Transcript cucumber mosaic, and tomato spotted wilt. The tomato mosaic virus

Section 2 Virus
Diseases of
Vegetable Plants
1. Virus Diseases of
Tomato
Several virus diseases of tomato are not as
prevalent as the wilt and foliar diseases. Three
of the more common virus diseases are tomato
mosaic (also referred to as tobacco mosaic),
cucumber mosaic, and tomato spotted wilt.
The tomato mosaic virus can attack a wide
range of plants, including tomato, pepper,
eggplant （茄子）, tobacco, spinach（菠菜）,
petunia（矮牵牛花）, and marigold（金盏草）.
1.1 Sympotom
On tomato, virus infection causes light and dark green
mottled areas on the leaves. The dark green areas tend to
be somewhat thicker than the lighter portions of the leaf.
The leaf mottling is seen more easily if the affected plant
surface is partially shaded. Stunting of young plants is
common, and often is accompanied by a distortion and
fern-like appearance of the leaves. Older leaves curl
downward and may be slightly distorted. Certain strains
of the virus can cause a mottling, streaking, and necrosis of
the fruits. Infected plants are not killed, but they produce
poor quality fruit and low yields.
The cucumber mosaic virus has one of the broadest
host ranges of any of the viruses. The disease affects a
number of important vegetables and ornamentals
including tomato, pepper, cucumber, melons, squash,
spinach, celery, beets, and petunia. Tomatoes infected
with the cucumber mosaic virus develop a slight
yellowing and mottling of the older leaves. Expanding
leaves typically become twisted, curl downward, and
develop a 'shoestring' appearance as a result of a
restriction of the leaf surface to a narrow band
around the midrib of the leaf. Diseased plants are
stunted and produce small quantities of fruit.
The tomato spotted wilt virus also has a
wide host range and can affect a number of
ornamental plants as well as tomato. Early
symptoms of spotted wilt on tomato are
difficult to diagnose. Young, infected plants
may show an inward cupping of leaves, and
the foliage may appear off-color or have a
slight bronze cast. In some cases, leaves
with exhibit dark purple flecks or small
necrotic concentric rings.
Leaf symptoms of tomato
mosaic virus.
Fruit symptoms of tomato mosaic
virus
Fruit symptoms of tomato spotted
wilt
Purple flecking of young leaves
caused by tomato spotted wilt.
As the disease progresses, plants may develop
dark brown to black streaks on the main stem.
Occasionally the top portion of the plant wilts.
The most characteristic symptom of spotted wilt
appears on the fruit. On young fruit, white to
yellow concentric rings, one-half inch in diameter,
develop on the fruit skin. a bumpy or warty
appearance. The bright yellow rings on red, mature
fruit are quite striking and are easily diagnosed as
spotted wilt. The area within the ring typically is
raised, which gives the fruit.
1.2 Causes
Tomato mosaic, cucumber mosaic, and spotted
wilt are incited by viruses. The tomato mosaic virus
is very stable and can persist in dry contaminated
soil, in infected tomato debris, on or in the seed coat.
The virus is transmitted readily from plant to plant
by mechanical means. This may simply involve
picking up the virus while working with infected
plant material, then inoculating healthy plants by
rubbing or brushing against them with contaminated
tools, clothing, or hands. Aphids are not vectors of
the tomato mosaic virus, although certain chewing
insects may transmit the pathogen.
The cucumber mosaic virus overwinters in
perennial weeds and may be transmitted to
healthy plants by aphid vectors (although
tomatoes are not the preferred host of aphids)
or by mechanical means.
The cucumber mosaic virus cannot
withstand drying, or persist in the soil. It also
is more difficult than tobacco mosaic to
transmit mechanically. Thus, cucumber
mosaic tends to progress more slowly than
tobacco mosaic in a field or garden.
The spotted wilt virus is transmitted from
plant to plant by several species of small
insects called thrips. Thrips are less than
one-quarter inch in length, light green to
brown, and are extremely difficult to find on
the plants. Several weedy hosts and
ornamental plants may serve as alternate
hosts for the virus.
1.3 Control
Virus diseases cannot be controlled once the plant is
infected. Therefore, every effort should be made to prevent
introduction of virus diseases into the garden. Many
varieties of tomato are available with resistance to tomato
mosaic virus. Sanitation is the primary means of controlling
the other two virus diseases. Infected plants should be
removed immediately to prevent spread of the pathogens.
