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Content

• 1. Viruses. Introduction
• 1.1 What are viruses
• 1.2 Why are viruses important
• 1.3 How are viruses transmitted
• 1.4 How are viruses classified
• 1.5 Genome properties: Important features include
• 1.6 Biological properties
• 2. General description of carrot
• 3. Carrot Virus Y
• 3.1 Distribution
• 3.2 Symptoms and Losses
• 3.3 Spread
• 3.4 Control
• 4. Carrot red leaf virus
• 4.1 Biological Properties
• 4.2 Virion Properties
• Literature
• 1. Viruses. Introduction
• 1.1 What are viruses
• Viruses are very small (submicroscopic) infectious particles (virions) composed of a protein coat and a nucleic acid core. They carry genetic information encoded in their nucleic acid, which typically specifies two or more proteins. Translation of the genome (to produce proteins) or transcription and replication (to produce more nucleic acid) takes place within the host cell and uses some of the host's biochemical "machinery". Viruses do not capture or store free energy and are not functionally active outside their host. They are therefore parasites (and usually pathogens) but are not usually regarded as genuine microorganisms.
• Most viruses are restricted to a particular type of host. Some infect bacteria, and are known as bacteriophages, whereas others are known that infect algae, protozoa, fungi (mycoviruses), invertebrates, vertebrates or vascular plants. However, some viruses that are transmitted between vertebrate or plant hosts by feeding insects (vectors) can replicate within both their host and their vector. This web site is mostly concerned with those viruses that infect plants but we also provide some taxonomic and genome information about viruses of fungi, protozoa, vertebrates and invertebrates where these are related to plant viruses.
• We also provide information about viroids, which are infectious RNA molecules that cause diseases in various plants. Their genomes are much smaller than those of viruses (up to 400 nucleotides of circular single-stranded RNA) and do not code for any proteins.
• 1.2 Why are viruses important
• Viruses cause many diseases of international importance. Amongst the human viruses, smallpox, polio, influenza, hepatitis, human immunodeficiency virus (HIV-AIDS), measles and the SARS coronavirus are particularly well known. While antibiotics can be very effective against diseases caused by bacteria, these treatments are ineffective against viruses and most control measures rely on vaccines (antibodies raised against some component of the virus) or relief of the symptoms to encourage the body's own defense system.
• Viruses also cause many important plant diseases and are responsible for huge losses in crop production and quality in all parts of the world. Infected plants may show a range of symptoms depending on the disease but often there is leaf yellowing (either of the whole leaf or in a pattern of stripes or blotches), leaf distortion (e.g. curling) and/or other growth distortions (e.g. stunting of the whole plant, abnormalities in flower or fruit formation).
• Yellow mosaic symptoms on lettuce caused by Lettuce mosaic virus.Yellow vein-banding symptoms on grapevine caused by Grapevine fanleaf virus.
• Fruit distortion on eggplant fruit caused by Tomato bushy stunt virus. Sometimes the virus is restricted to certain parts of the plant (e.g. the vascular system; discrete spots on the leaf) but in others it spreads throughout the plant causing a systemic infection. Infection does not always result in visible symptoms (as witnessed by names such as Carnation latent virus and Lily symptomless virus, both members of the genus Carlavirus). Occasionally, virus infection can result in symptoms of ornamental value, such as 'breaking' of tulips or variegation of Abutilon.
• Plant viruses cannot be directly controlled by chemical application. The major means of control (depending on the disease) include:
• Chemical or biological control of the vector (the organism transmitting the disease, often an insect): this can be very effective where the vectors need to feed for some time on a crop before the virus is transmitted but are of much less value where transmission occurs very rapidly and may already have taken place before the vector succumbs to the pesticide.
• Growing resistant crop varieties: in some crops and for some viruses there are highly effective sources of resistance that plant breeders have been using for many years. However, no such "natural" resistance has been identified for many others. Transgenic resistance has shown considerable promise for many plant-virus combinations following the discovery that the incorporation of part of the virus genome into the host plant may confer a substantial degree of resistance. For example, the use of this approach in Hawaii to control Papaya ringspot virus has been credited with saving the local papaya industry. However, this technology is controversial, particularly in Europe, and the extent to which it will be used commercially is currently uncertain.
• Use of virus-free planting material: in vegetatively propagated crops (e.g. potatoes, many fruit crops) and where viruses are transmitted through seed major efforts are made through breeding, certification schemes etc., to ensure that the planting material is virus-free.
• Exclusion: the prevention of disease establishment in areas where it does not yet occur. This is a major objective of plant quarantine procedures throughout the world as well as more local schemes.
• 1.3 How are viruses transmitted
• Some important animal and human viruses can be spread through aerosols. The viruses have the "machinery" to enter the animal cells directly by fusing with the cell membrane (e.g. in the nasal lining or gut).
• By contrast, plant cells have a robust cell wall and viruses cannot penetrate them unaided. Most plant viruses are therefore transmitted by a vector organism that feeds on the plant or (in some diseases) are introduced through wounds made, for example, during cultural operations (e.g. pruning). A small number of viruses can be transmitted through pollen to the seed (e.g. Barley stripe mosaic virus, genus Hordeivirus) while many that cause systemic infections accumulate in vegetatively-propagated crops. The major vectors of plant viruses are:
• Insects. This forms the largest and most significant vector group and particularly includes:
