245
July 1981
Family: Potyviridae
Genus: Potyvirus
Species:
Acronym:

Description 366 (Potyviridae) is a more recent description of the entire family

Potyvirus group

M. Hollings
Glasshouse Crops Research Institute, Littlehampton, Sussex, England

A. A. Brunt
Glasshouse Crops Research Institute, Littlehampton, Sussex, England

Contents

Type Member
Main Characteristics
Members
Geographical Distribution etc
Association with Vectors
Ecology
Relations with Cells and Tissues
Particle Properties
Genome Properties
Replication
Satellites
Defective-Interfering RNA
Relationships within the Taxon
Notes on Tentative Members
Affinities with Other Groups
Notes
References
Acknowledgements
Figures

Type Member

Potato virus Y

Main Characteristics

Flexuous filamentous particles, normally 720-900 nm long and c. 11 nm in diameter, sedimenting at c. 150 S, and with a buoyant density in CsCl of 1.31 g/cm3. The particles consist of up to 2000 subunits of a single protein species (M. Wt 32 to 34 x 103) arranged as a helix (pitch c. 3.4 nm) enclosing the genome. The genome is a single molecule of single-stranded RNA (M. Wt 3.0 to 3.5 x 106), and constitutes c. 5% of the particle weight.

Thermal inactivation point (10 min) 50 to 75°C (usually 55 to 60°C); longevity in sap 1 to 50 days (usually 2 to 4 days); and dilution end-point 10-1 to 10-6 (usually 10-3 to 10-4). Most potyviruses have restricted, or very restricted, host ranges, but the different viruses occur in a wide range of monocotyledonous and dicotyledonous plants. They induce mosaic or mottle symptoms in leaves; many also induce colour-breaking in flowers, mottled and/or distorted fruits and seeds, and some cause considerable losses of crop yield and quality. Definitive potyviruses are transmitted in the non-persistent manner by aphids, and epidemic levels of field spread often occur. Potyviruses are also transmitted by inoculation of sap, and at least 12 of them are carried in a small proportion of the seeds of some host species.

The virus particles occur, and are probably assembled, in the cytoplasm; those of many of the viruses reach moderately high concentrations in plant sap, but the particles of some are difficult to extract and purify. The viruses induce the formation of characteristic cytoplasmic inclusion bodies.

Members

Definitive and possible members of the group are listed in the table. A virus is considered to be a definitive member of the group if it:- (1) has filamentous particles of characteristic size and properties; (2) is transmitted non-persistently by aphids; (3) induces the formation of characteristic cytoplasmic inclusions (‘pinwheels’); (4) is clearly serologically distinguishable from morphologically similar viruses.

Serological relationship to other definitive potyviruses is not an essential criterion for membership of the group but is reliable confirmatory evidence.
Table 1. Definitive and possible members of the potyvirus group*
A. Members  
Amaranthus leaf mottle (Lovisolo & Lisa, 1976)
Araujia mosaic (Charudattan et al., 1980)
Bean common mosaic (BCMV; 73)
Bean yellow mosaic (BYMV; 40)
Bearded iris mosaic (147)
Beet mosaic (53)
Bidens mottle (161)
Blackeye cowpea mosaic (Lima et al., 1979)
Carnation vein mottle (CarVMV; 78)
Carrot thin leaf (218)
Celery mosaic (CeMV; 50)
Clover yellow vein (ClYVV; 131)
Cocksfoot streak (CkSV; 59)
Colombian datura (Kahn & Bartels, 1968)
Commelina mosaic (Morales & Zettler, 1977)
Cowpea aphid-borne mosaic (134)
Daphne Y (Forster & Milne, 1976)
Dasheen mosaic (191)
Datura shoestring (Giri & Agrawal, 1971)
Euphorbia ringspot (Bode & Lesemann, 1976)
Gloriosa stripe mosaic (Koenig & Lesemann, 1974)
Guinea grass mosaic (190)
Helenium Y (Kuschki et al., 1978)
Henbane mosaic (HMV; 95)
Hippeastrum mosaic (117)
Hyacinth mosaic (Derks & Vink-van den Abeele, 1980)
Iris mild mosaic (116) Iris fulva mosaic (IFMV; Barnett & Alper, 1977)
Iris severe mosaic (Brunt & Phillips, 1980)
Leek yellow stripe (240)
Lettuce mosaic (LtMV; 9)
Mungbean mottle (Sun et al., 1977)
Narcissus degeneration (Brunt, 1980)
Narcissus late season yellows (Brunt, 1980)
Narcissus yellow stripe (76)
Onion yellow dwarf (158)
Papaya ringspot (84)
Parsnip mosaic (91)
Passionfruit woodiness (PWV; 122)
Pea seedborne mosaic (PSbMV; 146)
Peanut mottle (141)
Pepper mottle (PeMV; Purcifull et al., 1975)
Pepper veinal mottle (PVMV; 104)
Pepper severe mosaic (Feldman & Garcia, 1977)
Plum pox (70)
Pokeweed mosaic (97)
Potato A (54)
Potato Y (PVY; 37, 242)
Primula mosaic (Lisa & Lovisolo, 1976)
Soybean mosaic (93)
Statice Y (Lesemann et al., 1979)
Sugarcane mosaic (SCMV; 88)
Tamarillo mosaic (Mossop, 1977)
Tobacco etch (TEV; 55)
Tobacco vein mottling (Sun et al., 1974)
Tomato (Peru) (Raymer et al., 1972)
Tulip breaking (71)
Turnip mosaic (TurMV; 8)
Watermelon mosaic (WMMV; 63)
Wisteria vein mosaic (Conti & Lovisolo, 1969; Bos, 1970)

