E U I V E R S T Y F L 0 ID
I N S T I T U T E OF FOOD AND AGRICULTURAL SCIENCES
Tomato spotted wilt virus
of agronomic, vegetable, and ornamental crops
Tom Kucharek, Larry Brown, Fred Johnson and Joe Funderburk*
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, dean
has been over 10%. In 1988, TSWV
ot mates in Dade County. Meanwh
In 1919, tomato spotted wilt virus (TSWV) was nations of western flowers thrips ha
first described in tomatoes in Australia. By 1920, central and south Florida. Thus, T,
TSWV was found in all Australian territories and tors are considered to be well estab
since that time TSWV has spread to many areas of the state of Florida. TSWV is a ma
the world. By 1926, TSWV was found in Hawaii in production of many crops with inci
pineapple. By 1935, TSWV became epidemic in Cali- being more than 50% by the fall of
fornia. In 1938, TSWV was found in greenhouse-pro- Georgia and north Florida area.
duced tomatoes in Cleveland, Ohio. Since the late
1960s, TSWV has caused significant damage to to- The case histories mentioned abc
matoes in Hawaii and in the 1980s TSWV has se- onstrate how TSWV can appear in
verely affected lettuce and pepper production there, over time increase to devastating p:
In 1971, TSWV was found in peanuts in Texas. In contrast, some areas.of the United
1985 and 1986, some peanut fields in Texas were serious problems with TSWV in so
destroyed by TSWV. However, the incidence of systems but over time TSWV becal
TSWV in Texas-produced peanuts was significant- lem for unknown reasons. Much re
ly less from 1987 to 1989. In 1990, the incidence inl;enrralerned about this enigmatic virus.
some peanut fields was again over 50%. Li4 ,t range
In Louisiana, TSWV was first found with certainty
in 1972. By 1988, incidences of TSWV in solanaceotN0o
crops ranged from 25% to 75% in Louisiana.
In Georgia, TSWV was identified in 1970 oncxeFSstY
it could not be found in a field survey in 1983. By-
1989, however, TSWV became a serious problem
in tobacco, peanuts, and tomatoes in south Georgia,
where incidences exceeded 50% in some fields. In
1990, some tobacco fields were plowed down because
In Florida, some evidence suggests that TSWV oc-
curred in tomatoes in 1974. TSWV was first identi-
fied in tomatoes and peanuts in the panhandle area
in May and June 1986, respectively. Since 1986,
TSWV has been found also in tobacco, peppers,
impatiens, gloxinia, and gladiolus (maybe in 1985
also) in Florida. By 1990 incidences of TSWV in
tobacco have increased to 7%. For peanuts, the inci-
dence in most fields has been between trace amounts
to 3%, but in some peanut plantings, the incidence
was identified in
ile, higher popu-
Ive been found in
3WV and its vec-
jor threat for the
dences in tomato
1990 in the south
>ve clearly dem-
some area and
States have had
ne a minor prob-
mains to be
1 jSXnfects many plant species. Some of the
com rops tht are susceptible to TSWV are
listed in Table 1. A major source of virus can be
'XeIA.*ti in practical terms it may not be possible
to eradicate the weed sources because they are nu-
merous and, very likely, are not even adjacent to
commercial crop fields. Also, at this time we do not
know what weeds function as hosts and sources for
TSWV in Florida. In other areas of the United States
and the world, many weeds have been identified as
potential hosts for TSWV. Certainly, weed control in
and around production fields and greenhouses is
Methods of spread
The primary movement of TSWV from one plant
to another is by the feeding of thrips (singular or
plural). Thrips are extremely small insects 0.5 to 5
mm (Figure 1) that inhabit flowers, leaves, and the
*Tom Kucharek is a Professor, Plant Pathology Department, IFAS, University of Florida, Gainesville, Florida 32611; Larry Brown is a
Plant Pathologist, Division of Plant Industry, Florida Department of Agriculture and Consumer Services, Gainesville, Florida 32608;
Freddie A. Johnson is a Professor, Entomology Department, IFAS, University of Florida, Gainesville, Florida 32611; Joe Funderburk is
an Associate Professor, Entomology and Nematology Department, North Florida Research and Education Center, Quincy, Florida 32351.