• Aphids: transmit viruses from many different genera, including Potyvirus, Cucumovirus and Luteovirus.
• Whiteflies: transmit viruses from several genera but particularly those in the genus Begomovirus.
• Hoppers: transmit viruses from several genera, including those in the families Rhabdoviridae and Reoviridae.
• Thrips: transmit viruses in the genus Tospovirus.
• Beetles: transmit viruses from several genera, including Comovirus and Sobemovirus
• The virus-vector relationships are of several types:
• At one extreme, the association occurs within the feeding apparatus of the insect, where the virus can be rapidly adsorbed and then released into a different plant cell. The feeding insect looses the virus rapidly when feeding on a non-infected plant. Such a relationship is termed "non-persistent". The best studied examples are of potyvirus transmission by aphids.
• At the other extreme, the virus is taken up into the vector, circulates within the vector body and is released through the salivary glands. The vector needs to feed on an infected plant for much longer and there is an interval (perhaps several hours) before it can transmit. Once it becomes viruliferous, the vector will remain so for many days and such a relationship is therefore termed "persistent" or "circulative". The best studied examples are of luteovirus transmission by aphids. In some examples of this type (e.g. some hoppers and thrips), the virus multiplies within the vector and this is termed "propagative".
• Nematodes: these are root-feeding parasites, some of which transmit viruses in the genera Nepovirus and Tobravirus.
• Plasmodiophorids: these are root-infecting obligate parasites traditionally regarded as fungi but now known to be more closely related to protists. They transmit viruses in the genera Benyvirus, Bymovirus, Furovirus, Pecluvirus and Pomovirus. Mites: these transmit viruses in the genera Rymovirus and Tritimovirus.
• 1.4 How are viruses classified
• The highest level of virus classification recognizes six major groups, based on the nature of the genome:
• Double-stranded DNA (dsDNA): there are no plant viruses in this group, which is defined to include only those viruses that replicate without an RNA intermediate (see Reverse-transcribing viruses, below). It includes those viruses with the largest known genomes (up to about 400,000 base pairs) and there is only one genome component, which may be linear or circular. Well-known viruses in this group include the herpes and pox viruses.
• Single-stranded DNA (ssDNA): there are two families of plant viruses in this group and both of these have small circular genome components, often with two or more segments.
• Reverse-transcribing viruses: these have dsDNA or ssRNA genomes and their replication includes the synthesis of DNA from RNA by the enzyme reverse transcriptase; many integrate into their host genomes. The group includes the retroviruses, of which Human immunodeficiency virus (HIV), the cause of AIDS, is a member. There is a single family of plant viruses in this group and this is characterized by a single component of circular dsDNA, the replication of which is via an RNA intermediate.
• Double-stranded RNA (dsRNA): some plant viruses and many of the mycoviruses are included in this group.
• Negative sense single-stranded RNA (ssRNA-): in this group, some or all of the genes are translated into protein from an RNA strand complementary to that of the genome (as packaged in the virus particle). There are some plant viruses in this group and it also includes the viruses that cause measles, influenza and rabies.
• Positive sense single-stranded RNA (ssRNA+): the majority of plant viruses are included in this group. It also includes the SARS coronavirus and many other viruses that cause respiratory diseases (including the "common cold"), and the causal agents of polio and foot-and-mouth disease.
• Within each of these groups, many different characteristics are used to classify the viruses into families, genera and species. Typically, a combination of characters are used and some of the most important are:
• Particle morphology: the shape and size of particles as seen under the electron microscope.
• Genome properties: this includes the number of genome components and the translation strategy. Where genome sequences have been determined, the relatedness of different sequences is often an important factor in discriminating between species.
• Biological properties: this may include the type of host and also the mode of transmission.
• Serological properties: the relatedness (or otherwise) of the virion protein(s).
• Particle morphology: Amongst plant viruses, the most frequently encountered shapes are: Isometric: apparently spherical and (depending on the species) from about 18nm in diameter upwards. The example here shows Tobacco necrosis virus, genus Necrovirus with particles 26 nm in diameter.
• Rod-shaped: about 20-25 nm in diameter and from about 100 to 300 nm long. These appear rigid and often have a clear central canal (depending on the staining method used). Some viruses have two or more different lengths of particle and these contain different genome components. The example here shows Tobacco mosaic virus, genus Tobamovirus with particles 300 nm long.
• Filamentous: usually about 12 nm in diameter and more flexuous than the rod-shaped particles. They can be up to 1000 nm long, or even longer in some instances. Some viruses have two or more different lengths of particle and these contain different genome components. The example here shows Potato virus Y, genus Potyvirus with particles 740 nm long.
• Geminate: twinned isometric particles about 30 x 18 nm. These particles are diagnostic for viruses in the family Geminiviridae which are widespread in many crops especially in tropical regions. The example here shows Maize streak virus, genus Mastrevirus.
• Bacilliform: Short round-ended rods. These come in various forms up to about 30 nm wide and 300 nm long. The example here shows Cocoa swollen shoot virus, genus Badnavirus with particles 28 x 130 nm.
• Further details can be found in the genus description pages and on the Rothamsted Electron Micrographs of Plant Viruses page.
• 15 Genome properties: Important features include
• Nature of the genome: circular (as in all known plant DNA viruses) or linear.
