Structure and biology
Viruses are extremely small, much smaller even than bacteria (see
Figure 15.2). The light microscope is unable to focus in on them, but they appear as rods or spheres when seen under an electron microscope.
The virus particle is composed of a DNA or RNA core surrounded by
a protective protein coat. On entering a plant cell, the virus takes over
the organization of the cell nucleus in order to produce many more virus
particles. Since the virus itself lacks any cytoplasm cell contents, it is
often considered to be a non-living unit. Some details of its classification
are given in Classification and naming
The virus’s close association with the plant cell nucleus
difficulties in the production of a curative virus control chemical that
does not also kill the plant. No established commercial ‘ viricide ’ has yet
been produced against plant viruses.
In recent years the broad area called ‘ virus diseases ’ has been closely
investigated. Virus particles have, in most cases, been isolated as the
cause of disease, e.g. cucumber mosaic. Other agents of disease to be
discovered are viroids
(e.g. in chrysanthemum stunt disease) and these
are smaller than viruses. Mycoplasmas
(the cause of diseases such as
aster yellows) are a group of bacteria that induce symptoms similar to
those produced by viruses.
A number of organisms (vectors
) spread viruses from plant to
plant and then transmit the viruses into the plant. Peach-potato aphid
capable of transmitting over 200 types of virus (e.g. cucumber mosaic) to
different plant species. The aphid stylet injects salivary juices containing
virus into the parenchyma and phloem tissues, enabling the virus to then
travel to other parts of the plant. ‘Persistent virus transmission ’ is seen
in some vector/virus combinations such as peach-potato aphid/potato
virus X, and Xiphinema
dagger nematode/arabis mosaic where the virus
is able to survive and increase within the vector’s body for several weeks.
In many vector/virus combinations such as plum pox, the virus survives
only briefly as a contaminant on the insect’s stylet. Other vector/virus
combinations include bean weevils/broad bean stain virus; and Olpidium
soil fungus/big vein agent on lettuce.
Other important methods of spread involve vegetative material (e.g.
chrysanthemum stunt viroid and plum pox), infected seed (e.g. bean
common mosaic virus), seed testa (e.g. tomato mosaic virus) and
mechanical transmission by hand (e.g. tomato mosaic virus).
The presence of a damaging virus in a plant is recognizable
to horticulturists only by means of its symptoms. For confirmation, they
may need to consult a virologist, whose identification techniques include
electron microscopy, transmission tests on sensitive plants such as Chenopodium
species, and serological reactions using specific antiserum
, a yellow mottling, is the most common symptom (e.g.
cucumber mosaic virus). Other symptoms include leaf distortion
feathery shapes (cucumber mosaic virus), flower colour streaks
tulip break virus), fruit blemishing
(tomato mosaic and plum pox), internal discolouration
of tubers (tobacco rattle virus causing ‘spraing’
in potatoes) and stunting
of plants (chrysanthemum stunt viroid).
Symptoms similar to those described above may be caused by misused
herbicide sprays, genetic ‘ sports ’ , poor soil fertility and structure (see deficiency symptoms
) and mite damage.
In the following descriptions of major viruses, Latin names of genus
and species are not included, since no consistent classification is yet
|Figure 15.19 Tomato mosaic
patches on the fruit
This disease may cause serious losses in tomatoes. Infected
seedlings have a stunted, spiky appearance. On more mature plants
leaves have a pale green mottled appearance, or sometimes a
bright yellow (aucuba) symptom. The stem may show brown
streaks in summer when growing conditions are poor, a condition
often resulting in death of the plant. Fruit yield and quality may
be lowered, the green fruit appearing bronze, and the ripe fruit
hard, making the crop unsaleable (see Figure 15.19).
Life cycle and spread:
The virus is a rod shaped virus. The
period from plant infection to symptom expression is about
15 days. The virus may survive within the seed coat (testa) or
endosperm of the tomato seed. It is very easily spread by human
contact as it is present in large numbers in the leaf hairs of
Heat treatment of dry seed at 70°C for 4 days by
seed merchants helps remove initial infection. Infected debris,
particularly roots, in the soil enables the virus to survive from
crop to crop, and soil temperatures of 90°C for 10 min are
normally required to kill the organism. Peat-growing bag and nutrientfilm methods enable the grower to avoid this source of infection. Hands
and tools should be washed in soapy water after working with infected
plants. Clothing may harbour the virus.
Cultivars and rootstocks containing several factors for resistance are
commonly grown, but newly arriving virus strains may overcome this
resistance. A mild strain spray inoculation method has been used at
the seedling stage to protect non-resistant cultivars from infection with
severe strains. Great care is required to avoid mosaic-contaminated
equipment when using this method.
|Figure 15.20 Cucumber mosaic virus. Note the leaf
Several strains of virus cause this disease. In addition to
cucumber, the following may also be affected: spinach, celery, tomato, Pelargonium and Petunia.
