Potato mop-top virus
B. D. Harrison
Scottish Horticultural Research Institute, Invergowrie, Dundee, Scotland
Contents
Introduction
-
Described by
Calvert & Harrison (1966) and
Harrison & Jones (1970).
-
A virus with straight tubular particles, occurring in small concentration in tissue extracts,
and transmissible by inoculation of sap. The host range is narrow. The soil-borne fungus
Spongospora subterranea is a vector. Found in western Europe and South America.
Main Diseases
In potato (
Solanum tuberosum), causes a wide range of symptoms, which depend greatly
on the cultivar
(
Calvert, 1968)
and the environmental conditions. The three commonest shoot symptoms
are yellow blotching or mottling
(
Fig.1), particularly of the lower leaves, chlorotic V-shaped
markings in the leaflets
(
Fig.2) and extreme stunting of the shoots,
known as ‘mop-top’
(
Fig.5).
Shoot symptoms develop best in cool conditions (5-15°C). In the year of infection from soil the
tubers of some cultivars, such as Arran Pilot, develop internal brown arcs
(‘spraing’) seen as
brown rings on the tuber surface
(
Fig.3).
The arcs develop at the boundary of the virus-infected
tissue in response to a specific change of temperature before or after harvest
but do not prevent
the virus spreading through the tuber
(
Harrison & Jones, 1971a).
The tubers of other
cultivars may develop superficial raised rings without internal symptoms or may show little evidence
of infection
(
Calvert, 1968).
In the year following infection from soil, the tubers of some cultivars
may be symptomless, or cracked and distorted
(
Fig.6), or they may show brown arcs centred on the
stolon. When infected tubers are planted, the virus is usually passed to only
half, or fewer, of
the resulting plants.
Geographical Distribution
Western Europe, Peru, and probably many other countries where potatoes are grown. In Scotland,
the prevalence of infection increases with the annual rainfall
(
Cooper & Harrison, 1973).
Host Range and Symptomatology
Transmitted by inoculation of sap to 26 species in the Solanaceae or Chenopodiaceae and to
Tetragonia expansa; species in 11 other families were not infected
(
Harrison & Jones, 1970).
Symptoms are greatly affected by the environmental conditions
(
Harrison & Jones, 1971b).
-
Diagnostic species
-
Chenopodium amaranticolor.
Concentric fine necrotic ringspot lesions develop in inoculated
leaves after a week or more at 13-16°C
(Fig.4).
A single lesion may spread to cover half a
leaf. Not systemic.
-
Nicotiana debneyi. Inoculated leaves develop necrotic spots, or necrotic or chlorotic
ringspots. The first systemically infected leaves show chlorotic or necrotic
‘thistle-leaf’ line
patterns
(Fig.8).
-
N. tabacum (tobacco) cv. Xanthi-nc or Samsun NN. Inoculated leaves develop necrotic or
chlorotic ringspots below 20°C, but usually no symptom at higher temperatures. Lesion type
can be altered by changing the environmental conditions
(Harrison & Jones, 1971b).
Systemic infection occurs predominantly in the winter months, causing necrotic or chlorotic
‘thistle-leaf’ line patterns, which appear first in a leaf immediately
above an inoculated leaf
and later in leaves nearer the shoot tip
(Fig.7).
-
Propagation species
-
N. tabacum
cv. Samsun NN (inoculated leaves) and N. debneyi can be used for
maintaining
cultures and as sources of virus.
-
Assay species
-
N. tabacum
cv. Xanthi-nc or Samsun NN is best for isolates that produce necrotic ringspot
lesions, and C. amaranticolor for other isolates.
-
N. debneyi seedlings (systemic symptoms) are useful for testing transmission by vectors
(Fig.8).
Strains
The type strain (isolate T;
Harrison & Jones, 1970)
produces necrotic lesions
in many
Nicotiana spp. and is more virulent than most other isolates. Several isolates
differing in virulence are reported
(
Harrison & Jones, 1970).
Transmission by Vectors
The only vector known is the plasmodiophoromycete fungus,
Spongospora subterranea
(
Fig.9).
The virus is carried, apparently internally, in the resting spores, in which it
persists for at least 2 years; transmission to roots is by zoospores released by virus-carrying
cultures of
S. subterranea. Virus-free
S. subterranea zoospores fail to acquire
transmissible virus when exposed to a virus suspension. Field soil is best tested for infectivity
by air-drying the soil at 20°C for 2 weeks, then moistening it and planting
N. debneyi
bait seedlings
(
Jones & Harrison, 1969).
Transmission through Seed
No reports.
Transmission by Dodder
No reports.
Serology
The virus seems moderately immunogenic but difficulty may be experienced with most isolates
in obtaining enough virus for antiserum production. Precipitin tests in mixed liquids
can be used
but some virus preparations precipitate with normal serum
(
Kassanis, Woods & White, 1972).
Relationships
A distant serological relationship to
tobacco mosaic virus
is reported
(
Kassanis et al., 1972).