B. Potyviruses regarded as strains of members listed in A,
or of uncertain relationship to those members
Aquilegia mosaic (Marani & Pisi, 1976)
Azuki bean mosaica (Tsuchizaki et al., 1970)
Bidens mosaic (Kuhn et al., 1978)
Bryonia mottle (Lockhart & Fischer, 1979)
Clover (Croatian) mosaic (Taraku et al., 1977)
Crinum (Pares & Bertus, 1978)
Datura 437 (Damsteegt, 1974)
Datura mosaic (Qureshi & Mahmood, 1978)
Dendrobium mosaic (Inouye, 1976)
Desmodium mosaic (Edwardson et al., 1970)
Dioscorea greenbanding (Hearon et al., 1978)
Dioscorea trifida (Migliori & Cadilhac, 1976)
Freesia mosaic (Brunt, 1974)
Garlic yellow streak (Mohamed & Young, 1981)
Guar symptomless (Hansen & Lesemann, 1978)
Groundnut eyespot (Dubern & Dollet, 1980)
Kennedya V (Dale et al., 1975)
Lupin mottleb (Hull, 1968)
Maize dwarf mosaicc (MDMV; 88)
Malva vein-clearing (Schmidt & Schmelzer, 1964)
Mungbean mosaicd (Kaiser et al., 1968)
Nothoscordum mosaic (McKinney, 1929; Gold et al., 1957)
Ornithogalum mosaic (Smith & Brierley, 1944)
Passionfruit ringspot (De Wijs, 1974)
Pea necrosisb (Bos et al., 1974)
Pea mosaicb (40)
Sweet potato A (Sheffield, 1957; Hollings & Stone, 1977)
Sweet potato feathery mottle (Campbell et al., 1974)
Sweet potato russet crack (Campbell et al., 1972)
Teasel mosaic (Gemignani, 1965)
Tradescantia virus (Lockhart & Betzold, 1980)
Wild potato mosaic (Jones & Fribourg, 1979)

C. Possible members
Anthoxanthum mosaic (Catherall, 1970)
Carrot mosaic (Chod, 1965)
Celery latent (Brandes & Luisoni, 1966)
Dock mottling mosaic (Juretic et al., 1976)
Fern virus (Nienhaus et al., 1974)
Holcus streak (Catherall & Chamberlain, 1975)
Maclura mosaice (239)
Palm mosaic (Mayhew & Tidwell, 1978)

        * Acronym and Description number or other reference given in parentheses

aregarded as a strain of cowpea aphid-borne mosaic
boften regarded as a strain of BYMV
cregarded as a strain of SCMV
dregarded as a strain of BCMV
ea virus of uncertain affinity, with properties resembling both carlaviruses and potyviruses.

Geographical Distribution etc

Potyviruses usually occur wherever their principal host plants are grown, but they are especially prevalent in tropical and sub-tropical countries.