Perennial weeds, which may serve as alternate hosts,
should be controlled in and adjacent to the garden. Avoid
planting tomatoes next to cucurbits, spinach, or other
vegetables and flowers susceptible to these diseases.
Control of insects, especially aphids and thrips, will help
reduce the likelihood of cucumber mosaic and spotted wilt.
1.4 Major Tomato Viruses
1.4.1Tobacco mosaic virus (TMV) is
distributed worldwide and may cause
significant losses in the field and greenhouse.
TMV is one of the most stable viruses known,
able to survive in dried plant debris as long
as 100 years. Many strains of TMV have been
reported and characterized. TMV can be
seedborne in tomato, is readily transmitted
mechanically by human activities, and may be
present in tobacco products. The virus is not
spread by insects commonly occurring in the
greenhouse or field.
The symptoms in tomato vary greatly in intensity
depending on the variety, virus strain, time of
infection, light intensity, and temperature. High
temperatures, for example, may mask foliar
symptoms. The most characteristic symptom of the
disease on leaves is a light- and dark-green mosaic
pattern (fig. 1). Some strains (referred to as the acuba
strains) may cause a striking yellow mosaic, whereas
other strains may cause leaf malformation and
“fernleafing“(蕨叶). With the use of TMV resistant or tolerant varieties, plants may be infected by some
strains whose symptoms are latent. Ordinarily the
fruit from infected plants do not show mosaic
symptoms, but may be reduced in size and number.
Seeds suspected of carrying TMV should be
treated with a solution of trisodium phosphate
(Na3PO4) for 15 minutes, rinsed in running tap
water, dried, and treated with a seed fungicide.
Control of TMV spread in the greenhouse is
especially difficult because plants are handled
more often than in the field. Hygienic measures
must be taken to prevent the introduction and
spread of this virus. Choose only varieties
resistant to TMV and remove any suspicious
plants as soon as they are observed.
Single and double virus streak and tomato leaf rolling are
three separate diseases, which involve strains of TMV, mixed
infections of TMV with other viruses, or interaction of TMV with
a specific tomato gene. Single virus streak is caused by a strain
of TMV that causes the same foliar symptoms as other TMV
strains, but under certain environmental conditions the plant
develops brown streaks（褐条病） on the stems and petioles(叶
柄）. On the fruit the symptoms appear as slightly sunken
brown rings (fig. 2) Double virus streak is the result of a mixed
infection of TMV with potato virus X (PVX). Both viruses are
readily transmitted mechanically and, when they occur together,
result in brown streaks along the petioles and stems, and small,
irregular brown areas on fruits. Leaf rolling is the result of TMV
infection interacting with the wilt gene (wt) found in some
tomato varieties.
1.4.2Cucumber mosaic virus (CMV) is the second most
important virus disease of tomato. CMV has an
extensive host range and is transmitted by aphids in a
nonpersistent manner. Unlike TMV, CMV is not
seedborne in tomato and does not persist in plant
debris in the soil or on workers’ hands. CMV has been
found in greenhouse plantings. Seedlings grown
outdoors and left unprotected by isolation before
moving indoors are one likely source of infection. Other
sources of inoculum are the spread of CMV by aphids
from infected plants in adjoining （邻近的）
greenhouses (weeds under benches, ornamentals, or
other vegetables) and by viruliferous （带病毒的）
aphids entering through non-insect-proof vent windows.
 The symptoms of CMV on tomato can be
spectacular, with the most characteristic
symptom being shoestringlike leaf blades (fig.
3) The symptoms of CMV can be transitory,
and bottom leaves or newly developed top
leaves may show a mottle similar to that
caused by TMV (fig. 4).
 Because of the wide host range for CMV,
sources of inoculum for field plantings are
numerous. Avoid planting near weedy border
areas or isolate tomatoes from such areas by
growing taller, nonsusceptible barrier crops
such as corn. No CMV-resistant or -tolerant
varieties are currently available.
1.5 Minor Tomato Viruses
1.5.1Tobacco etch virus (TEV) and potato virus Y (PVY),
are two common tomato viruses in some areas. The
symptoms of TEV and PVY can be confused with TMV.
