curl disease (TYLCD) is first found in Italy in the late 1980s, later was found in Spain and was transmitted by local B. tabaci biotypes. TYLCD is one of the most devastating and widespread virus infections of tomato production. The symptoms are marginal chlorosis and reduce in size of apical young leaves, upward curling, reduced size of young leaves, shortened internodes, and severe stunting. Floral distortion, usually observed, causes complete yield loss. It is transmitted by whiteflies and is a member of Begomovirus genus of Geminiviridae which has ssDNA genome. The complex is referred to as TYLCV-like viruses and includes approximately 15 accepted and proposed species. TYLCV is present in Mediterranean Basin, Far East, American continent, and Australia (Navas Castillo et al., 2011). Common bean crops do not play as a bridge crop for Tomato yellow leaf curl Sardinia virus (TYLCSV), whereas it is important for the epidemy of TYLCV. Mixed infection of tomato plants with TYLCV and TYLCSV led to the appearance of two recombinant viruses: Tomato yellow leaf curl Malaga virus (TYLCMaV) and Tomato yellow leaf curl Axarquia virus (TYLCAxV). Although TYLCV is worldwide, TYLCSV is present only in Mediterranean countries. African Cassava Mosaic Disease and Latent Indian Cassava Mosaic Virus Cassava is important for African countries and the crop is affected by a number of begomoviruses which cause mosaic disease of cassava.

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Infection is prevalent in Africa and Indian subcontinent. Leaves of infected plants are distorted, smaller, and usually chlorotic spots are observed. Tubers number and size are greatly reduced because of the virus infection (Navas Castillo et al., 2011). Yield reduction is severe. Cassava mosaic disease is first reported from Tanzania and it is named African cassava mosaic virus (ACMV). The virus has bipartite genome and belongs to Begomovirus genus of Geminiviridae family. Recently, more begomoviruses are detected and involved for cassava mosaic virus disease in Eastern and Western Africa: East African cassava mosaic virus (EACMV), East African cassava mosaic Kenya virus, East African cassava mosaic Cameroon virus, East African cassava mosaic Malawi virus, South African cassava mosaic virus, and so. Two begomoviruses are reported from India and Sri Lanka in Indian subcontinent and they are identified and named Indian cassava mosaic virus and Sri Lankan cassava mosaic virus according to their localization. The virus is not present in South America although cassava cultivations have great importance. Southern Rice Black-Streaked Dwarf Virus The virus was first discovered in 2001 and caused devastating outbreak in Vietnam and China in 2009 10, threatening the rice production of Asia (Xu and Zhou, 2017). The virus is present only in East and South-East Asia and transmitted by Sogatella furcifera (white-backed plant hopper) as persistent circulative manner. Southern rice blackstreaked dwarf virus besides rice infects maize and Chinese sorghum. Major symptom of infected seedlings is severe dwarfing. Excessive tillering is observed and small spikes and barren grains occur. Infected plants develop brown roots; therefore, great yield loss is evaluated in the infected areas (Zhou et al., 2013). The virus particles have 10 double-stranded RNA genome and belong to Fijivirus genus of Reoviridae. Raspberry Bushy Dwarf Virus It is the most important virus infection of raspberry (Rubus idaeus) and found in Scotland (United Kingdom) in 1961 (Jones et al., 1996). It causes symptomless infection on some varieties of raspberry or mild infection but causes severe infection on fruits and reduces the fruit quality. The infection is seed and pollen transmitted and can be transmitted by mechanical inoculation and grafting. The infection can be as mixed infection raspberry leaf mottle virus, raspberry latent virus, and in this case crumbling, of fruits and stunting of canes are more severe (QuitoAvilo et al., 2014). The virus is eliminated by heat therapy of infected plants. The virus is widespread in the United States, Australia, New Zealand, and Russia.

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Genome consists of two RNAs, RNA1 encodes the polymerase gene and shorter RNA, RNA2 encodes the movement and coat protein. Tomato Necrotic Spot Virus It is a new virus infection infecting field-grown tomatoes in Central Valley of California (United States) (Batuman et al., 2011). It causes necrotic lesions on stems, petioles, leaves, and fruits of infected plants. The virus belongs to Ilarvirus genus and reduces the quality of the production. Blueberry Shock Ilarvirus It is pollen borne virus infection of blueberry (Vaccinium corymbosum). Honeybees play a major role in dissemination of infection and transfer the pollen to flowers of healthy plants in spring (Bristow and Martin, 1999). Recovery sometimes occur and the infected plants do not exhibit any symptom but they continue to serve as inoculum source. Cranberries are generally symptomless. Flowers and young shoots die at the beginning of blossom and blighted tissues drop. Small and less fruits are produced by the infected plants. Thin red spots occur on both sides of the leaves. The virus is detected in the United States. Blueberry Scorch Virus The virus belongs to genus Carlavirus and aphid transmitted in nonpersistent manner. It is widespread in the United States and Canada and recently found in Europe. The virus also infects cranberry and wild black huckleberry without any symptoms. Blueberry aphid Ericaphous fimbriata is detected as inefficient vector of the virus in the United States and the other vectors involved in transmission of the virus are unknown (Martin et al., 2009). Symptoms usually develop 1 2 years after infection. Leaves and the blossoms are blighted but they remain attached to the stem. Leaf mottling, overall pale color, oakleaf pattern, and low number of blossoms are the major symptoms of infected plants. Infection causes great yield loss in the orchards. Blueberry Red Ringspot Virus Blueberry red ringspot virus (BRRSV) of the Soymovirus genus in the family Caulimoviridae causes red ringspot diseases in highbush blueberry (V. corymbosum L.) on leaves, stems, and fruits. The virus has been identified in the United States, Japan, Czech Republic, Slovenia, Poland, and Korea (Cho et al., 2012). Symptoms appear as red rings 4 6 mm in diameter on 1-year-old stems, leaves, and the fruits. These lesions are prominent on the upper surface but on some cultivars, they can be visible on both surfaces (Martin et al., 2009). Purple colored rings, 2 3 mm,

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develop on the infected fruits. Fruits of some infected cultivars are misshapen and unmarketable.

CONCLUSION Virus diseases of cultivated plants, ways of transmission, prevalence of the disease, and the damage they cause in different parts of the world are of great importance. This is because the food security as the world population increases. The adverse effects of viral diseases are both causing losses in the yield of the cultivated plants and quality of production. The damage is severe in tropics and subtropics and protected and organic cropping systems (Jones and Barbetti, 2012). Virus-infected plants are usually stunted and exhibit special leaf symptoms of that infection. Devastating pandemics of cassava mosaic virus in East and Central Africa forced many farmers to stop growing cassava. Same occurred in Australia, ZYMV epidemics have forced farmers to stop growing cucurbits (Coutts et al., 2011). According to the reports of Plant Virus Epidemiology Group, many other virus infections are threatening the global food security including Cacao swollen shoot virus in cacao, Rice tungro bacilliform virus in rice, potyviruses of cucurbits, soil-borne viruses of cereals and causing great epidemics (Jones and Barbetti, 2012). Viruses appear to cause a greater proportion of emerging infecting diseases of plants than the other pathogens, and the only method to control it is through breeding resistant or tolerant varieties although plant breeding for resistance against viruses are not short-term processes.

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