Association with Vectors

Most of the definitive potyviruses are transmitted by aphids in a non-persistent, stylet-borne manner. Virus is acquired most efficiently after probes of only a few minutes during which there is merely superficial insertion of the stylets into the epidermis, and acquisition is enhanced if the aphids are previously starved for 1 to 3 h. Infection likewise follows feeding of a few minutes; the viruliferous aphid usually remains infective for less than 1 h if it continues to feed, or 4 h if starved. There is no evidence for a latent period between acquisition and transmission, nor for virus multiplication in the vector. Although a given potyvirus may be transmitted by several aphid species, considerable specificity occurs, and there are marked differences in efficiency of transmission between different clones or colonies of one aphid species, or between different strains of the same virus. All potyviruses tested so far require a ‘helper factor’ for their transmission by aphids (Govier & Kassanis, 1974); this factor is labile proteinaceous material of apparent M. Wt 100 to 200 x 103, which is probably coded for by the viral genome and perhaps assists in attaching virus particles to the mouth-parts of the vector aphid (Govier, Kassanis & Pirone, 1977). Additional factors may also be involved (Pirone, 1979).

Members of very few other invertebrate taxa have been reported to transmit definitive potyviruses: WMMV and CeMV were reported to be inefficiently transmitted by the leaf miner Liriomyza satur (Diptera: Agromyzidae) (Zitter & Tsai, 1977); apparent soil-transmission of SCMV in the absence of root contact, suggested the involvement of a soil-living vector (Bond & Pirone, 1970).

Ecology

Potyviruses are a very successful group of pathogens which flourish in a wide range of crops and environmental conditions; they are most successful in tropical or sub-tropical countries, where continuous or successive crops are grown throughout the year. Many different factors affect their rate of spread and their severity in crops, but the most important are the proximity of virus sources, and the number, activity and occurrence of the alate forms of vector species. A low incidence of infection early in the life of the crop often ultimately results in a high incidence and severe disease. Potyviruses do not survive in dead leaves or other host debris, nor for long periods in the vector. In temperate climates, they survive in perennial or vegetatively propagated crops. Because most have narrow, often extremely restricted, host ranges comparatively few are able to survive in alternative host species; even where this does occur, it is often of only minor importance. The most dangerous virus sources are infected planting material, or infected volunteer plants from previous crops.

A consequence of the extreme host-specialisation of different potyviruses is the occurrence of numerous strains, or pathotypes, which may have survived through the absence of inter-strain competition in common hosts (Hollings & Brunt, 1981). However, in a number of host species, two or more different potyviruses are quite often found in the same individual plant.

Relations with Cells and Tissues

Potyvirus particles are sometimes found scattered randomly throughout the cytoplasm of infected cells and occasionally within plasmodesmata (e.g. Weintraub, Ragetli & Lo, 1974); they also occur in uniseriate arrays parallel to the tonoplast or cytoplasmic lamellae, or in cytoplasmic strands projecting into or bridging the cell vacuole (e.g. Lawson & Hearon, 1971). Discrete membrane-associated aggregates present in negatively-stained sap are possibly either fragments of the virus-containing strands or tonoplast-associated particles entrapped during cellular disruption by pieces of the adjacent membrane (Brunt & Atkey, 1974). Infected cells sometimes contain unusual aggregates of mitochondria or chloroplasts (Weintraub, Agrawal & Ragetli, 1973; Kitajima & Lovisolo, 1972; Kitajima & Costa, 1973).

Cytoplasmic inclusions. Potyviruses induce the formation of conical or cylindrical cytoplasmic inclusions (CCI), the structure of which has been elucidated from ultrastructual studies of infected plants (Edwardson, 1966; Rubio Huertos & Lopez-Abella, 1966), freeze-etch electron microscopy (McDonald & Hiebert, 1974a) and, more recently, computer-assisted analytical geometry (Mernaugh, Gardener & Yocom, 1980). Fully-formed inclusions each have a central core from which radiate 10-20 thin striated rectangular or triangular curved plates (so-called ‘arms’, ‘lamellae’ or ‘septa’). CCI seen in transverse section are described as ‘pin-wheels’ and in longitudinal section as ‘bundles’. Sections of the ‘lamellae’, when not obviously associated with the ‘pin-wheel’ core, are described as ‘laminated inclusions’ if straight or unfolded, as ‘scrolls’ or ‘tubes’ if rolled inwardly, and as ‘laminated aggregates’ if two or more are closely associated or partially fused. CCI are generally considered to be either cylindrical (Edwardson, 1966) or conical (Andrews & Shalla, 1974) in shape, but computer-assisted mathematical interpretations of serial transverse sections of TEV-CCI suggest that some might also be hour-glass-shaped (elliptic hyperboloids) (Mernaugh et al., 1980).