Figure 1. Thrips.
Figure 2. Discoloration from TSWV and deformation from thrips in
Figure 3. Discoloration of peanut leaves.
soil. Six species of thrips can transmit TSWV.
Western flower thrips, onion thrips, and tobacco
thrips are among the vectors for TSWV that occur in
Thrips arise from eggs deposited in tender tissues
of stems, leaves, or flowers. The immature forms
(larvae) begin feeding as soon as they hatch. When
larvae (wingless) feed on infected tissue for 15 min-
utes or more, they may acquire the virus internally.
As the feeding period lengthens, the probability for
the thrips to acquire the virus is increased. Full-
grown larvae fall to the soil and become pupae that
do not feed on the plant. Later, winged adults
emerge from the pupae. Only adults are capable of
transmitting the virus to a plant. Adult thrips may
carry the virus for its entire life, but the virus is not
passed on to the egg stage. The next generation ac-
quires the virus by feeding upon infected plants. The
time from the egg to the adult stage varies with
many factors but has been measured at 14 days at
85*F (29"C) for western flower's thrips.
In north Florida, populations of thrips found in
flowers have been highest from late April to early
June. It is during and shortly after this time that
symptoms of TSWV begin to appear in tomatoes,
peanuts, peppers, and tobacco in north Florida. Dur-
ing the summer, active populations of thrips in flow-
ers have declined followed by a small increase in
populations in the fall. Thrips that inhabit leaves
feed actively on peanuts, tomatoes, and other plants
during the summer months in north Florida.
Up to the spring of 1990, the occurrence of TSWV
in tomatoes, for example, has been highest in the
spring when populations of flower-inhabiting thrips
are highest. Fall plantings of tomatoes, when popula-
tions of flower-inhabiting thrips are lower, had lower
incidences of TSWV than spring plantings in 1986,
1987, 1988, and 1989. However, in summer plant-
ings of tomatoes in 1990 in the south Georgia and
north Florida area, incidences in some fields exceed-
ed 50%. In Hawaii, populations of thrips tend to be
high during most of the year, which may explain
partially why TSWV has been so severe there for
Plant sap from an infected plant can be a source of
TSWV. Thus, any kind of mechanical injury to an
;ure 5. Early leaf wilting in tomatoes.
infected plant that moves infectious sap to another
plant can serve as a method of transmission. Me-
chanical transmission does not appear to be a major
method of spread for TSWV in commercial situations
when compared to spread by thrips. However, one
should avoid contact with infected plants where field
operations such as suckering, topping, tying, and
transplanting are used. It may be beneficial to have
separate individuals rogue infected plants before
other field operations that require plant contact are
Seed transmission of TSWV has not been docu-
mented with certainty. Clonally propagated plants
from shoots, suckers, bulbs, stems, and tubers are
likely to be infected with TSWV if the mother plant
is infected because, as with most viral diseases, infec-
tions become systemic to some degree. Transplants
of infected plants have been a source of TSWV and
many other diseases.
Probably no other plant pathogen causes such an
array of symptoms as TSWV. Some plants or variet-
ies of crops may be infected without expressing
symptoms. Until you become familiar with the com-
mon symptoms of TSWV associated with specific
crop species, it is recommended that laboratory diag-
noses be relied upon.
Symptoms caused by TSWV are variable. Several
strains of TSWV have been identified around the
world and each strain may cause different symptoms
in different crop species and varieties of crop species.