Because some varieties carry partial resistance to TMV
and none have been bred for resistance to either TEV or
PVY, plants with a general mosaic suggest TEV or PVY
infection. TEV symptoms are usually more severe (fig. 5),
causing foliage distortion and severely stunted plants.
 Because of the lack of virus resistance and because
both viruses are transmitted by aphids in a
nonpersistent manner, most control measures are
developed around the removal of solanaceous weed
reservoirs. In southern states where tomato viruses are
particularly troublesome, commercial growers have
successfully used mineral oil sprays to delay virus
spread in their crops.
1.5.2Potato leafroll virus (PLRV) is more commonly
associated with potato, but a tomato-specialized
isolate of PLRV has been identified. Leaf rolling and
marginal leaf chlorosis are the typical foliar symptoms
seen with this virus (fig. 6). Because this virus is
transmitted by aphids in a persistent manner, aphid
control should reduce the amount of virus spread.
1.5.3Tomato spotted wilt virus (TSWV) can cause a
severe disease of tomato, especially in tropical areas
where thrips are plentiful. In addition to causing foliar
necrosis and brown streak of the stems and petioles,
fruit are severely mottled, making them unmarketable
(fig. 7). Use of insecticides to control the vector
reduces disease incidence.
1.5.4 Tomato big-bud mycoplasma (TBB) is a
disorder of tomato more closely identified with
subtropical areas of the world where this
disease with the leafhopper as vector is more
commonly found. As the name implies, the
bud’s size is greatly accentuated by this
mycoplasma-like organism (fig. 8). Leafhoppers
are generally not difficult to control in
temperate climates; thus, vector control should
reduce the incidence of this disease.
2. Virus Diseases of Cucurbits
 Many viruses affect cucurbits and cause mosaic
diseases. The most important of these viruses are
cucumber mosaic virus (CMV), squash mosaic virus
(SqMV，南瓜花叶病毒), watermelon mosaic virus
(WMV，西瓜花叶病毒), zucchini yellow mosaic virus
(ZYMV，小胡瓜黄花叶病毒), and papaya ring spot
virus (PRSV，番木瓜环斑病毒). The viruses differ in
the range of host plants they infect.
 The symptoms caused by different cucurbit
viruses are commonly very similar. It is impossible
to identify these viruses with certainty based on
symptoms alone. Usually special laboratory tests
are required to correctly identify a cucurbit virus.
zucchini yellow mosaic virus (ZYMV，小胡瓜黄花叶病毒)
Leaves of virus-infected plants often appear
mottled and distorted. Pumpkin plants
infected early in their development (near or
before the time of flowering) are severely
affected and produce few fruit, and most of
the pumpkins that are produced are likely to
be misshapen（畸形的） or off-color. However,
plants infected after fruit reach full size may
not show any effect on yield or quality. Late-
season pumpkins are especially prone to
losses associated with virus disease.
watermelon mosaic virus (WMV，西瓜花叶病毒),
squash mosaic virus (SqMV，南瓜花叶病毒) ，
papaya ring spot virus (PRSV，番木瓜环斑病毒)
 Viruses survive in infected weed hosts. SqMV is seedborne. These pathogens are spread by insect vectors
(especially aphids) and mechanical operations that disturb
plants and bruise leaves and vines.
 Virus diseases of cucurbits can be controlled by
planting resistant varieties (wherever available), using
virus-free seed, cultural practices, and insect control. Early
planted field tend to have less damage than those that are
planted later. Weed control within and around field is
important. Avoiding mechanical transmission of viruses is
important. Insect control for reducing incidence of virus
diseases is effective. However, Attempts to control insects
for virus disease control may be futile, because insects
may transmit the virus before insecticides are effective.
2.1 Mosaic Virus of Cucurbits
Mosaic virus is caused by the cucumber mosaic virus. This
virus is one of the most widespread and destructive
pathogens on cucumbers, muskmelons(香瓜）, and squash
（南瓜）. The virus is found worldwide and infects many
different types of plants including various vegetables, flowers,
and weeds. Much of the early infection on cucurbits comes
from floral crops which harbor the virus. Ornamentals as
asters（紫苑）, chrysanthemums（菊花）, geraniums（天竺
葵）, gladiola（剑兰）, larkspur（翠雀）, marigolds（万寿菊）,
petunias（矮牵牛花）, phlox（夹竹桃）, zinnias（鱼尾菊）.
