Plant Pathology Fact Sheet Circ-914

Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean

Tomato Spotted Wilt Virus of Agronomic,Vegetable, and Ornamental CropsTom Kucharek, Larry Brown, Fred Johnson and Joe Funderburk,Respectively,Professor, Plant Pathology Department, University of Florida,Gainesville, Florida 32611; Former Plant Pathologist, Division of Plant Industry,Florida Department of Agriculture and Consumer Services, Gainesville, Florida32608; Professor, Entomology Department, University of Florida, Gainesville,Florida 32611; Professor, North Florida Research and Education Center, Quincy,Florida 32351.November, 1990. Copied March, 2000.

Introduction

In 1919, tomato spotted wilt virus (TSWV)was first described in tomato in Australia. By1920, TSWV was found in all Australian terri-tories and since that time, TSWV has spread tomany areas of the world. By 1926, TSWV wasfound in Hawaii in pineapple. By 1935, TSWVbecame epidemic in California. In 1938, TSWVwas found in greenhouse-produced tomatoesin Cleveland, Ohio. Since the late 1960’s, TSWVhas caused significant damage to tomatoes inHawaii and in the 1980’s, TSWV has severelyaffected lettuce and pepper production there.In 1971, TSWV was found in peanuts in Texas.In 1985 and 1986, some peanut fields in Texaswere destroyed by TSWV. However, the inci-dence of TSWV in Texas-produced peanuts wassignificantly less from 1987 to 1989. In 1990, theincidence in some peanut fields was over 50%.

In Louisiana, TSWV was first found withcertainty in 1972. By 1988, incidences of TSWVin solanaceous crops ranged from 25% to 75%in Louisiana.

In Georgia, TSWV was identified in 1970,once, but it could not be found in a field sur-

vey in 1983. By 1989, however, TSWV becamea serious problem in tobacco, peanut, and to-mato in south Georgia, where incidences ex-ceeded 50% in some fields. In 1990, some to-bacco fields were plowed down because ofTSWV.

In Florida, some evidence suggests thatTSWV may have occurred in tomato in 1974.TSWV was first identified in tomato and pea-nut in the panhandle area in May and June 1986,respectively. Since 1986, TSWV has been foundalso in tobacco, pepper, watermelon, potato,impatiens, gloxinia, and gladiolus (maybe in1985 also) in Florida. By 1999 incidences ofTSWV in a few tobacco fields have increased togreater than 50%. For peanut, the incidence inmost fields has been between trace amounts to7%, but in some peanut plantings, the incidencehas been 60-90%. In 1988, TSWV was identifiedin tomato in Dade County. Meanwhile, higherpopulations of western flower thrips have beenfound in central and south Florida. Thus, TSWVand its vectors are considered to be well estab-lished throughout the state of Florida. TSWVis a major threat for the production of manycrops with incidences in tomato being morethan 50% by the fall of 1990 in the south Geor-gia and north Florida area.

The case histories mentioned above clearly

demonstrate how TSWV can appear in somearea and over time, increase to devastating pro-portions. In contrast, some areas of the UnitedStates have had serious problems with TSWVin some production systems but over timeTSWV became a minor problem for unknownreasons. Nearly total crop destruction has oc-curred in South Georgia in tobacco and peanutsin some fields in recent years. Much remains tobe learned about this enigmatic virus.

In this publication, tomato spotted wiltvirus is used to designate the causal agent forthe disease Tomato Spotted Wilt. However,some of the original photographs, particularlyon some ornamentals, may have been impa-tiens necrotic spot, a disease caused by a re-lated, but distinct, virus. Impatiens necroticspot virus has a considerably narrower hostrange.

Host range

TSWV infects many plant species. Some ofthe common crops that are susceptible to TSWVare listed in Table 1. A major source of viruscan be weeds, but in practical terms it may notbe possible to eradicate the weed sources be-cause they are numerous and, very likely, arenot even adjacent to commercial crop fields.Also, at this time we do not know what weedsfunction as hosts and sources for TSWV inFlorida. In other areas of the United States andthe world, many weeds have been identifiedas potential hosts for TSWV. Certainly, weedcontrol in and around production fields andgreenhouses is encouraged.