• Number of genome components: This varies from a single component (e.g. in the genera Potyvirus and Tobamovirus) to 11 (in some members of the genus Nanovirus). Individual components vary in size from about 1kb (Nanovirus components) to about 20 kb (in the genus Closterovirus).
• Number of genes: These vary considerably. Most plant viruses have at least 3 genes: 1 (or more) concerned with replication of the nucleic acid, 1 (or more) concerned with cell-to-cell movement of the virus and 1 (or more) encoding a structural protein that is assembled into the virus particle (usually called the "coat" or "capsid" protein). There may also be additional genes that have a regulatory function or which are required for transmission between plants (association with a vector).
• Translation strategy: A variety of strategies are employed to translate the genes from the genome components either directly or via mRNA intermediates and (in some cases) to permit different amounts of protein to be produced from the different genes. These are summarised for each genus in the genus description pages but 3 examples here serve to illustrate some of the variety:
• Genus Potyvirus: in this very large genus, there is one ssRNA component that encodes one large (c. 350 kDa) polyprotein. This is cleaved by 3 different proteases (all encoded by the virus itself) into 10 different mature proteins. The two proteins at the C-terminus of the polyprotein are respectively an RNA-dependent RNA polymerase (NIb, involved in replication of the virus) and the (single) coat protein (CP). Many of the proteins have multiple functions. The genome organisation of a typical member is shown here, indicating the 10 mature proteins and the nine cleavage sites (arrowed).
• Genus Furovirus: in this genus there are two ssRNA components. The 5'-proximal gene on each RNA is translated directly from the genomic RNA: on RNA1 (the larger RNA component) this gene encodes a replication protein and on RNA2 it is the coat protein. The stop codons of both of these genes are "leaky" and in a small percentage of cases, translation continues to produce a larger ("readthrough") protein. On RNA1, the replication protein is extended to include an RNA-dependent RNA polymerase (RdRp) while the readthrough region of the coat protein is probably required for particle assembly and for transmission by the plasmodiophorid vector. There is a further (3'-proximal) gene on each of the RNAs and these are translated from shorter RNA molecules transcribed from the 3'-end of the genomic RNA ("subgenomic" mRNAs). That from RNA1 is a cell-to-cell movement protein (MP) that enables the virus to move between adjacent plant cells via the plasmodesmata while the function of the product from RNA2 is uncertain but may involve supression of the host plant defence reaction. The genome organisation of a typical member is shown here
• Genus Fijivirus: in this genus there are 10 components of dsRNA. Most of the components encode a single protein and at least 3 of these are structural proteins assembled into the complex virion.
• Genome relatedness: the degree of nucleotide identity (or amino acid identity in the protein sequence) between sequences is often used to examine the relationship between different viruses or isolates. For example, recent studies in the genus Carlavirus show that when different species are compared, they have less than 73% nucleotide identity (or 80% amino acid identity) in their coat proteins.
• 1.6 Biological properties
• In some families, the type of host is a useful feature for classification. For example, in the family Reoviridae, there are currently 3 genera with plant-infecting members (Fijivirus, Oryzavirus, Phytoreovirus), 1 genus of mycoviruses (Mycoreovirus), 1 genus containing viruses of fish and cephalopods (Aquareovirus), two genera that are restricted to insects (Cypovirus and Entomoreovirus) and 5 genera of vertebrate viruses that sometimes also infect insects.
• The mode of transmission is also a useful characteristic of some groups of plant viruses. For example in the family Potyviridae, members of the largest genus (Potyvirus) are transmitted by aphids, while viruses in the genera Rymovirus and Tritimovirus are transmitted by mites of the genus Abacarus or Aceria respectively, those in the genus Ipomovirus are transmitted by whiteflies and those in the genus Bymovirus by plasmodiphorids (root-infecting parasites once considered to be fungi but probably more closely related to protists).
• Serological properties: Many viruses are good antigens (elicit strong antibody production when purified preparations are injected into a mammal) and this property has been widely exploited to produce specific antibodies that can be used for virus detection and for examining relationships between viruses. Earlier studies used agar diffusion plates but in the last 20 years these have been largely superseded by ELISA (enzyme-linked immunosorbent assay) procedures. Although serological properties are still important, their significance in taxonomy has declined to some extent now that nucleotide sequence data are available.
• 2. General description of carrot
• The carrot (Daucus carota sativus, Etymology: Middle French carotte, from Late Latin carфta, from Greek karфton. Originally from the Indoeuropean root ker- (horn), due to its horny shape) is a root vegetable, usually orange or white, or red-white blend in colour, with a crisp texture when fresh. Daucus carota is a variable biennial plant, usually growing up to 1 m tall and flowering from June to August. The umbels are claret-coloured or pale pink before they open, then bright white and rounded when in full flower, measuring 3-7cm wide with a festoon of bracts beneath; finally, as they turn to seed, they contract and become concave like a bird's nest. This has given the plant its British common or vernacular name, Bird's Nest. Very similar in appearance to the deadly Water Hemlock, it is distinguished by a mix of bi-pinnate and tri-pinnate leaves, fine hairs on its stems and leaves, a root that smells like carrots, and occasionally a single dark red flower in its center. The edible part of a carrot is a taproot. It is a domesticated form of the wild carrot Daucus carota, native to Europe and southwestern Asia. It has been bred for its greatly enlarged and more palatable, less woody-textured edible taproot, but is still the same species.
• Table 1. Carrot viruses. Resistance and irresistance of carrrot