Weeds may also harbor the virus; examples include wild
ground cherries（野生酸浆草）, milkweed（马利筋）,
pokeweed（美洲商陆）, and spurge（大戟）.
2.1.1 Symptoms
Plants can be infected at any stage of growth. When
seedlings become infected they remain small, turn yellow,
and die. Symptoms on older plants include dwarfing of
plants, mottling, distortion, and downward curling leaves.
Infected cucumber fruit reveals patterns of white blotches
interspersed with dark green spots that are raised into
conspicuous blisters. Infected fruit has a bitter taste（苦味）
when eaten or becomes soggy （黏湿的）when pickled（腌
制）. The fruit of infected summer squash plants is knobby
（多节的） and distorted. Muskmelon and winter squash fruit
rarely shows symptoms but usually is off-color and bittertasting. Watermelons infected with the virus usually show
less stunting and mottling than cucumbers and muskmelons.
2.1.2 Disease Cycle
The virus survives the winter in roots of
susceptible plants, in greenhouses, and
possibly in seeds of wild cucumbers. Aphids are
the principle means of dissemination of the
virus. Mosaic also can be transmitted by the
cucumber beetle and by workers picking
cucumbers. As aphids feed on virus-infected
hosts, they become contaminated with the virus
and subsequently spread it to healthy plants.
2.1.3 Control
Weed control around and within the planting is
important, since the virus can survive in many
weed plants. Most effective method of control
of common mosaic on cucumber is planting
resistant varieties. These types of cucumbers
are resistant: Marketmore 70, Marketmore,
Gemini F1, Sweet Slice, and Victory. Cucumber
picking types that are resistant include
Pioneer, Spartan, Valor, and Sulty.
3. Virus Diseases of Leafy Vegetables and
Celery
Lettuce, endive (escarole，苣荬菜), spinach
（菠菜）, and celery（芹菜） are members of
different plant families, but share many of
the same viral problems. most of these crops
are produced on organic soils, but they may
also be grown on mineral soils. Because
these crops are often planted near one
another, the viruses or other agents infecting
the crops are the same, and they are
transmitted by the same insect vectors.
The viruses infecting lettuce （莴苣/生菜） or
endive(苣荬菜） include lettuce mosaic virus (LMV)，
cucumber mosaic virus (CMV), turnip（芜菁） mosaic
virus (TuMV), broadbean wilt virus (BBWV，蚕豆萎蔫
病毒), Bidens mottle virus (BiMV，鬼针草斑驳病毒),
and beet western yellows virus (BWYV，西部甜菜黄疸
病毒). Additional organisms infecting lettuce are
lettuce big vein agent (LBVA) and aster（紫苑）
yellows mycoplasma (AY).
The two most important virus diseases of celery are
CMV and celery mosaic virus (CeMV); the occurrence
of aster（紫苑 ）yellows mycoplasma (AY) depends
upon the prevalence of the leafhopper vector.
3.1 Lettuce and Endive Diseases
Lettuce mosaic virus (LMV) can infect all
lettuce types (crisphead, cos, Boston, bibb,
and leaf) as well as escarole and endive.
LMV is seedborne in all lettuce types, but
not in endive; and infected seed probably
serves as the chief source of inoculum,
although weed hosts are also recognized.
 Plants infected through seed are referred to as
seedborne "mother" plants because they can serve
as virus reservoirs from which aphids can spread the
virus to surrounding healthy plants. The mother
plants show early mosaic symptoms, are stunted, and
never develop marketable heads. Secondary infected
plants show mosaic, leaf puckering, and deep or
accentuated serration of the leaf margins in most
lettuce types (fig. 1). Endive heads may be stunted,
but generally show only mild mosaic symptoms.
Two methods for controlling LMV are currently in use.
First, because of the prevalence of seedborne virus,
lettuce seed is tested by three methods (direct
reading of lettuce seedlings, inoculation of ground-up
seed with a sensitive indexing host, or, more recently,
a serological technique) to ensure that each lettuce
seed lot contains no infected seeds in a sample of
30,000 seeds (MTO or mosaic tolerance 7ero). A more
successful control measure is the incorporation of
virus resistance into the principal lettuce types grown
on both mineral and organic soils.