Methods of spread

The primary movement of TSWV from oneplant to another is by the feeding of thrips (sin-gular or plural). Thrips are extremely small in-sects 0.5 to 5 mm (Figure 1) that inhabit flow-ers, leaves, and the soil, Six species of thripscan transmit TSWV. Western flower thrips, on-ion thrips, and tobacco thrips are among the

vectors for TSWV that occur in Florida.

Thrips arise from eggs deposited in tendertissues of stems, leaves, or flowers. The imma-ture forms (larvae) begin feeding as soon as theyhatch. When larvae (wingless) feed on infectedtissue for 15 minutes or more, they may acquirethe virus internally. As the feeding periodlengthens, the probability for the thrips to ac-quire the virus is increased. Full grown larvaefall to the soil and become pupae that do notfeed on the plant. Later, winged adults emergefrom the pupae. Only adults are capable oftransmitting the virus to a plant. Adult thripsmay carry the virus for its entire life, but thevirus is not passed on to the egg stage. The nextgeneration acquires the virus by feeding uponinfected plants. The time from the egg to theadult stage varies with many factors but hasbeen measured at 14 days at 85°F for westernflower’s thrips,

In north Florida, populations of thrips foundin flowers have been highest from late April toearly June. It is during and shortly after this timethat symptoms of TSWV begin to appear in to-mato, peanut, pepper, and tobacco in northFlorida. During the summer, active populationsof thrips in flowers have declined followed bya small increase in populations in the fall.Thrips that inhabit leaves feed actively on pea-nut, tomato, and other plants during the sum-mer months in north Florida.

Up to the spring of 1990, the occurrence ofTSWV in tomato, for example, has been high-est in the spring when populations of flower-inhabiting thrips are highest. Fall plantings oftomato, when populations of flower-inhabitingthrips are lower, had lower incidences of TSWVthan spring plantings in 1986, 1987, 1988, and1989. However, in summer plantings of tomatoin 1990 in the south Georgia and north Floridaarea, incidences in some fields exceeded 50%.In Hawaii, populations of thrips tend to be highduring most of the year, which may explainpartially why TSWV has been so severe there

for many years.

Plant sap from an infected plant can be asource of TSWV. Thus, any kind of mechanicalinjury to an infected plant that moves infectioussap to another plant can serve as a method oftransmission. Mechanical transmission does notappear to be a major method of spread forTSWV in commercial situations when com-pared to spread by thrips. However, one shouldavoid contact with infected plants where fieldoperations such as suckering, topping, tying,and transplanting are used. It may be benefi-cial to have separate individuals rogue infectedplants before other field operations that requireplant contact are conducted.

Seed transmission of TSWV has not beendocumented with certainty. Clonally propa-gated plants from shoots, suckers, bulbs, stems,and tubers are likely to be infected with TSWVif the mother plant is infected because, as withmost viral diseases, infections become systemicto some degree. Transplants of infected plantshave been a source of TSWV and many otherdiseases.

Symptoms

Probably no other plant pathogen causessuch an array of symptoms as TSWV. Someplants or varieties of crops may be infectedwithout expressing symptoms. Until you be-come familiar with the common symptoms ofTSWV associated with specific crop species, itis recommended that laboratory diagnoses berelied upon.

Symptoms caused by TSWV are variable.Several strains of TSWV have been identifiedaround the world and each strain may causedifferent symptoms in different crop speciesand varieties of crop species. Symptoms in cer-tain crops are evident only under certain envi-ronmental conditions (e.g., no symptoms occurin Stephanotis during hot temperatures). In to-

bacco, fewer lesions and later-development oflesions occurred at 60°F when compared to68°F. Usually, as plants become older, infectionresults in progressively reduced symptoms. Forexample, the degree of stunting in tobacco isless when older plants are infected. Also, to-mato fruit may not express severe symptomswhen infection occurs after those fruit are set.