Susceptible to:	Insusceptible to:
Alfalfa mosaic alfamovirus Arabis mosaic nepovirus Beet pseudo-yellows closterovirus Carrot latent nucleorhabdovirus Carrot mosaic potyvirus Carrot mottle mimic umbravirus Carrot mottle umbravirus Carrot red leaf luteovirus Carrot temperate 1 alphacryptovirus Carrot temperate 2 betacryptovirus Carrot temperate 3 alphacryptovirus Carrot temperate 4 alphacryptovirus Carrot thin leaf potyvirus Carrot yellow leaf closterovirus Cassava green mottle nepovirus Celery mosaic potyvirus Celery yellow net virus Clover yellow vein potyvirus Coriander feathery red vein nucleorhabdovirus Cucumber mosaic cucumovirus Galinsoga mosaic carmovirus Heracleum latent trichovirus Lettuce infectious yellows closterovirus Oat blue dwarf marafivirus Okra mosaic tymovirus Parsnip mosaic potyvirus Parsnip yellow fleck sequivirus Poplar mosaic carlavirus Potato black ringspot nepovirus Strawberry latent ringspot nepovirus Tobacco ringspot nepovirus Tomato black ring nepovirus Tulip X potexvirus	Anthriscus yellows waikavirus Araujia mosaic potyvirus Arracacha A nepovirus Artichoke latent potyvirus Asparagus 1 potyvirus Aucuba ringspot badnavirus Barley yellow streak mosaic virus Beet mosaic potyvirus Belladonna mottle tymovirus Cacao necrosis nepovirus Cacao yellow mosaic tymovirus Cassava Ivorian bacilliform ourmiavirus Celery latent potyvirus Celery yellow mosaic potyvirus Celery yellow spot luteovirus Chicory yellow mottle nepovirus Chino del tomat‚ bigeminivirus Clitoria mosaic potexvirus Cole latent carlavirus Commelina diffusa potyvirus Commelina mosaic potyvirus Datura mosaic potyvirus Dulcamara mottle tymovirus Elderberry latent carmovirus Groundnut rosette umbravirus Helenium S carlavirus Helenium Y potyvirus Hop mosaic carlavirus Lettuce speckles mottle umbravirus Marigold mottle potyvirus Mulberry ringspot nepovirus Ononis yellow mosaic tymovirus Panicum mosaic sobemovirus Patchouli mosaic potyvirus Pea seed-borne mosaic potyvirus Physalis mosaic tymovirus Purple granadilla mosaic virus Scrophularia mottle tymovirus Squash leaf curl bigeminivirus Squash mosaic comovirus Sweet potato sunken vein closterovirus Tobacco etch potyvirus Tobacco mosaic tobamovirus Tobacco necrosis necrovirus Tobacco rattle tobravirus Tobacco stunt varicosavirus Tomato ringspot nepovirus Turnip yellow mosaic tymovirus Watermelon curly mottle bigeminivirus