3.1.1Cucumber mosaic virus (CMV) is the most
important virus disease of lettuce and the most
difficult to control. Because the virus can infect
more than 775 plant species, including many weed
species (chickweed[繁缕], milkweed[马利筋],
purslane[马齿苋], dayflower[鸭跖草], etc.), many
reservoirs（蓄水池） exist to overwinter the virus
near growers‘ fields. Winged aphids（有翅蚜）
transmit the virus from bordering weedy areas to
the crop to create primary infection sites.
 Secondary infections account for most virus
spread and occur when migrant aphids spread the
virus from infected primary lettuce plants to
surrounding healthy plants. When large migrantaphid flights occur. an entire field may become
almost totally infected in a matter of weeks.
 Although the symptoms of CMV bear some
resemblance to those of LMV, the mosaic is more
intense with veinal chlorosis and, frequently, veinal
browning and necrosis when temperatures in the
mid-50s or lower occur (fig 2) .
Because weed hosts harboring CMV are common, a
concerted effort must be made by adjoining lettuce
growers to make weed sanitation and other
programs effective. Weed control must be done
before or soon after the crop is planted. Because
migrant aphids account for much of the virus spread
and, normally, do not colonize lettuce, insecticidal
sprays are only minimally effective in reducing
spread. Reducing primary inoculum in the crop is so
critical in delaying virus epidemics that major
emphasis should be applied at this point. Make
earliest plantings downwind from weedy border
areas, so that older lettuce plantings do not serve as
reservoirs of virus for subsequently planted crops.
Although not extensively tested on lettuce, mineral
oil sprays are known to significantly reduce
secondary virus spread in many nonpersistent,
aphid-transmitted virus diseases such as CMV,
provided the treatment is begun before primary
inoculum exceeds 8-10%, and heavy virus pressure
is not present in adjoining fields. Spraying border
areas with mineral oils is also beneficial. The
technique used is sophisticated; consult extension
personnel for complete details.
3.1.2Turnip mosaic virus (TuMV) is not a common
disease of lettuce varieties, but escarole and endive are
very susceptible. Susceptibility to TuMV was
inadvertently transferred to some lettuce varieties by
breeding for resistance to downy mildew. TuMV
typically causes veinal chlorosis and necrosis of lower
leaves of both lettuce (fig. 3) and endive, severely
stunting the plants and making the heads unmarketable.
Cruciferous weed hosts (shepherdspurse, etc.) are the
principal source of inoculum for this aphid-transmitted
virus, and controls discussed under CMV are applicable.
3.1.3Broadbean wilt virus (BBWV), like CMV, can
cause a serious disease of lettuce in susceptible
varieties, resulting in large patches of infected plants
(fig. 4). Symptoms are similar to those of LMV and
CMV, but with low field temperatures the infected
plants develop severe veinal necrosis. Some
varieties are very susceptible whereas others display
good tolerance to infection.
BBWV is transmitted by aphids in a nonpersistent
manner. The major weed hosts in New York have
been identified as broadleaf plantain (Plantago ma/or)
and buckhorn plantain (P. Ianceolata).
3.1.4 Bidens mottle virus (BiMV) has been
recovered from lettuce and endive in Orange
County, but is not widely distributed in the state.
Symptoms caused by BiMV are similar to those of
LMV in both lettuce and endive. The lettuce variety
Valmaine (a romaine or cos type) is naturally
resistant to BiMV and has been used in breeding
programs for joint resistance to BiMV and LMV.
BiMV is not seedborne in either lettuce or endive,
and principal weed hosts for this aphid-transmitted
virus are beggarticks, pepperweed, and others.
3.1.5 Beet western yellows virus (BWYV) is an important
virus disease of lettuce and endive in California, Arizona,
and Florida. Although not formally recognized in this
state, weed hosts such as shepherdspurse are present.
BWYV causes a pronounced chlorosis of the outer
leaves of lettuce (fig. 5) and escarole.
Although spread by many of the same aphid species that
are vectors for the other common lettuce viruses, BWYV
is transmitted in a persistent manner, meaning that a
longer time is required to acquire and then transmit the
virus. Insecticidal sprays would presumably help to delay
the spread of this particular virus disease.