Symptoms may develop from 3 to 14 daysafter inoculation, but in some situations symp-toms may not occur for weeks. An array ofsymptoms are presented in Figures 2-24. Othersymptoms can occur and may mimic other dis-eases. In Figure 2 are pictured small etchingsin leaves of peanuts accompanied by leaf curl-ing, which are caused by the feeding of thrips.Damage from feeding of thrips can cause leafdeformations but in peanut such damage is notconsidered to be of economical importance. Theprimary damage from thrips to peanut is theirtransmission of TSWV. However, feeding andegg-laying of thrips in blossoms and on recentlyformed fruit of tomato can cause fruit abortionand cosmetic fruit scars, respectively.

Control and tactical decision making

In some situations, control of TSWV will not bepossible. Control measures for TSWV are notyet available for many situations, particularlyin outdoor plantings. The following lists con-tain information that relate to the control ofTSWV.

Field and greenhouse considerations.

• Identify the presence of TSWV as soonas possible. Make a strong effort to famil-iarize yourself with the many symptomscaused by TSWV in the crops you grow.Control actions that begin early are likelyto be most effective.

• Plantings of susceptible crops should beseparated from each other as much as pos-sible on a geographical and a time basis.

• Roguing of infected plants is not discour-aged, but some studies have shown rogu-ing is not effective in field situations. Forgreenhouses, roguing has been helpful forcontrol when coupled with other methods.

• Immature fruit that is harvested (e.g., greentomatoes) should be inspected prior toshipment for subtle light-colored ringspotsor spots that will not ripen normally. Forexample, green tomato fruit with suchspots will ripen with yellow spots orblotches. To minimize such occurrences,known infected plants can be marked in thefield and pickers should be instructed notto harvest from such plants.

• Inspect plantings for thrips by using accept-able sampling methods such as yellow orblue sticky traps. This may be one way ofdetermining how often to spray insecti-cides. In greenhouses, insecticidal spraysevery three or four days may be necessary.

• Minimize field and greenhouse opera-tions such as cultivation during periodswhen thrips are abundant if possible. Dis-turbance of plants at these times may fur-ther encourage movement of thrips.

• Use resistant varieties if available. Cur-rently, several varieties (Southern Runner,Georgia Green, Virugard) of peanut areresistant to TSWV. With ornamentals, somevarieties are notably more susceptible thanothers. For example, New Guinea impa-tiens is highly susceptible. Also, chrysan-themum varieties differ in reaction type toTSWV. Certainly if you observe that onevariety is more resistant than another, thatinformation should be considered.

• Use of currently available insecticideseither as granular soil treatments or foliarsprays has reduced damage from thrips

in peanuts and tomatoes. However, theuse of insecticides as foliar sprays has notbeen successful in reducing TSWV on aconsistent basis. In some studies moreTSWV occurred where foliar insecticidalsprays were used. Possibly, insecticidesstimulate movement of winged formswhich results in additional spread of dis-ease. Perhaps, spray intervals would haveto be shortened to attain some control ofTSWV. If so, production costs would in-crease and integrated pest managementprograms would be jeopardized. Anotherconsideration is the lack of control ofthrips in adjacent crops. If thrips are con-trolled in one field with insecticides,thrips from outside the field may infestyour sprayed field and the thrips mustfeed for at least one hour on sprayedplants before they die. Remember, fifteenminutes or more of feeding is required fortransmission of TSWV. In furrow treat-ment with Thimet during planting of pea-nut has reduced TSWV. Use of Admireinsecticide in the transplant water for to-bacco has reduced TSWV.

• Pyrethroid insecticides have been notedfor flushing out thrips. Thrips tend todwell in hidden places making it difficultto reach with sprays. Combinations ofpyrethroid insecticides with other typesof insecticides may provide better control.Resistant strains of thrips to insecticidesmay occur if the same chemical is usedrepeatedly. Alternating different chemicaltypes during the crop period can reducethe occurrence of resistant strains and in-creases the possibility of control of imma-ture stages which are more difficult to killwhen compared to adults.

• Sprays of crop oils have not proven suc-cessful for control of TSWV.

