Using Virus Indicator Plants

Impatiens Necrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV) have wide host ranges including many greenhouse ornamentals and vegetables. These viruses are transmitted by several species of thrips including western flower thrips. Diagnosis of these viruses is sometimes difficult due to widely variable symptoms on a variety of plants.

Virus epidemics usually start in one of two ways: from carried-over virus and thrips within crops or weeds, or by transmission from infected plant material newly introduced to the greenhouse. Spring crops may be infected while in the propagation house if viruliferous thrips are present, or through the propagation of infected mother plants. Although these scenarios are arguably less common than they once were due to greater awareness of sanitation and careful screening of mother plants by producers of cuttings and plugs, outbreaks can still occur. Some of the spring crops implicated as virus/thrips sources include begonias, double-flowered impatiens, and New Guinea Impatiens. Neither virus is associated with seeds.

Pinpointing the origin of a problem can also be difficult. Virus from one source and thrips from another may interact to produce an epidemic involving many plant species. Growers concentrating all of their plants in a single house during winter months run a greater risk of mixing INSV-free seed crops with either leftover fall crops, or newly acquired plant material that may carry thrips and/or virus.

Once symptoms suggest that INSV or TSWV may be present, growers can confirm their suspicions using their own test kits or by sending samples to a diagnostic lab. Test kits are commercially available from Agdia. These kits are easy to use and some growers find them to be a convenient and valuable tool. Other growers prefer to send samples to a laboratory for confirmation. For information on sending samples to the UMass Extension Plant Diagnostic Lab, please see http://ag.umass.edu/services/plant-diagnostics-laboratory

Petunias and fava beans have been successfully used to detect the presence of thrips infected with TSWV and INSV in greenhouses. Both petunias and fava beans are very attractive to thrips and readily show signs of feeding injury when western flower thrips are present. The petunia cultivars 'Calypso', 'Super Blue Magic', and 'Summer Madness' have been shown to make good indicator plants for several reasons. First, these varieties are highly attractive to western flower thrips. Secondly, infected petunia plants do not serve as source of virus in the greenhouse because INSV does not become systemic within these petunias. Infected leaves can be picked off and discarded and petunia indicator plants can remain in the greenhouse to continue monitoring. Another reason is that the virus lesions show up very soon after thrips feed. Thrips feeding injury on the foliage leaves distinct white feeding scars. If the thrips are carrying INSV, a brown rim and, later, a circular lesion can be seen around the white feeding scars as early as 2-3 days after exposure to thrips.

It is important to attract and encourage thrips feeding on the foliage of indicator plants. If thrips don't feed on the foliage, you won't see the distinctive viral lesions. To encourage thrips to feed on the foliage of indicator plants, keep the petunia flowers removed from the plants and place a blue non-sticky card in each pot at plant height. The blue card will attract thrips to the vicinity and increase the chances that they will actually land on the petunia plants. Blue plastic picnic plates cut in half work well for this purpose.

Petunia indicator plants should be placed among crops at bench or floor level at a rate of one plant every 20 to 30 feet. It has been shown that they should be placed in areas with higher thrips populations. Though thrips are weak fliers, they may be carried throughout an entire crop on air currents, plant material, or greenhouse workers. If possible, isolate incoming plant material with indicator plants for at least three to four days to allow time for thrips scars to develop and show viral lesions.

The fava bean cultivar 'Aquadulce' also works well as an indicator plant. The same general principles apply for using fava bean plants except for one important difference - do not leave virus infected bean plants in the greenhouse. Any thrips that develop on them can spread the virus to other plants. Remove entire pots once symptoms are observed and replace them with new pots planted with bean seeds. Do not carry infested plants through the greenhouse; instead, place them directly into a lidded trash bin. Fava beans do not have a lot of foliage and therefore more plants are needed than when using petunias. One suggestion is to plant one bean seed per 4" pot and place the pots throughout the greenhouse at a rate of 12 pots per 1,000 sq.ft.

Check the bean plants daily. White scars will indicate thrips feeding and a brown rim around the feeding injury indicates that thrips may be carrying INSV. Do not confuse leaf spots caused by fungus with those caused by virus. Watch thrips populations and feeding injury as clues. Indicator plants can be a helpful tool for detecting INSV and TSWV early and for evaluating your cleanup program's effectiveness after a virus has been detected. A grower's best defense against INSV and TSWV is to detect it before it destroys a crop.

For more information on INSV and TSWV, see the fact sheet Impatiens Necrotic Spot Virus and Tomat Spotted Wilt Virus.

References

• Allen W.R and J.A. Matteoni. 1991. Petunia as an indicator plant for use by growers to monitor for thrips carrying the tomato spotted wilt virus in greenhouses. Plant Disease. 75(1).
• Daughtrey, M. 1991. Spring disease control: your guarantee for profits. GrowerTalks. Jan. issue. pp 32-40.
• Daughtrey, M. 1992. Procedure for using indicator plants for detecting tomato spotted wilt virus. Fact Sheet.
• Pundt L., J. Sanderson and M. Daughtrey. 1992. Petunias are your tip-off for TSWV. GrowerTalks. Nov. issue. pp 69-72.