Tomato spotted wilt virus (TSWV) is so named because it was initially described on tomatoes where it caused a wilt and spotting. Since the initial 1919 description, it had been found in roughly 350 plant species including vegetables, ornamentals, field crops, and weeds. Many greenhouse plants, however, tested differently when antibody-based assays were used to detect the virus. This virus was called TSWV–Impatiens strain in 1989. By 1992, it was found to be an entirely separate virus from TSWV and was named Impatiens necrotic spot virus (INSV).
These two viruses are transmitted by thrips while they feed on host plants. (Note that the name “thrips” is always plural: one thrips or many thrips.) Thrips are the only known insect vector. Six species of thrips have been shown to vector the viruses. The most common vector in Oregon is the western flower thrips (WFT). Plants propagated from infected stock plants may also contain the virus. In two cases the virus was reported to have been seedborne.
The seriousness of TSW and INS has been realized in many areas of the country and world. Many greenhouse operators have suffered severe losses from INSV- and/or TSWV-infected bedding and floral plants. These viruses have eliminated production of certain vegetable and field crops in some areas. Retailers are affected because of reduced return business from homeowners who have experienced stunted, spotty, and unthrifty plants that die prematurely.
TSWV and INSV have wide, overlapping host ranges and produce a broad range of symptoms on each plant species. Although TSWV is often found in tuberous dahlias and chrysanthemums, INSV is commonly found on other greenhouse crops such as impatiens, cyclamen, and gloxinia. Vegetable transplants such as tomato, eggplant, peppers, lettuce, and basil can also get both viruses.
Symptoms of both diseases vary according to host, time of infection, and environmental and cultural conditions. TSW and INS symptoms can mimic those of fungal, bacterial, or other virus infections. Both viruses can produce necrotic (dead) spots on leaves, discolored or black areas that follow leaf veins, general or spotty chlorosis (yellowing), black or purple stem streaks or leaf spots, falling of leaves or buds, leaf distortion, blackening of young leaves and growing points, stunting, and colored spots, stripes, or rings on petals, leaves, or fruit. The ringspot symptom is one that is often seen.
Symptoms range from dramatically obvious to no symptoms at all under certain conditions. Most infected plants tend to show the most obvious symptoms during cool weather or low light. An infected plant may not display symptoms at all during warm, high light, rapid growth periods. It is also common to find infected plants with only a few symptomatic portions while the majority of the plant looks normal.
Symptoms on gloxinia, garden impatiens, and tomato are described here. These diseases also are common in the Pacific Northwest on verbena, lobelia, and peppermint and less often on tuberous begonia, ageratum, dahlia, and chrysanthemum. Close attention should be paid to these crops for symptoms of TSW or INS.
Some greenhouse growers use gloxinia as an early indicator plant. Symptoms include stunting and browning or necrosis of the central part of the plant resulting in death. Ringspots and veinal necrosis may develop on the leaves.
Impatiens necrotic spot is a frequent problem on impatiens. Dark purple ringspots or arcs are commonly seen on the leaves. Veinal necrosis, stem streaking, general chlorosis, stunting, leaf drop, and white concentric rings on the flower petals are also common. The ringspot symptom is more pronounced in the spring and fall and tends to disappear in the summer.
INSV and several strains of TSWV have been described on tomatoes. A severe strain of the virus causes young leaves to look bronzed; then numerous small necrotic spots appear and leaves roll downward and stiffen. Terminals often die back, and stems may be streaked. Green fruit from infected plants develop faint concentric rings with raised centers. Ripe fruit have more dramatic white and red or yellow and red concentric ring patterns. Other visible symptoms on tomatoes are stunting, yellowing, leaf distortion, and vertical cracks or corkiness on ripe fruit.