• Crops that are produced during the pe-riods of time that thrips are active (e.g.,

April, May, and June in North Florida) aremore likely to be infected by TSWV.

• Consider less intensive crop productionof susceptible crops, particularly on anoverlapping basis. This has been benefi-cial for control of TSWV in greenhouses.Sometimes diseases can become so severeevery year that laws have been establishedthat prohibit the planting of that crop fora given period of time (e.g., one-monthlettuce-free period). This is an act of des-peration but may be necessary. Produc-tion of vegetables in Florida has becomemore intensive over a wider geographi-cal area. Such situations enhance croppests if there is no down time for the crops.

• Determine what the financial break-evenpoint is for a crop at various times in theseason. If the severity of TSWV becomeshigh early in the season, it may be best,financially, not to continue producing thatcrop. This is an important decision asmuch of the cost of production with somecrops occurs toward the middle to the endof the season, particularly harvesting andpost-harvest costs. This tactic has beenused in Hawaii for lettuce production.

Additional considerations for fieldplantings.

• Use crop rotation with non-susceptiblecrops (see Table 1) if possible. Also, avoiddouble cropping with susceptible crops(e.g., tomato followed by pepper). Thripspupate in the soil.

• Plowing and thorough seedbed prepa-ration should aid in the reduction ofpopulation of pupae in the soil. Also, aseedbed that is prepared early reducesseedling blights which inhibit young plantvigor. For peanut, minimum tillage hasreduced TSWV compared with conven-

tional plow-plant systems.

• Purchase certified disease-free trans-plants or grow your own disease-freetransplants.

In field situations, every effort should bemade to encourage high vigor of seedlingsand young plants. TSWV seems to bemore common, at least in peanut, whereplant stands are thin. Thus, young planthealth should be given the highest prior-ity. Seed with a high germination rate andhigh vigor will aid in attaining rapidgrowth of young plants. High seedingrates should be used for crops such aspeanut. Thick stands aid in the control ofTSWV. However, thicker plantings mayencourage buildup of foliar diseases suchas peanut leaf spot. If you choose to usethick plantings, be prepared to enhanceyour peanut leaf spot control programwith earlier and more frequent fungicideapplications.

• For tobacco and tomato, increasing theplant population in the row to compen-sate for infected plants may offset someyield loss. In Florida, in-row spacing oftobacco should not be less than 16 inches.Quality in down-stalk tobacco leaves isenhanced with increased in-row spacing.For tomato, plant spacing can be reducedin the row to 12 inches to compensate forinfected plants. However, more rigorouspruning and spray programs for pestsmay be necessary because of the thickerleaf canopy.

• Weed control may or may not be impor-tant for the control of TSWV in field situ-ations. First, the number of weed speciesthat are susceptible is enormous. Sec-ondly, the pertinent weed species thatharbor TSWV in Florida are not knownat this time. Because thrips can migratefrom beyond the confines of a field, in-

fected weeds or crops away from the pro-duction field may be the source of virus.Regardless, a good weed control pro-gram in and around the field is recom-mended.

• Field plantings of susceptible crops(Table 1) should not be next to green-house or transplant production sites. Ithas been observed in some localities inthe United States that TSW tends to bemore severe in fields that are near orna-mental plantings and greenhouse pro-duction areas.

Additional considerations for the green-house.

• Use certified, disease-free plant material.

• For greenhouse production areas, donot reuse soil from infested plantingsunless it has been sterilized. Thisshould reduce pupae of thrips.

• Destruction of weeds and infected cropplants has aided in the control of TSW

in the greenhouse. Sanitation in andaround greenhouses is imperative.

• Adjusting greenhouse temperaturesmay provide delays in expression ofsymptoms. For example, it has beennoted that cooler temperatures delaysymptom expression in chrysanthemum.However, because infected, symptom-less plants are a source of virus, it mightbe best to avoid cooler temperature sothat infected plants express symptomssooner which would allow for earlierroguing.