CCI are initially closely associated with plasmodesmata (e.g. Lawson & Hearon, 1971), and this suggests that they may be concerned with the intercellular transport of virus particles and/or their nucleic acid and protein components (Andrews & Shalla, 1974). Potyviruses have been sub-grouped according to the predominant type of CCI they induce (Edwardson, 1974).

CCI are detectable by light microscopy in suitably stained epidermal leaf strips (Christie & Edwardson, 1977). Amorphous inclusions or X-bodies (masses of cytoplasmic inclusions aggregated with ribosomes, virus particles, endoplasmic reticulum, mitochondria and/or other organelles) are also commonly induced by potyviruses in chronically-infected plants.

The lamellae of the CCI have surface striations with a periodicity of about 5 nm. The lamellae and/or their fragments can be extracted and purified from infected plants; with some viruses, their yields (16 to 18 A280 units/kg leaf tissue) are consistently greater than those of virus particles (Hiebert & McDonald, 1973). CCI contain a single virus-specific polypeptide species (M. Wt 67 to 70 x 103).

Nuclear inclusions. A few potyviruses, notably TEV, also induce the formation of crystalline nuclear inclusions (CNI) (Kassanis, 1939) which are readily detectable by light microscopy in suitably stained epidermal strips (Christie & Edwardson, 1977). CNI appear to be flat crystals 6 to 8 µm square when viewed from above, but to be slightly curved plates in side view.

TEV-CNI can be extracted and purified, yields of 2 to 8 x 108 inclusions/kg leaf tissue being readily obtained (Knuhtsen, Hiebert & Purcifull, 1974). Extracted CNI each consist of thin rectangular plates which may be present in regular stacks but are often offset at 45° to each other. The plates have a clearly discernible periodic substructure with striations 10.2 nm apart and having primary axes intersecting at 90°; this periodicity is also clearly visible in CNI seen in situ by freeze-etch electron microscopy (McDonald & Hiebert, 1974b). In transverse section, the CNI appear to be multilayered pyramids.

TEV-CNI contain two polypeptide species (M. Wt 49.8 and 54.5 x 103) which are serologically unrelated to those of the associated virus particles or CCI, or to healthy plant protein (Knuhtsen et al., 1974).

CNI of various shapes and sizes induced by other potyviruses (Christie & Edwardson, 1977) have yet to be similarly characterised.

Particle Properties

The modal length of potyvirus particles was originally defined as 730 to 760 nm (Brandes & Wetter, 1959), but the size range was later extended slightly to 720 to 770 nm (Brandes, 1964); although most potyviruses have particles within these limits, those of HMV (Lovisolo & Bartels, 1970) and several potyviruses from flower bulbs (Brunt & Atkey, 1971) were subsequently found to be straighter and mostly 850 to 950 nm long. Particles of HMV, BYMV and PVMV are rigid, straight and c. 850 nm long in the presence of Mg++ ions, but flexuous and c. 750 nm long in the absence of Mg++ ions (Govier & Woods, 1971); such morphological changes are sometimes reversible. Similar changes occur with particles of other potyviruses such as CkSV (Chamberlain & Catherall, 1977) and CarVMV (Hollings & Stone, 1971), but those of IFMV are unusual in reacting in the reverse manner to the presence or absence of Mg++ (Barnett & Alper, 1977). Particles of some potyviruses easily fragment during purification, giving apparent shorter modal lengths; e.g. 590 to 700 nm with C1YVV (Singh & Lopez-Abella, 1971; Hollings & Stone, 1974) and 680 nm with TurMV (Shepherd & Pound, 1960). Potyvirus particles usually show very little substructure, although some have a discernible central canal 2-3 nm diameter in negatively stained preparations. The protein subunits are arranged in a helix with a pitch of c. 3.4 nm (Varma et al., 1968). A particle weight of 60 to 70 x 106 daltons, estimated from data for nucleic acid and protein contents, indicates that each particle contains 1700-2000 protein subunits that are possibly arranged 8-9 per turn of the helix.