Early Detection with Indicator Plants and ELISA
Indicator plants are one method of early detection. A good indicator plant has several characteristics: It is attractive to the thrips vector; it will produce conspicuous symptoms of infection within a few days after the virus-infected thrips feed on the leaf; and the virus does not become systemic in the plant, so the indicator plant can’t serve as a reservoir for the virus. A nonsticky blue or yellow card is attached to the indicator plant pot to enhance its attractiveness to thrips. Flowers must be removed promptly to force thrips to feed on foliage; if flowers are present, thrips will feed in them, and no symptoms will be seen. A number of petunia cultivars including ‘Calypso’, ‘Summer Madness’, and ‘Super Blue Magic’ are good INSV and TSWV indicators. The fava bean ‘Toto’ also can be used. It has the advantage of being faster than petunia to produce from seed. If an INSV-infected thrips feeds on the indicator plant, a small papery tan spot appears on the leaf within a few days.
Place indicator plants in the greenhouse several weeks before susceptible crops arrive as transplants or seedlings. Maintain the indicator plants with the crop throughout the crop cycle. Water and fertilize them so they will serve as attractive food sources for thrips. Check indicator plants carefully once or twice a week for symptoms of virus infection and thrips feeding. Early morning is the best time to check for small new spots.
Because it is not possible to distinguish between TSW and INS based on symptoms, a more reliable way to detect these viruses is to send a plant (sample) to a laboratory. Laboratory methods currently used to detect the virus are to look for virus particles with the electron microscope; to inoculate indicator plants (bioassay); to use serological methods (ELISA and immunoblots which are the techniques used for routine samples); and to analyze symptoms. On-site test strips are commercially available to detect both viruses.
One reason that TSWV and INSV are difficult to find is that the virus is not evenly distributed in the plant. A false negative result can occur if the part of the plant tested does not contain the virus even though the rest does. A useful sample might include symptomatic plant parts from the crop of indicator plants. Some laboratories may be able to test thrips right off sticky cards. If the wrong antiserum is used, the virus will not be detected. Also, a plant may be symptomless in the summer even though it is infected. During this period it is very difficult to find the virus by any method.
Western flower thrips’ feeding is the main way TSWV and INSV are vectored. This tiny insect feeds on leaves and flowers. All feeding stages probe plant tissue with a knifelike mouth structure, sucking up cell sap and leaving air-filled cells that impart a silvery appearance to the plant surface. The female lays eggs in tiny slits in plant tissues. They hatch in 2 to 4 days into translucent larvae that begin to feed. Larvae molt to a second stage and, after 2 to 4 days, to the prepupal stage. The prepupae leave the plant and seek shelter in the soil (in the pot or under the bench) to pupate. The generation time, depending on temperature, is 7 to 13 days. The adult female may live 30 to 45 days and lay 150 to 300 eggs during her lifetime.
Immature thrips (larvae) acquire the virus when feeding on diseased plants; the insects then spread the virus as adults when feeding on healthy plants. The virus multiplies in the plant and may be through-out it. Its uneven distribution is the reason that some parts of the plant may look perfectly healthy while other parts show symptoms.
INSV survives the winter in infected plants, including those held over the winter in greenhouses, and in many species of weeds. These plants act as reservoirs of the virus from which it can again spread in the spring by the feeding activity of thrips.
Managing WFT and INSV in the Greenhouse
The key to control is to prevent the introduction of the virus in the first place. A single infected plant may serve as a source of INSV for the entire greenhouse. Once INSV is in a greenhouse, no treatment will eradicate it short of destroying every infected plant and infective thrips. Losses can be as much as the entire greenhouse planting if the virus is not detected early and eliminated.
Successful management of WFT or INSV in the greenhouse is a four-step process: (1) monitor for thrips and viruses; (2) construct barriers to thrips’ movement; (3) eliminate sources of the virus; and (4) suppress the thrips population. Each step is important.
Monitor for Thrips and Virus
An effective thrips monitoring program, conducted year-round, may be the most important single element of your control program. A good monitoring program will alert greenhouse managers to the presence of WFT so they can take control measures before populations build to the point that control is difficult. It can help locate WFT within the greenhouse range so that spot treatments may be applied.