• 3. Carrot Virus Y
• Carrot Virus Y (CarVY) is spread by aphids and causes mild leaf and severe root symptoms in carrots. It seriously diminishes quality of carrots if plants are infected at an early growth stage. CarVY infects some other plants belonging to the same plant family as carrots (Apiaceae), such as anise, chervil, coriander, cumin, dill, and parsnip. It does not infect celery, fennel, parsley, and several other related herbs.
• Figure 1. Healthy carrot leaf (left), carrot leaf infected with CarVY showing symptoms of mild mottle and chlorosis (right).
• 3.1 Distribution
• CarVY has only been detected in Australia. It is found in carrot crops in New South Wales, Queensland, South Australia, Tasmania, Victoria and Western Australia. It is detected at higher incidences when carrot crops are grown throughout the year at the same site without a break in production.
• 3.2 Symptoms and Losses
• In carrot plants, leaf symptoms of CarVY include chlorotic mottle (see Fig. 1), marginal necrosis, increased subdivision of leaflets giving a feathery appearance, and affected plants show mild stunting. In the roots, the most severe symptoms are seen when infection occurs early (carrot seedlings infected up to six weeks after germination) and include stubby or shortened roots, knobbliness and severe distortion rendering them unmarketable (see Fig. 2). Later infection (carrot plants infected more than six weeks after germination) produces milder symptoms of thinner carrots with only slight distortion but still with a substantial overall yield loss (around 30%). In some cases crops have been abandoned due to large-scale early infection and the severe root symptoms that result. All commonly grown carrot cultivars are susceptible to CarVY.
• Figure 2. Carrots with severe root symptoms caused by CarVY.
• 3.3 Spread
• CarVY is spread by aphids in a non-persistent manner, i.e. they rapidly acquire the virus when feeding on infected carrot plants and then just as rapidly lose the virus from their mouthparts after feeding on a healthy or non-host plant. A range of aphid species can transmit CarVY, including species that do not normally colonise carrots. The green peach aphid (Myzus persicae) is a very efficient vector. Volunteer carrots and nearby carrot crops infected with CarVY are the main sources of virus for spread to newly planted crops.
• Other Apiaceous host crops such as anise, chervil, dill, coriander, cumin and parsnip are potential alternative virus sources. Some native Apiaceaous plants may possibly be other sources, but this has yet to be demonstrated. As CarVY has a very narrow host range, it is unlikely that weeds and crop plants that do not belong to the Apiaceae family are a source of infection. Whether CarVY is seed-borne at very low levels is yet to be confirmed but other similar viruses belonging to the same virus family are seed-borne at low levels. If it is seed-borne, this would provide a means of introduction of the virus to new sites.
• Figure 3. Healthy carrot (left), carrot infected with CarVY late showing mild symptoms (middle) and carrot infected with CarVY early showing severe symptoms (right).
• 3.4 Control
• carrot virus biological genome

