Tomato Spotted Wilt virus and Western Flower Thrips: Update, Vol ...

ORNAMENTALSNORTHWESTARCHIVESJuly-August 1989Vol.13, Issue 4Pages 4-5J.L. Green, Horticulture Department,Oregon State UniversityTOMATO SPOTTED WILT VIRUS AND WESTERN FLOWER THRIPS: UPDATESituationPrevious Reports: The first report of tomato spotted wilt virus (TSWV) was from Australia in1915 (Brittlebank, 1919). In 1935, TSWV was reported in Oregon by McWhorter and Milbrath(1935). Background information and reports of current research and managementrecommendations pertaining to tomato spotted wilt virus and the western flower thrips have beenpresented in previous issues of the Ornamentals NorthWest Newsletter (Green, 1986; Green,1988; Green, et al., 1988; DeAngelis, 1989; Ormrod, 1989).TSWV infects woody plants as well as herbaceous: Not only has TSWV been reported onmore than 200 species of herbaceous plants, including many ornamentals and associated weeds,it has also been isolated from several woody plants, including: Nerium Oleander (evergreen,erect, woody shrubs), Carica papaya (succulent trees, or rarely vines), Stephanotis floribunda(Green, et al. 1988) and Duboisia leichhardtii (Francki and Hatta, 1981). Symptoms onDuboisia leichhardtii, a woody tree of Australia, include leaf lines, vein banding, lowerbranch collapse, apical dieback, leaf fall and wilt and main stem stunting (Francki andHatta, 1981).TSWV Incidence in Both Greenhouse and Field Crops Has Increased: Because of thepotential threat to the ornamentals industry, the American Floral Endowment sponsored asymposium hosted by Yoder Brothers, Inc. on TSWV and the western flower thrips in June1987. The symposium was successful in providing a multidisciplinary meeting for research,extension, and industry to discuss the problem and develop research priorities. In March, 1989,the TSWV symposium was expanded to include discussion on all crops affected by the disease(Cho, et al. 1989).Dr. Ron Jones, North Carolina State University, reported that surveys in North Carolina foundTSWV in only one greenhouse in 1986; it has now been confirmed to be in 18 greenhouses inNorth Carolina with sometimes heavy losses (Baker and Jones, 1988).The first report of TSWV in the field this season was posted from Florida on April 21,1989: The first tomato spotted wilt virus symptoms of the season were found on Floridatomatoes the week of April 3 in several fields in Gadsden County. One field checked April 21had an incidence of 0.4%. Symptoms included stunting and necrotic areas and ring spots onleaves (Sprenkel, 1989).

TSWV frequently causes 50-90% crop losses in lettuce during any season of the year in Hawaii.TSWV has become a limiting factor in lettuce production in Hawaii, especially when fields arereplanted before all the thrips that have pupated in the soil after acquiring TSWV from theprevious crop have emerged. In field experiments in Hawaii, thrips continued to emerge from thesoil for 2-3 weeks after crop residues were plowed and rototilled. More than a million thrips perhectare of crop emerged during this period (Cho, et al. 1989). Heavy damage to the young plantscan occur: Initial symptoms on lettuce are small brown necrotic spots on young leaves followedby meristem stunting, systemic wilting of the entire plant, and premature death within 2 weeksafter appearance of the initial symptoms. Today, major losses on Maui in lettuce, tomato, andpepper have forced several growers to not plant TSWV-susceptible crops (Cho, et al. 1989)Management Strategies• Genetic Resistance to TSWVHost resistance to TSWV offers the best long-term solution, but development of suitablecultivars requires several years of research.Researchers in Hawaii (Cho, et al. 1989) have screened 609 lettuce lines, mostly Lactuca sativaand Lactuca serriola L. In further evaluations, an additional 223 lines of wild and cultivatedlettuce were screened. Several L. sativa lines appeared to be resistant. Progeny tests indicatedthat none of the L. sativa lines were homozygous resistant. Results of test crosses and TSWVresistancetests of the Fl and F2 plants suggested that two cultivars ('Tinto' and 'Ancora', lines ofLactuca sativa) had the same genes for resistance and that this resistance is partially dominant(O'Malley and Hartmann, 1989).At present, several tomato lines highly resistant to TSWV and with reasonable fruit size havebeen selected; commercially suitable cultivars should become available within 2-3 years (Cho, etal. 1989).• Insecticides:26 insecticides were evaluated for control of the Western Flower Thrips (WFT) -the insecticidesdid not kill rapidly enough to prevent virus transmission; none of the insecticide treatmentssuppressed TSWV disease occurrence(Cho, et al. 1989). Dr. Ron Mau, University of Hawaii stated that, "Pesticides alone could notcontrol the virus. Even at 200 gallons per acre twice a week, there was not enough effect towarrant reliance solely on pesticide treatment" (Baker and Jones, 1988).• Cultural Practices:It is essential that all propagative plant material be free of tomato spotted wilt virus. In addition,exclusion of thrips, a program of thrips eradication, and removing infected plants are essential tohold losses to an acceptable level (Baker and Jones, 1988).

