TOMATO SPOTTED WILT VIRUS (TSWV).   By now you have probably heard about the tomato spotted wilt virus problems in Maryland and neighboring states.  TSWV, a member of the thrips- transmitted tospovirus group, affects vegetable and field crops such as tomatoes, peppers, peanuts, potatoes, and tobacco.  However, it is occasionally found in many ornamental and greenhouse crops. This growing season, TSWV was first detected in newly transplanted tobacco fields in St. Mary's and Prince George's Counties.  I've included below a news release prepared by Dave Conrad, which summarizes what we currently know about the situation on tobacco.  Since the mid-80s, epidemics of tomato spotted wilt virus have been troublesome throughout the South.  TSWV has been a serious disease problem on tobacco in the flue-cured regions of Florida, Georgia and southern South Carolina. The disease has already caused major losses in Georgia fields and more cases are being reported every day. So far this season, the disease has showed up earlier than normal and further north in the eastern states.  North Carolina tobacco and potato growers have reported serious losses due to TSWV which spread across the state rapidly.  Moderate levels of the disease have been detected in potato plots at the Painter Research Station on the VA Eastern Shore according to Dr. Sam Alexander (Plant Pathology), and have occurred in tomatoes on the VA Eastern Shore and Tidewater Area, according to Clifton Slade (Virginia Cooperative Extension) and my counterpart, Dr. Tom Kuhar, at Painter.

In the past, Maryland growers have had sporadic problems with TSWV on processing tomatoes due to transplants already infected or infested with the thrips vector shipped in from the south.  We solved this problem by producing our transplants in local greenhouses with strict thrips management.  However, this is the first time that TSWV has occurred in commercial fields with transplants produced locally in outdoors seedbeds, suggesting the possibility of overwintering of thrips that vector the disease.

TSWV is vectored exclusively by thrips. Only nine of the more than 5000 described thrips species are proven vectors. Three of these, Frankliniella fusca (tobacco thrips), F. occidentalis (western flower thrips, WFT) and Thrips tabaci (onion thrips) are the chief vectors in the southern United States.  Of these, the western flower thrips, has the broadest host range affecting a diverse variety of ornamental and vegetable crops.  WFT is the chief TSWV vector in greenhouse settings around the world, as well as in  vegetable growing regions in the south where it can overwinter. Tobacco thrips is the major vector in tobacco and peanut fields, whereas the onion thrips vector a strain of tospoviruses in onion and garlic crops.  Both species are widely distributed in tropical, warm, and cool temperate areas around the world.  In the mid-Atlantic states, WFT can overwinter in greenhouses, whenever temperatures are favorable for their development and host plants (including weeds) are available for food.  Field infestations appear to be somewhat local and often occur later in the season in the proximity of greenhouses with a history of high incidence of WFT.  A recent study showed that adult thrips can overwinter outdoors in nectarine orchards in southeastern Pennsylvania. This spring capture followed high WFT population densities in the fall and a milder than normal winter.  Obviously, the record mild winter that just passed has probably accounted for the unusual occurrence of TSWV problems.  Successful overwintering of WFT and other thrips species in outdoors habitats can result in the spread of TSWV to perennial weed hosts early in the spring. In one study, Canadian researchers collected weeds both in and near greenhouses where there was a history of TSWV.  Many weeds were found to be both susceptible to TSWV and suitable hosts for thrips to lay eggs. Fifty species, including such common weeds as redroot pigweed, chickweed, lambsquarters, bindweed, thistle, galinsoga, pineapple weed, burdock, shepherd's purse, purslane, and black nightshade may be susceptible to TSWV.

Hopefully, our TSWV problems are the result of an unusual mild winter and thus probably would not occur following a normal or severe winter. However, in a typical scenario, TSWV will appear sporadically in an area, perhaps for years, then suddenly surge to epidemic proportions in agriculturally important host plants.  Wherever TSWV incidence has increased enough to cause economic losses, it has remained a chronic problem. This pattern has been repeated in India, Australia, Hawaii, and in the southern United States.  In some areas of California and Hawaii, tomato can no longer be grown due to TSWV infection.