The u.v. absorbance spectra of potyviruses are typical of nucleoproteins, with maximum at 260 to 262 nm and minimum at 240 to 246 nm. The A260/A280 ratios of 1.14-1.25, and Amax/Amin ratios of 1.11-1.27 are typical of nucleoproteins containing 5-6% nucleic acid (Layne, 1957). The absorption coefficient (A0.1%, 1cm at 260 nm) which has been determined for only a few potyviruses, ranges from 2.4 to 2.9. Several potyviruses are stable in CsCl at 20 to 25°C, and have buoyant densities of 1.318 to 1.336 g/cm3 (Huttinga & Mosch, 1974; Damirdagh & Shepherd, 1970).

The M. Wt of the single capsid protein of potyviruses is now generally regarded as being between c. 32 x 103 for TEV (Hiebert & McDonald, 1973) and c. 36.5 x 103 for MDMV (strain of SCMV) (Hill, Ford & Benner, 1973). Values c. 26-28 x 103 reported earlier are probably due to proteolytic degradation of the protein (Huttinga & Mosch, 1974), for increasing amounts of lower M. Wt material are commonly detectable by electrophoresis in polyacrylamide/SDS gels following storage of virus preparations at c. 2°C (Brunt & Kenten, 1971; Huttinga, 1975; Moghal & Francki, 1976). Other explanations for protein heterogeneity, however, have been proposed (Hill & Benner, 1980). Amino acid analyses indicate that the structural subunits of potyviruses such as BYMV, LtMV, BCMV, PWV, PVY and SCMV contain c. 290 amino acids (Moghal & Francki, 1976); earlier reports probably greatly underestimated the number of residues. Protein from PVY particles, dissociated with LiCl, polymerises in 1 to 100 mM phosphate solutions (pH 6 to 9) to form long flexuous particles (McDonald, Beveridge & Bancroft, 1976; Goodman et al., 1976). The 3 S and 10 S protein components in such preparations first polymerise to form stacked rings, each about 40 nm long. These then reassemble into filaments c. 10.5 nm in diameter, and of various lengths up to several µm. PVY-protein polymerises with PVY-RNA or papaya mosaic virus-RNA to form helical virus-like particles that are non-infective (McDonald & Bancroft, 1977).

Genome Properties

Potyviruses contain 5 to 6% single-stranded positive-sense RNA (Matthews, 1979); infective RNA preparations have been obtained from some potyviruses such as MDMV (Pring & Langenberg, 1972), PVY, TurMV and TEV (Makkouk & Gumpf, 1974, 1975; Hill & Benner, 1976). Native and formaldehyde-treated PVY-RNA sediment at 39 S and 24 S respectively (Pring & Langenberg, 1972; Makkouk & Gumpf, 1974, 1975). By electrophoresis in polyacrylamide gels, the estimated M. Wt (x 106) of the genome RNA were: TurMV-RNA, 3.5; PVY-RNA, 3.2; and TEV-RNA, 3.15 (Hill & Shepherd, 1972; Hinostroza-Orihuela, 1975; Hari et al., 1979). By density-gradient centrifugation, the M. Wt of MDMV-RNA was estimated at 2.7 x 106 and that of PVY-RNA at 3.1 x 106 (Pring & Langenberg, 1972; Makkouk & Gumpf, 1974, 1975). TEV-RNA is reported to contain strands with or without poly(A) which are equally infective (Hari et al., 1979); the activity of poly(A) polymerase is greatly enhanced in TEV-infected tissues (Hari, 1980).

Nucleotide base ratios have been determined for several potyviruses; most values lie in the ranges G 21-26; A 23-30; C 20-27; U 18-29, but that of PSbMV is reported as G22.8; A44.0; C17.6; U15.6 (Hampton & Mink, 1975).

Replication

Although little is known about their replication, potyviruses are thought to replicate within the cytoplasm. In recent in vitro translation experiments, Dougherty & Hiebert (1980a, 1980b, 1980c) found that the 39 S single-stranded RNA molecules of TEV and PeMV each induce the formation of six gene products of which one is the coat protein and another is the cytoplasmic inclusion (CCI) protein; the TEV products also include the two nuclear inclusion (CNI) proteins. The estimated total M. Wt of the products were 340 x 103 (TEV) or 324 x 103 (PeMV); these values account for 95% and 93% of the estimated coding capacity of the TEV-RNA and PeMV-RNA, respectively. Although TEV-RNA and PeMV-RNA induce the production of several products in in vitro translation experiments, each nucleic acid is considered to act as a monocistronic messenger (Dougherty & Hiebert, 1980c). Such studies have also permitted the tentative genetic mapping of both nucleic acids, although there is some disagreement about the location in TEV-RNA of the coat protein gene (Dougherty & Hiebert, 1980c; Hellman et al., 1980).