3.1.6 Lettuce big vein agent (LBVA) is probably caused
by a virus, although the virus particle has not been
seen with the electron microscope. This disease differs
greatly from the other lettuce virus diseases; its vector
in nature is a soilborne fungus (Olpidium brassicae).
The virus is acquired by the fungus protoplast in virusinfected roots and is carried by fungal zoospores,
which infect healthy roots and release the virus.
Infected lettuce plants are striking, with leaves
showing enlarged and lightened veins and petioles (fig.
6). Leaves are also thickened and distorted, and heads
are smaller than normal.
Because of the soilborne nature of the disease, no
practical control measures are available.
3.1.7Aster yellows mycoplasma (AY),
formerly thought to be caused by a virus,
can infect a wide range of plants in addition
to lettuce and celery. The disease results in
strikingly yellowed and stunted plants (fig. 7),
which are unmarketable. This disease may
occur annually, its extent being determined
by the preponderance of the aster leafhopper
vector (Macrosteles J:ascifrons). Controls
are based upon the use of insecticidal
sprays to reduce the only known vector.
3.2 Virus Diseases of Spinach
Cucumber mosaic virus (CMV) infection of
spinach has long been called "spinach blight" by
growers, but should not be confused with fungal
diseases for which the term blight is more
commonly used. The disease is caused by CMV,
which is covered in detail in other sections of
this fact sheet. Infected spinach plants may show
a variety of symptoms including stunting,
yellowing, and mottling of the older leaves and
malformation of the younger leaves.
Good resistance to CMV is available, but this
resistance is temperature dependent; at
temperatures above 80° F crown necrosis
will develop (fig. 8), which is similar to
infection with broad bean wilt virus (BBWV)
without the need for high temperatures (fig. 9).
With the exception of the high temperature
response for CMV infection, resistance for
CMV in spinach has provided an effective
control measure for over 60 years.
3.2.1Broadbean wilt virus (BBWV) has previously been
mentioned in this report. This virus does cause a major
disease of spinach, particularly in the autumn crop
when aphid vectors are most plentiful and much
inoculum is present from earlier plantings.
Symptoms of BBWV closely resemble those caused by
CMV, and therefore, identification is impossible under
field conditions (fig. 9).
Control involves the removal of the principal weed
hosts P. major and P. lanceolata. Mineral oil sprays to
delay virus spread have been used only on a trial basis
and have not been tested under heavy disease pressure.
3.2.2Lettuce mosaic and turnip mosaic viruses (LMV
and TuMV), in addition to infecting lettuce, can also
infect spinach. Neither LMV nor TuMV is seedborne in
spinach, but an adjoining lettuce crop could
inadvertently lead to infection of spinach. LMV
symptoms on spinach consist of bright yellow circular
spots that coalesce into a diffuse dull chlorotic mottle.
Young infected leaves are small, mottled, and
distorted. Older plants are stunted, and old infected
leaves die prematurely. TuMV-infected plants show
diffuse mottle, leaf distortion, and stunting. Lower
leaves become necrotic and abscise prematurely.
3.3. Celery Diseases
Cucumber mosaic virus (CMV), also called southern
celery mosaic, is the principal virus disease of celery in
New York. The chief characteristics of this virus disease
and control measures have already been discussed.
CMV symptoms consist of general mosaic, which, on
older leaves infected earlier, develops into chlorotic
yellowing and veinal necrosis (fig. 10). Plants infected
when young will also be stunted. The petioles on
similarly infected plants may also show slightly sunken,
buffcolored lesions (fig. 11). If plants are infected at an
early age, inner petioles are also affected, making the
plant unmarketable.
3.3.1 Celery mosaic virus (CeMV), also called western celery
mosaic because it was first identified in California, can also be a
serious disease of celery. The virus is transmitted by several
aphid species in a nonpersistent manner and is limited only to
umbelliferous plants (celery, carrot, parsley, etc.). Plants infected
at an early age are greatly stunted, and the foliage, after showing
early mosaic, becomes cupped and malformed (fig. 12). Because
of the limited host range, destruction of umbelliferous weeds is
the recommended control measure.
3.3.2 Aster yellows mycoplasma (AY) may also infect celery on
an annual basis. This disease is described under "Lettuce and
Endive Diseases."