Potato mop-top virus seems also to have affinities with
soil-borne wheat mosaic virus
(
Brakke, 1971);
the two viruses have similar particles of two predominant lengths and are
transmitted in similar ways by plasmodiophoromycete vectors.
Stability in Sap
In tobacco sap, the thermal inactivation point (10 min) is 75 to 80°C and dilution
end-point from 10
-2 to 10
-4. In sap at 20°C, the virus loses most of
its infectivity in 1 day but retains a little for 10 weeks.
Purification
Kassanis et al. (1972).
For partial purification, centrifuge sap for 20 min at 9000
g and resuspend pellets in 0.5-1 vol. 0.5 M borate buffer at pH 7.5. Clarify by
centrifuging 3 min at 9000
g, emulsify supernatant fluid with 1 vol. diethyl ether,
centrifuge at low speed and emulsify aqueous phase with 1 vol. carbon tetrachloride, then
centrifuge at low speed. Concentrate virus from aqueous phase by centrifuging for 2 h at 100,000
g and resuspending in a small volume of water, keeping 24 h at 4°C before
clarifying at 9000
g. Colour may be removed from preparations by emulsifying with
ethylene dichloride.
Properties of Particles
Sedimentation coefficients (
s20, w) of components in partially purified
preparations were 126, 171 and 236 S. The 236 S component may consist of dimers of
171 S particles
(
Kassanis et al., 1973).
Infectivity was associated with the longer
(300 nm) particles.
Particle Structure
Particles are straight, helically constructed, with a pitch of 2.4-2.5 nm and a hollow core.
Particle width is 18-20 nm and length usually 100-150 or 250-300 nm. The helix is loosely coiled
at the ends of many particles in sap
(
Fig.10)
(
Harrison & Jones, 1970;
Kassanis et al., 1972).
Particle Composition
Nucleic acid: Probably single-stranded RNA.
Protein: Polyacrylamide gel electrophoresis of coat protein revealed one polypeptide
of M. Wt 18,500-20,000.
Relations with Cells and Tissues
The virus particles aggregate in sheaves in the cytoplasm of tobacco cells
(
Fig.11;
White, Kassanis & James, 1972).
The pattern of systemic invasion of test plants suggests that the
virus may move from cell to cell in parenchyma tissue and not through sieve tubes.
Notes
Symptoms induced by potato mop-top virus in potato resemble those caused by
tobacco rattle virus,
although most potato cultivars react somewhat differently to the two viruses. Also, the
two viruses have different effects on
Chenopodium amaranticolor, and potato mop-top virus
seems not to infect
Phaseolus vulgaris. The vectors of tobacco rattle virus are nematodes
(
Trichodorus spp.), which are killed by air-drying infested soil.
Potato mop-top virus can be distinguished from other viruses occurring in potato by its
particle size and shape, and by the reaction of C. amaranticolor, Nicotiana debneyi, N.
tabacum and P. vulgaris. In Scotland, potato seems to be its only important host and
the virus survives between potato crops in resting spores of S. subterranea
(Jones & Harrison, 1972).
The virus is gradually self-eliminating in potato stocks grown on virus-free soil.
References
- Brakke, CMI/AAB Descriptions of Plant Viruses 77, 4 pp., 1971.
- Calvert, Rec. agric. Res. Minist. Agric. N. Ire. 17: 31, 1968.
- Calvert & Harrison, Pl. Path. 15: 134, 1966.
- Cooper & Harrison, Pl. Path. 22: 73, 1973.
- Harrison & Jones, Ann. appl. Biol. 65: 393, 1970.
- Harrison & Jones, Ann. appl. Biol. 68: 281, 1971a.
- Harrison & Jones, Ann. appl. Biol. 67: 377, 1971b.
- Jones & Harrison, Ann. appl. Biol. 63: 1, 1969.
- Jones & Harrison, Ann. appl. Biol. 71: 47, 1972.
- Kassanis, Woods & White, J. gen. Virol. 14: 123, 1972.
- White, Kassanis & James, J. gen. Virol. 15: 175, 1972.
Yellow mottling in leaf of potato cv. Red Craig’s Royal.
Naturally infected plants.
Chlorotic V-shaped marking in leaf of potato cv. Arran Pilot.
Naturally infected plants.
External (above) and internal (below) symptoms of
spraing in potato tubers
of cv. Arran Pilot. Naturally infected plants.
Lesions consisting of concentric necrotic etched rings in inoculated leaf of
Chenopodium amaranticolor.
Healthy (left) and mop-top affected (right) plants of potato cv.
Alpha. Naturally infected plants.
Cracked and distorted potato tubers of cv. Alpha produced in second year of
infection. Naturally infected plants.
Systemically infected plant of Nicotiana tabacum cv. Xanthi-nc.
Systemically infectcd N. debneyi bait seedlings grown in infective soil.
Spongospora subterranea zoosporangium in root hair of potato cv. Arran Pilot.
Virus particles 20 nm wide in sap, showing loose helical structure at one end.
Sheaf of virus particles in cytoplasm of leaf cell of N. tabacum cv. Xanthi-nc.
Bar represents 500 nm.