Relationships within the Taxon

Serological relationships among potyviruses are complex. Most of the definitive potyviruses are serologically related to at least one other member of the group, and in many instances to several other potyviruses. But present techniques have failed to detect any serological relationship between many pairs of potyviruses, and relationships may be only indirect among several members. Thus, relationships were found between intact particles of LtMV and BYMV, and between BYMV and BCMV, but not between BCMV and LtMV (Alba & Oliveira, 1976). Also, different strains of one potyvirus may differ considerably in their serological relationships to other potyviruses. Although some potyviruses react adequately in agar-gel immunodiffusion, many must first be fragmented into small lengths, e.g. by sonication (Tomlinson et al., 1965), or the coat protein dissociated into subunits by chemical treatments (e.g. with SDS and/or pyrrolidine (Shepard et al., 1974)). However, dissociated proteins are very poor immunogens, and may not react with antisera to intact particles (Moghal & Franki, 1976). Serological relationships observed between intact particles of two potyviruses may not occur between their dissociated particle proteins.

The proteins of the characteristic CCI induced by potyviruses are antigenically unrelated to the coat protein, and those of serologically unrelated potyviruses show considerable antigenic differences even when the viruses are propagated in the same plant species; however, CCI proteins of serologically closely related potyvirus strains appear to be immunochemically indistinguishable (Hiebert et al., 1971; Purcifull, Hiebert & McDonald, 1973).

Cross-protection occurs between a number of potyviruses (e.g. TEV, PVY and HMV; Bawden & Kassanis, 1941) but not between others, even when the latter are recognised as strains of the same potyvirus (e.g different strains of PVY (Horváth, 1969) or of MDMV (Gillaspie & Koike, 1973)). Two or more potyviruses can occur, and have synergistic effects, in naturally infected plants (Hollings & Brunt, 1981).

Several attempts have been made to evaluate criteria for differentiating individual potyviruses, for defining virus strains, and for dividing potyviruses into sub-groups. The properties used have included host range, cross-protection, particle length and serology (Bos, 1970; Lindsten, Brishammar & Tomenius, 1976), host range and serology (Jones & Diachun, 1977), serology and amino acid analyses of the particle protein (Moghal & Francki, 1976), and morphology of CCI (Edwardson, 1974). Suggested sub-groupings of potyviruses have so far shown various anomalies and inconsistencies, and none has been generally accepted. Serological comparisons, using both intact and dissociated particles, seem to offer the most practical method at present for establishing the identity of new isolates; but among potyviruses there exists a multitude of strains and pathotypes that differ mainly in biological properties, such as host range or pathogenicity, and which form an almost continuous range of variants for which a species concept is not applicable (Bos, 1970).

Affinities with Other Groups

By a combination of properties, potyviruses are readily differentiated from potexviruses, carlaviruses and closteroviruses. However, they show some similarities to at least nine other viruses which also have filamentous particles and induce the formation of cytoplasmic inclusions but, unlike potyviruses, have vectors other than aphids. The affinities of such viruses are uncertain; until the taxonomic significance of vector type and inclusion formation have been further considered, they are probably best excluded from the potyvirus group.

The better known viruses of this type, ryegrass mosaic, agropyron mosaic and wheat streak mosaic, infect cereal crops in America and Europe; they have filamentous particles mostly c. 700 nm long, induce CCI indistinguishable from those of authentic potyviruses but are transmissible by eriophyid mites. It has been suggested previously that such viruses should be included in the carlavirus group (Brandes & Bercks, 1965), the potyvirus group (e.g. Edwardson, 1974), or in a separate group or a sub-group of the potyvirus group (Gibbs, 1969; Lapierre & Spire, 1969).

Another CCI-forming virus, sweet potato mild mottle, has filamentous particles mostly c. 800 nm long; as it is transmissible by whiteflies and is serologically unrelated to 40 potyviruses, it is also for the present probably best excluded from the group (Hollings, Stone & Bock, 1976).

Some cereal viruses, namely barley yellow mosaic, oat mosaic, rice necrosis mosaic, wheat spindle streak mosaic and wheat yellow mosaic, induce CCI but are transmissible by the fungus Polymyxa graminis and have rigid filamentous particles of two modal lengths (Inouye & Fujii, 1977); these viruses also are probably best excluded from the potyvirus group.

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