On cucumbers, a mottling of young leaves
occurs (see Figure 15.20) followed by a twisting and curling of the
whole foliage, and fruit may show yellow sunken areas. On the shrub Daphne oderata,
a yellowing and slight mottle is commonly seen on
infected foliage, while Euonymus
leaves produce bright yellow leaf
spots. Infected tomato leaves are reduced in size (fern-leaf symptom).
The virus may be spread by infected hands, but more
commonly an aphid (e.g. peach-potato aphid) is involved. Many crops
(e.g. lettuce, maize, Pelargonium
and privet) and weeds (e.g. fat hen and
teasel) may act as a reservoir for the virus.
Since there are no curative methods for control, care must be
taken to carry out preventative methods. Choice of uninfected stock is
vital in vegetatively propagated plants, e.g. Pelargonium.
control of aphid vectors may be important where susceptible crops
(e.g. lettuce and cucumbers) are grown in succession or next to other
susceptible species. Removal of infected weeds, particularly from
greenhouses, may prevent widespread infection.
The petals of infected tulips produce irregular coloured streaks and may
appear distorted. Leaves may become light green, and plants become
stunted after several years’ infection. The virus is spread mechanically
by knives, while three aphid vectors are known: the bulb aphid in stores,
the melon aphid in greenhouses, and the peach-potato aphid outdoors
and in greenhouses.
Preventative control must be used against this disease. Removal
of infected plants in the field prevents a source of virus for aphid
transmission. Aphid control in field, store and greenhouse further
reduces the virus’s spread.
This disease, also called ‘Sharka’, has increased in importance
in the British Isles since 1970 after its introduction from mainland Europe. Plums, damsons, peaches, blackthorn and ornamental plum
are affected, while cherries and flowering cherries are immune. Leaf
symptoms of faint interveinal yellow blotches can best be seen on leaves
from the centre of the infected tree.
The most reliable symptoms, however, are found on fruit, where sunken
dark blotches are seen. Ripening of infected fruit may be several weeks
premature, yield losses may reach 25 per cent, and the fruit is often sour.
The virus is spread by several species of aphids. The speed
of spread is quite slow because the virus is not able to live and multiply
in the aphid. Movement of infected young plants is an important method
Preventative control is the only option open to growers. Clean
Ministry-certified stock should be used. Routine aphid-controlling
insecticides should be applied in late spring, summer and autumn.
Suspected infected trees should be reported and infected trees removed
Chrysanthemum stunt viroid
This disease, found only on plants of the Asteraceae family
and mainly on the chrysanthemum, produces a stunted plant, often only
half the normal size but without any distortion. Flowers often open one
week earlier than normal, and may be small and lacking in colour.
The virus enters gardens and nurseries through infected
cuttings, and is readily transmitted by leaf contact and by handling.
Symptoms may take several months to appear, thus seriously
reducing the chance of early removal of the disease source. The grower
must use preventative control. Certified planting material
heat-treated meristem stock (see tissue culture) reduces the risk of this
|Figure 15.21 Reversion disease of
the infected leaf
(bottom) has fewer main veins and
than the healthy leaf (top)
This virus infects a wide range of horticultural crops. On
strawberries, yellow spots or mottling are produced on the leaves, and
certain cultivars become severely stunted. On ornamental plants, e.g.
Daphne odorata , yellow rings and lines are seen on infected leaves, and
the plants may slowly die back, particularly when this virus is associated
with cucumber mosaic inside the plant.
Several weeds, e.g. chickweed and grass spp., may harbour
this disease, and in strawberries severe attacks of the disease may occur
when planted into ploughed-up grassland. The virus is spread by a
common soil-inhabiting nematode, Xiphinema diversicaudatum , which
may retain the virus in its body for several months.
Control of this disease can be achieved by preventative
methods. Certified virus-free soft fruit planting material is available. Fumigation of soil with chemicals such as dichloropropene,
applied well before planting time,
eliminates many of the eelworm vectors. No curative
chemical is available to eliminate the virus inside the
Reversion disease on blackcurrants
This virus disease, caused by blackcurrant
reversion virus can seriously reduce blackcurrant yields.
Flower buds on infected bushes are almost hairless and
appear brighter in colour than healthy buds. Infected
leaves often have fewer main veins than healthy ones
(see Figure 15.21). After several years of infection, the
bush may cease to produce fruit.
Life cycle and spread:
The virus is spread by the
blackcurrant gall mite, and reversion infected plants are
particularly susceptible to attack by this pest.
Removal and burning of infected plants is an
important form of control. Use of certified plant material,
raised in areas away from infection and vectors, is strongly