• Small mesh screens (100-400) should beused for greenhouse ports to minimizeentrance of thrips. Also, entrance portsshould have a separate foyer type en-trance to reduce direct movement ofthrips from the outside. Fans that ex-haust air should also reduce the entranceof thrips into greenhouses.

Figure 1. Thrips. Figure 2. Discoloration from TSWV anddeformation from thrips in peanut leaves.

Table 1. Some common plant species (not including weeds) that are infected by TSWV.________________________________________________________________________________________________Common name Scientific Name Common name Scientific Name________________________________________________________________________________________________Agronomic and Vegetable Crops Ornamentals (continued)Tobacco Nicotiana tobacum Nasturtium Tropaelourn majus and othersTomato Lycopersicon esculentum Impatiens Impatiens sppPepper Capsicum annum Petunia Petunia spp.Potato Solanum tuberosum Zebra plant Aphelandra squarrosaEggplant Solanum melongena var esculentum Gloxinia Sinningia speciosaLettuce Lactuca sativa (many varieties) Statice Limonium latifoliumEndive Cichorium endivia Verbena Verbena litoralisCelery Apium graviolens Strawflower Gomphrena globosaPeanut Arachis hypogaea African violet Saintpaulia ionanthaSpinach Spinacia oleracea Ageratum Ageratum spp.Bean Phaseolus vulgaris Amaranthus Amaranthus spp.English pea Pisum sativum Anemone Anemone spp.Southern pea Vigna sinensis Begonia Begonia spp.Soybean Glycine max Calceolaria Calceolaria spp.Watermelon Citrullus vulgaris Calenclula Calendula officinalisCucumber Cucumis sativus Exacum Exacum spp.Cauliflower Brassica oleracea var botrytis Geranium Geranium spp.Broccoli Brassica oleracea var botrytis Snapdragon Antirrhinum spp.

Dusty miller Senecio cinerariaFruit crops Madagascar-jasmine Stephanotis floribunclaPapaya Carica papaya Ranunculus Ranunculus spp.Pineapple Ananas sativus Cyclamen Cyclamen spp.

Cineraria Hydrangea Hydrangea spp.Ornamentals Gypsophila GypsophilaAmaryllis Amaryllis spp. Gerbera daisy Gerbera jamesoniiGladiolus Gladiolus spp. Peony Peony spp.Calla lilly Zantecleschia spp. Sage Salvia spp.Lillies (various) Forget-me-not Myosotis scorpiodesDahlia Dahlia spp. Morning glory lpomea spp.Marigold Tagetes spp. Coleus Coleus spp.Chrysanthemum Chrysanthemum spp. Larkspur DelphiniumAster Aster spp. Lupine Lupinus spp.Zinnia Zinnia spp. Evening Primrose Oenothera spp.Coreopsis Coreopsis spp____________________________________________________________________________________________________

Figure 3. Discoloration of peanut leaves. Figure 4. Plant stunting and ringspots inleaves of peanut.

Figure 5. Early leaf wilting in tomato. Figure 6. Severe leaf wilting in tomato.

Figure 7. Petiole (leaf stem) and leaf discol-oration in tomato.

Figure 8. Brilliant leaf discoloration in to-mato.

Figure 9. Light colored fruit spots in tomato. Figure 10. Dark colored fruit spots in tomato.

Figure 11. Severe fruit spots in cherry-typetomato.

Figure 12. Severe fruit spotting and deforma-tion in tomato.

Figure 13. Severe wilt in young tobaccoplant.

Figure 14. Severe wilt in old tobacco plant.

Figure 15.Discoloration in outer stem tissueof tobacco.

Figure 16. Discoloration in outer and innerstem of tobacco.

Figure 17. Leaf discoloration along leaf veinsin tobacco.

Figure 18. Leaf discoloration on one side oftobacco plant.

Figure 19. Leaf symptoms in Gloxinia. Figure 20. Leaf symptoms in Aphelandra.

Figure 21. Mosaic pattern in leaves of NewGuinea Impatiens.

Figure 22. Ringspotting in leaves of Impa-tiens.

Figure 23. Petiole discoloration in Ciniaria. Figure 24. Flower mosaic in Gloxinia.