Once a carrot plant becomes infected with CarVY, there is no cure. The best means of control is adopting management practices that minimize the reservoir of infection. If the following integrated control strategy is followed, infection will be greatly reduced:

Avoid side-by-side plantings of carrots, grow an intervening non-host crop or have the bordering area under fallow - Having side by side plantings of carrots of different ages leads to the younger crops being infected from older, nearby virus-infected ones. Planting a barrier non-host crop or having a fallow area around the crop will reduce virus spread. Planting an intervening non-host barrier crop helps more than leaving the intervening area fallow because virus-carrying aphids may land on the non-host crop before arriving at the carrot crop. By probing a non-host plant first they lose the virus from their mouthparts and so lose their potential to spread virus when they move on to the carrot crop.

Destroy all volunteer carrots and any finished crops - Carrots that remain after the crop has been harvested may be infected with CarVY and act as a source for its spread to newly sown carrot crops. Crops that are finished or abandoned should immediately be ploughed in, well below the soil surface.

Monitor aphid numbers, manipulate the sowing date so that young carrot seedlings are not present during peak aphid periods and protect the crop with insecticides while the crop is young - It is important to monitor the area for aphid numbers by trapping. Try to avoid planting during peak aphid population times because, if young seedlings (up to 6 weeks old) are infected with CarVY, they develop severe root symptoms (Fig. 3). Peak aphid population times occur for short periods in spring and autumn. Chemical control of aphids in carrot field trials is being researched and preliminary results have shown a reduction in virus spread with fortnightly applications of one `new chemistry' insecticide.

Introduce a carrot-free period - Sequential plantings of carrots all year round are often associated with CarVY outbreaks, so this type of planting should be avoided. Having a fallow period after harvest will greatly reduce the likelihood of an epidemic. If a non-host crop is grown in rotation, the area can still be productive while removing the source of CarVY infection.

4. Carrot red leaf virus

4.1 Biological Properties

Viral hosts belong to the Domain Eucarya.