Cho, et al (1989) reached the following conclusions:1. Don't Grow TSWV-Susceptible Crops in High Incidence Areas: Management is not totallyeffective if virus and vector occurrence is high throughout the area; in this situation, TSWVsusceptiblecrops should not be planted.2. Areawide Cooperation Is Essential: Because of substantial movement of thrips betweencrops and farms, areawide cooperation is essential. Alternate hosts of the virus and thrips mustbe controlled, sequential planting of susceptible crops avoided, virus-free plants planted, andharvested or abandoned crops immediately plowed.3. Crop isolation reduces TSWV incidence: Separate blocks of susceptible crops with blocksof nonsusceptible crops.References:Baker, James R and Ronald K. Jones. 1988. Sixth Ornamental Workshop, October 1988. NorthCarolina Flower Growers Bulletin 33(6):1-4, December 1988.Baker, James R and Ronald K. Jones. 1988a. Update on tomato spotted wilt virus and westernflower thrips. Ornamentals Northwest Newsletter 12(3):12-14.Brittlebank, C.C. 1919. Tomato Diseases. J. Agric. Victoria 17:231-235.Cho, JJ., R.F.L. Mau, T.L. German, RW. Hartmann, L.S. Yudin, D. Gonsalves and RProvvidenti. 1989. A multidisciplinary approach to management of tomato spotted wilt virusin Hawaii. Plant Disease 73(5):375-383.DeAngelis, Jack. 1989. Greenhouse management of western flower thrips and tomato spottedwilt virus. Ornamentals Northwest Newsletter 12(6):7-11.Francki, R.I.B. and T. Hatta. 1981. Tomato spotted wilt virus, Chapter 17, pp 492-511. In:Handbook of Plant Virus Infections and Comparative Diagnosis, edited by E. Kurstak.Elsevier/North-Holland Biomedical Press, 943 pp.Green, J.L. 1986. Tomato spotted wilt virus transmitted by western flower thrips. OrnamentalsNorthwest Newsletter 10(2):8-9.Green, J.L. 1988. Tomato spotted wilt virus in greenhouse crops. Ornamentals NorthwestNewsletter 12(3):10.Green, J.L., T.C. Allen and S. Fischer. 1988. Symptoms of stephanotis infected with tomatospotted wilt virus. Ornamentals Northwest Newsletter 12(3):10-11.McWhorter, F.P. and J.A. Milbrath. 1935. The interpretation of Oregon tip blight on a basis ofcausal viruses. Phytopath. 20:897-898.O'Malley, P.J. and RW. Hartmann. 1989. Resistance to tomato spotted wilt virus in lettuce.HortScience 24(2):360-362.Ormrod, DJ. 1989. Plant diagnostic clinic, BC Ministry of Agriculture & Fisheries. OrnamentalsNorthwest Newsletter 13(3):8.Sprenkel, RK. 1989. First TSWV report of season in Florida - posted April 21, 1989. NationalAgricultural Pest Information System, NAPIS.

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