Adult WFT can be monitored with sticky card traps. Yellow sticky card traps, in a variety of sizes, are now available from suppliers. Place three traps per 100 feet of bench, just above plants; skip every other bench or row. Place additional traps near doors and vents. Traps should be checked at least every 7 days. Number each trap so that accurate records can be kept of locations and what is caught. Replace traps after pesticide applications.
Visually inspecting plants is the only way growers can detect the presence of the virus. Use indicator plants in each house where thrips may land. Suspected INSV-infected plants should be sent to a diagnostic lab for confirmation.
A large open range where many different varieties are grown at many different stages is the worst possible situation in terms of WFT and INSV management. Once introduced, the virus becomes almost impossible to eradicate short of removing and destroying all plants and then fumigating. Therefore, if possible, partition the range with barriers of plastic sheets or plastic screen. Segregate propagation material purchased elsewhere from the rest of your culture, preferably in a separate house until you are sure it is healthy.
Thrips enter greenhouses mainly through doors and vents and on plant material and workers. Screen all vents to exclude adult thrips. Fit doorways with a double entry constructed of the same screening material. Some greenhouse operators have found that screening adequate to exclude thrips reduces air circulation to the point that larger fans are necessary to draw in additional air. Finally, since thrips are attracted to light colors, especially yellow and blue, avoid wearing these colors while in the range because thrips can be carried on clothing from points of infestation into clean areas.
Eliminate Virus Sources (Sanitation)
Weeds and discarded, deteriorating plants are sources of both WFT and INSV. Do not leave discarded plants or soil mix in the greenhouse. As plants and soil dry out, the thrips move to fresh plant material. If possible, empty a house of all plant material, refuse, and weeds and fumigate to control any WFT that may be lying in wait for the new plants. A regular weed control program in and around the range, and under benches, is also very important.
Suspect that all purchased propagation material is infected and, if possible, isolate it until you are sure it is virus-free. If you are not able to isolate new material, watch it closely for symptoms and be prepared to rogue out any plants that show signs of INS, especially if WFT is present.
Suppress the Thrips Population
The thrips management program you choose depends on whether INSV is present in your greenhouse. If you have detected INSV in your greenhouse, you must maintain a more aggressive thrips control program. Keep the following points in mind.
- At any given time, some portion of the thrips population is hidden in the soil (pupating) and so is not susceptible to chemical controls; therefore, you must reapply sprays at 3- to 7-day intervals to catch the new adults as they emerge.
- WFT often hides in flower petals or other protected areas of the plants, so thorough spray coverage is the key to successful management. This means low-volume/high-pressure mist application if possible.
- Rotate chemicals to avoid building pesticide resistance.
WFT has developed resistance to several insecticides in many parts of the country. If you experience a control failure, try switching to an insecticide from a different chemical group.
Spot-treat areas of the range where WFT has been trapped or seen. It is a waste of time and money to treat an entire range if the population of WFT is confined to a small area. Overtreatment is likely to promote the development of insecticide resistance. WFT normally will not move unless its host plant begins to dry out.
Under-bench Treatment Fungus gnats and WFT pupate in the soil under and around benches. Apply a long-residual insecticide in these areas to effectively reduce these pests. Also treat the underside of benches because these will be resting spots for both pests. (Remember to avoid getting residues on plants.)
Natural Enemies In recent years, research in Canada and Europe on controlling thrips in greenhouses has used a predaceous mite, Amblyseiius cucumeris. This mite has been highly effective in cucumber production houses where spider mites and whiteflies are controlled with natural enemies. The mite now is available commercially and can be reared on-site for release into houses.
Inspect bedding plants and vegetable transplants before purchase. Reject plants that have purple ringspots or streaks on leaves or stems, those that are off-color, are dropping leaves, and generally appear unhealthy. Buy plants only from a reputable nursery or garden center. Your best protection is prevention, since it is not possible to cure infected plants. Spraying insecticides to control thrips outdoors is probably ineffective.
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