Scouting for specific vectors of TSWV for control decisions is not feasible.  Identification of different thrips species is very difficult in the field because most species are similar in appearance.  Western flower thrips adults are slender, about 1.5 mm long, yellowish, and hold their fringed wings over their backs. Larvae are smaller and wingless, but otherwise resemble adults. Thrips are commonly found in the underside of leaves and in flowers and generally move quickly to shelter when disturbed.

The vector-virus relationship between thrips and TSWV is important to understanding how virus spread occurs.  The tospoviruses replicate in their thrips vectors, thus the insects not only spread the virus, but serve as a virus host. Thrips cannot transmit tospoviruses unless they acquire the virus during their immature stages.  When larvae feed on infected plants, ingested virus crosses the midgut barrier and enters the salivary glands.  A midgut barrier in adults prevents virus ingested during this stage from moving to the salivary gland.  While the insects remain infective for life, there is no evidence of transovarial passage from one generation of thrips to the next.  The adult thrips is the critical stage for transmitting the virus between plants, since this winged stage is more mobile that the wingless larval stage which tends to remain on the same plant upon which it emerged.  Thrips feed upon plants by rupturing leaf epidermal cells and slurping up the contents with a mixture of their saliva. It is at this point that the virus, being released with their saliva, has the opportunity to enter damaged plant cells and infect the plant. The thrips life cycle varies from 7 to 14 days at fluctuating temperatures between 68 to 98 F, so there are multiple generations on weed hosts and during the growing cycle of crop plants.

A systemically infected plant cannot be cured. Upon finding infected plant material the best course of action is to dispose of the plant as soon as possible. One infected plant can serve as a source to any other susceptible plant nearby.  However, roguing out symptomatic plants is not always effective as a control for secondary infections because TSWV has often spread before symptoms develop.  Generally, controlling weeds, avoiding contaminated host plants near the vegetable crop, and eliminating thrips in greenhouses is the best way to manage this problem.

To repeat, once symptoms start developing in the field, it is often too late to head off an epidemic. In general, the use of insecticides to control thrips has been an ineffective means of suppressing TSWV.  In theory, lowering overall thrips populations with insecticides should effectively reduce in-field spread of TSWV. However, insecticides have proven to be ineffective at suppressing primary infection, which accounts for most virus transmission.  There are several reasons why insecticides have not been effective for control of TSWV.  Thrips tend to be hidden in flowers and buds (resulting in incomplete spray coverage), have a rapid life cycle with a high reproductive potential (eggs inserted in plant tissue are unaffected and hatch quickly replenishes the population), and have a wide host range including many weeds (thrips are constantly being blown into fields from these external virus reservoirs). In addition, widespread resistance has made chemical control more difficult.  For example, resistance to certain pyrethroids, carbamates, organophosphates and abamectin, (not labeled for thrips) has been documented in certain thrips populations in the field.  In my studies of nontarget effects of Bt and conventionally-sprayed nonBt sweet corn, high populations of thrips are relatively unaffected by Warrior treatments.  This should give you an idea of how difficult it is to control a virus vector which only have to feed for 5 minutes to transmit the disease.

For more information on tomato spotted wilt, there is plenty of information via the Internet.  In particular, good images of disease symptoms on tomato can be found at
http://vegetablemdonline.ppath.cornell.edu/PhotoPages/Tomatoes/Tom_SpWilt/Tom_SpWiltPhotoList.htm.

Also, Ethel Dukty in our Plant Diagnostic Lab on campus has TSWV ImmunoSTRIP Tests that can be used for the detection of TSWV.

Additional information on purchasing  kits can be obtained online at http://www.agdia.com/cgi_bin/catalog.cgi/39300