Symptoms in certain crops are evident only under
certain environmental conditions (e.g., no symptoms
in Stephanotis during hot temperatures). In tobacco,
fewer lesions and later-development of lesions oc-
curred at 60'F (15C) when compared to 68F
(20C). Usually, as plants become older, infection
results in progressively reduced symptoms. For ex-
Figure 7. Petiole (leaf stem) and leaf discoloration in tomato.
Figure 8. Brilliant leaf discoloration in tomato
Figure 8. Brilliant leaf discoloration in tomato.
ample, the degree of stunting in tobacco is less if
older plants are infected. Also, tomato fruit may not
express severe symptoms when infection occurs after
those fruit are set.
Symptoms may develop from 3 to 14 days after
inoculation, but in some situations symptoms may
not occur for weeks. An array of symptoms are pic-
tured in Figures 2-24. Other symptoms can occur
severe Trunl spotting ana aeiormaton in romaro.
rigure lu. uarK colored Truli spots in tomato.
11. Severe fruit spots in cherry-type tomatoes.
and may mimic other diseases. In Figure 2 are pic-
tured small etchings in leaves of peanuts accompa-
nied by leaf curling, which are caused by the feeding
of thrips. Damage from feeding of thrips can cause
leaf deformations but in peanuts such damage is not
considered to be of economical importance. The
primary damage from thrips to peanuts is their
transmission of TSWV. However, feeding and egg
laying of thrips in blossoms and on recently formed
fruit of tomatoes can cause fruit abortion and cos-
metic fruit scars, respectively.
Control and tactical decision making
In some situations, control of TSWV will not be
possible. Control measures for TSWV are not yet
available for many situations, particularly in outdoor
plantings. The following lists contain information
that relate to the control of TSWV.
Field and greenhouse considerations.
* Identify the presence of TSWV as soon as possi-
ble. Make a strong effort to familiarize yourself
with the many symptoms caused by TSWV in the
crops you grow. Control actions that begin early
are likely to be most effective.
* Plantings of susceptible crops should be separated
from each other as much as possible on a geo-
graphical and a time basis.
* Roguing of infected plants is not discouraged, but
some studies have shown roguing is not effective
in field situations. For greenhouses, roguing has
been helpful for control when coupled with other
* Immature fruit that is harvested (e.g., green to-
matoes) should be inspected prior to shipment for
subtle light-colored ringspots or spots that will
not ripen normally. For example, green tomato
fruit with such spots will ripen with yellow spots
or blotches. To minimize such occurrences,
known infected plants can be marked in the field
and pickers should be instructed not to harvest
from such plants.
* Inspect plantings for thrips by using acceptable
sampling methods such as yellow or blue sticky
traps. This may be one way of determining how
often to spray insecticides. In greenhouses, insec-
ticidal sprays every three or four days may be
* Minimize field and greenhouse operations such as
cultivation during periods when thrips are abun-
dant if possible. Disturbance of plants at these
times may further encourage movement of thrips.
The spraying of an insecticide after such activity
may reduce spread of TSWV.
* Use resistant varieties if available. Currently, the
Southern Runner peanut variety has some resis-
tance to TSWV. With ornamentals, some varieties
are notably more susceptible than others. For ex-
ample, New Guinea impatiens is highly suscep-
tible. Also, chrysanthemum varieties differ in
reaction type to TSWV. Certainly if you observe
that one variety is more resistant than another,
that information should be considered.
* Use of currently available insecticides either as
granular soil treatments or foliar sprays has re-
duced damage from thrips in peanuts and toma-
toes. However, the use of insecticides has not
been successful in reducing TSWV on a consis-
tent basis. In some studies more TSWV occurred
where insecticidal sprays were used. Possibly, in-
secticides stimulate movement of winged forms
which results in additional spread of disease.
Perhaps, spray intervals would have to be short-
ened to attain some control of TSWV. If so, pro-
duction costs would increase and integrated pest
management programs would be jeopardized. An-
other consideration is the lack of control of thrips
in adjacent crops. If thrips are controlled in one
field with insecticides, thrips from outside the
field may infest your sprayed field and the thrips
must feed for at least one hour on sprayed plants
before they die. Remember, fifteen minutes or
more of feeding is required for transmission of
* Pyrethroid insecticides have been noted for flush-
ing out thrips. Thrips tend to dwell in hidden
places making it difficult to reach with sprays.