Domain Eucarya - Kingdom Plantae. Kingdom Plantae Phylum Magnoliophyta (Angiosperms, Class Magnoliopsida (Dicotyledonae).

Severity and Occurrence of Disease - Host: Signs and symptoms persist and vary seasonally.

Transmission and Vector Relationships - virus is transmitted by a vector. Virus is not transmitted by mechanical inoculation; not transmitted by contact between hosts; not transmitted by seeds; not transmitted by pollen.

Virus is transmitted by arthropods, by insects of the order Hemiptera, family Aphididae; Cavariella aegopodii. Virus is transmitted in a persistent manner; retained when the vector moults; does not replicate in the vector; not transmitted congenitally to the progeny of the vector; can facilitate the vector transmission of another virus (carrot mottle virus).

Experimental Hosts and Symptoms. Under experimental conditions susceptibility to infection by virus is found in few families. Susceptible host species are found in the Family Umbelliferae. The following species were susceptible to experimental virus infection: Anethum graveolens, Anthriscus cerefolium, Anthriscus sylvestris, Apium leptophyllum, Coriandrum sativum, Daucus carota.

Host:Experimentally infected hosts mainly show symptoms of yellowing, reddening.

Diagnostic host species and symptoms: Anthriscus cerefolium, Apium leptophyllum, Coriandrum sativum -- yellowing and reddening of older leaves. Diagnostic host: insusceptible host species Apium graveolens, Petroselinum crispum.

Histopathology: Virus can be best detected in phloem and companion cells. Virions are found in the cytoplasm and cell vacuole.

Cytopathology: Inclusions are present in infected cells. Inclusion bodies in the host cell are found in the cytoplasm. Cytoplasmic inclusions are amorphous X-bodies and membranous bodies. Inclusions contain mature virions.

The virus occurs in Australia, Canada, Germany, Japan, New Zealand (Aotearoa), the United Kingdom, and the United States of America.

4.2 Virion Properties

Morphology. Virions consist of a capsid. Virus capsid is not enveloped, round with polyhedral symmetry. The isometric capsid has a diameter of 25 nm. Capsids appear round, or hexagonal in outline.

Electron microscopic preparation and references: Virus preparation contains few virions. Reference for electron microscopic methods: Waterhouse and Murant (1981).

Physicochemical and Physical Properties. Virions have a buoyant density in CsCl of 1.403 g cm-3. There are 1 sedimenting component(s) found in purified preparations. The sedimentation coefficient is 104 S20w.

Nucleic Acid. The Mr of the genome constitutes 28% of the virion by weight. The genome is not segmented and contains a single molecule of linear positive-sense, single-stranded RNA. The complete genome is 5750 nucleotides long. Genome is sequenced, but only an estimate is available, complete sequence is 5750 nucleotides long.

GenBank records for nucleotide sequences; complete genome sequences.

Proteins constitute about 72% of the particle weight. The viral genome encodes structural proteins and non-structural proteins. Virions consist of 4 structural protein(s).

Lipids are absent.

Antigenicity. The virus is serologically related to barley yellow dwarf-RPV and beet western yellows viruses moderately closely; tobacco necrotic dwarf, potato leafroll and bean leaf roll viruses distantly; and soybean dwarf, barley yellow dwarf-MAV viruses very distantly.

Diagnostics and Reference Collections. The best tests for diagnosis are This virus is the only luteovirus known to infect Apiaceae and has a very narrow host range. It is usually found in a complex with CMotV, which unlike CRLV is sap transmitted.

Literature

1. Murant, A.F. Vira. Viruses No. 137, 1974, 4 pp.

2. Murant, AF and Roberts, I.M. Carrot virys Y. Ann. appl. Biol. 92: 343, 1979

3. Murant, AF, Waterhouse, P.M., Raschke, J.H. and Robinson, D.J. Invisible life. J. gen. Virol. 66: 1575, 1985

4. Waterhouse, P.M.. Viruses of carrot. Thesis, University of Dundee, U.K., pp. 244. 1981

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