Combinations of pyrethroid insecticides with
other types of insecticides may provide better
control. Resistant strains of thrips to insecticides
may occur if the same chemical is used repeated-
ly. Alternating different chemical types during the
;ure 17. Leaf discoloration along leaf vein,
Crops that are produced during the periods of
time that thrips are active (e.g., April, May, and
June in North Florida) are more likely to be in-
fected by TSWV.
Consider less intensive crop production of sus-
ceptible crops, particularly on an overlapping
basis. This has been beneficial for control of
TSWV in greenhouses. Sometimes diseases can
become so severe every year that laws have been
established that prohibit the planting of that crop
for a given period of time (e.g., one-month let-
tuce-free period). This is an act of desperation but
may be necessary. Production of vegetables in
Figure 18. Leaf discoloration on one side of tobacco plant. Florida has become more intensive over a wider
geographical area. Such situations enhance crop
pests if there is no down time for the crops.
Determine what the financial break-even point is
for a crop at various times in the season. If the
severity of TSWV becomes high early in the sea-
son, it may be best, financially, not to continue
producing that crop. This is an important deci-
sion as much of the cost of production with some
crops occurs toward the middle to the end of the
season, particularly harvesting and post-harvest
costs. This tactic is being used in Hawaii for let-
.* : ~ Additional considerations for field plantings.
Figure 19. Leaf symptoms in Gloxinia. U Use crop rotation with nonsusceptible crops (see
Table 1) if possible. Also, avoid double cropping
crop period can reduce the occurrence of resistant with susceptible crops (e.g., tomatoes followed by
strains and increases the possibility of control of peppers). Thrips pupate in the soil.
immature stages which are more difficult to kill U Plowing and thorough seedbed preparation
when compared to adults. should aid in the reduction of population of pupae
0 Sprays of crop oils have not proven successful for in the soil. Also, a seedbed that is prepared early
control of TSWV. reduces seedling blights which inhibit young
* Purchase certified disease-free transplants or
grow your own disease-free transplants.
* In field situations, every effort should be made to
encourage high vigor of seedlings and young
plants. TSWV seems to be more common, at least
in peanuts, where plant stands are thin. Thus,
young plant health should be given the highest
priority. Seed with a high germination rate and
high vigor will aid in attaining rapid growth of
young plants. Recommendations in some states
suggest that higher seeding rates should be used
for crops such as peanuts to compensate for in-
fected plants. Normally, most growers have used
higher than necessary seeding rates already. It
would be better to purchase the highest quality
seed possible and prepare a well manicured seed
bed. While thicker stands may aid in the control
of TSWV, thicker plantings may encourage build-
up of foliar diseases such as peanut leafspot. If
you choose to use thick plantings, be prepared to
enhance your peanut leafspot control program
with earlier and more frequent fungicide appli-
* For tobacco and tomatoes, increasing the plant
population in the row to compensate for infected
plants may offset some yield loss. In Florida,
in-row spacing of tobacco should not be less than
16 inches (0.4 m). Quality in down-stalk tobacco
leaves is enhanced with increased in-row spacing.
For tomatoes, plant spacing can be reduced in the
row to 12 inches (0.3 m) to compensate for infect-
ed plants. However, more rigorous pruning and
spray programs for pests may be necessary be-
cause of the thicker leaf canopy.
* Weed control may or may not be important for
the control of TSWV in field situations. First, the
number of weed species that are susceptible is
enormous. Secondly, the pertinent weed species
Figure 24. Flower mosaic in Gloxinia.
that harbor TSWV in Florida are not known at
this time. Because thrips can migrate from be-
yond the confines of a field, infected weeds or
crops away from the production field may be the
source of virus. Regardless, a good weed control
program in and around the field is recommended.
* Field plantings of susceptible crops (Table 1)
should not be next to greenhouse or transplant
production sites. It has been observed in some
localities in the United States that TSWV tends
to be more severe in fields that are near orna-
mental plantings and greenhouse production
Additional considerations for the greenhouse.
* Use certified, disease-free plant material.
* For greenhouse production areas, do not reuse
soil from infested plantings unless it has been
sterilized. This should reduce pupae of thrips.
* Destruction of weeds and infected crop plants has
aided in the control of TSWV in the greenhouse.
Sanitation in and around greenhouses is impera-
* Adjusting greenhouse temperatures may provide
delays in expression of symptoms. For example, it
has been noted that cooler temperatures delay
symptom expression in chrysanthemum. Howev-
er, because infected, symptomless plants are a
source of virus, it might be best to avoid cooler
temperature so that infected plants express symp-
toms sooner which would allow for earlier rogu-
* Small mesh screens (100-400) should be used for
greenhouse ports to minimize entrance of thrips.
Also, entrance ports should have a separate foyer-
type entrance to reduce direct movement of
thrips from the outside. Fans that exhaust air
should also reduce the entrance of thrips into
Table 1. Some common plant species (not including weeds) that are infected by TSWV.
Common name Scientific Name
Agronomic and Vegetable Crops
Tobacco Nicotiana tobacum
Tomato Lycopersicon esculentum
Pepper Capsicum annuum
Potato Solanum tuberosum
Eggplant Solanum melongena var esculentum
Lettuce Lactuca sativa (many varieties)
Endive Cichorium endivia
Celery Apium graviolens
Peanut Arachis hypogaea
Spinach Spinacia oleracea
Bean Phaseolus vulgaris
English pea Pisum sativum
Southern pea Vigna sinensis
Soybean Glycine max
Watermelon Citrullus vulgaris
Cucumber Cucumis sativus
Cauliflower Brassica oleracea var botrytis
Broccoli Brassica oleracea var botrytis
Nasturtium Tropaeloum majus and others
Impatiens Impatiens spp,
Petunia Petunia spp.
Zebra plant Aphelandra squarrosa
Gloxinia Sinningia speciosa
Statice Limonium latifolium
Verbena Verbena litoralis
Strawflower Gomphrena globosa
African violet Saintpaulia ionantha
Ageratum Ageratum spp.
Amaranthus Amaranthus spp.
Anemone Anemone spp.
Begonia Begonia spp.
Calceolaria Calceolaria spp.
Calendula Calendula officinalis
Exacum Exacum spp.
Geranium Geranium spp.
Snapdragon Antirrhinum spp.
Dusty miller Senecio cineraria
Madagascar-jasmine Stephanotis floribunda
Ranunculus Ranunculus spp.
Cyclamen Cyclamen spp.
Cineraria Hydrangea Hydrangea spp.
Gerbera daisy Gerbera jamesonii
Peony Peony spp.
Sage Salvia spp.
Forget-me-not Myosotis scorpiodes
Morning glory Ipomea spp.
Coleus Coleus spp.
Lupine Lupinus spp.
Evening Primrose Oenothera
Coreopsis Coreopsis spp.
COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES, John T. Woeste,
director, in cooperation with the United States Department of Agriculture, publishes this information to further the purpose of the May 8 and June 30, 1914
Acts of Congress; and is authorized to provide research, educational information and other services only to individuals and institutions that function without
regard to race, color, sex, age, handicap or national origin. Single copies of extension publications (excluding 4-H and youth publications) are available
free to Florida residents from county extension offices. Information on bulk rates or copies for out-of-state purchasers is available from C.M. Hinton,
Publications Distribution Center, IFAS Building 664, University of Florida, Gainesville, Florida 32611. Before publicizing this publication, editors should ..........
contact this address to determine availability. Printed 11/90.