Tomato Spotted Wilt Virus (TSWV), [genus Tospovirus of the family Bunyaviridae (Murphy, et al., 1995)], is of worldwide importance (Peters, 1998). This virus is a single-stranded ribonucleic acid (ssRNA)(Murphy, et al., 1995).

TSWV is best recognized for creating yellow splotches and yellow bulls-eye like rings on ripe tomato fruit. Other vegetables affected by this disease organism include potato, lettuce, pepper, eggplant, celery, endive, spinach, and pea. Some ornamentals also are affected.

The first symptoms are small, orange-colored flecks on some of the middle or lower leaves. As new spots appear, the older leaves turn brown, die and droop. Similar spots as well as streaks occur on the main stems and leaf petioles. Younger leaves usually turn bronze and later develop numerous small, dark spots. The tip of the plant may die back and stems may be streaked. The entire plant becomes dwarfed; with its drooping leaves it resembles a plant affected by Fusarium or Verticillium wilt.

Yellowish spots up to 0.5 inch in diameter appear on green tomato fruit. These spots often have distinct concentric zones or shades of yellow or brown alternating with green and later with pink or red. The mature fruit retains this bulls-eye appearance of concentric rings of pale red, yellow or white. The spots can be an inch or more in diameter.

TSWV infected potatoes have zonate (target-like) dead spots on their upper leaves, streaks on the stems. They wilt from the top down. The plants are stunted and yield is reduced. Infected lettuce is yellow, is retarded in its growth, and has brown blemishes on the central leaves. Affected spots are parchment-like with brown margins.

Infected peas have purplish necrotic spots on the stems and leaves along with circular spots and wavy lines on the pods. Spots on outer stalks of celery are first yellow, then necrotic with pockets of dead tissue inside the petioles. Infected celery is stunted and worthless.

Infected Ornamentals:

Ornamentals and flowers known to be susceptible to TSWV include dahlia, sweet pea, zinnia, nasturtium, poppy, calla lily, petunia, orchid, chrysanthemum, aster, salpiglossis, cineraria, gloxinia, and schizanthus. In many cases, zonate spots are present on infected plants.

TSWV-infected china asters have dead tan areas in the leaves, and brown surface blotches on the stems. Calla lilies have whitish, then brown spots and streaks. On sweet pea, reddish brown to purple streaks may run the full length of the stem. Circular to oval leaf spots with diffuse margins are followed by yellowing and death of leaves and stems. Blossoms sometimes develop a circular pattern in the pigment. Delphiniums may have numerous distinct double rings.

The virus persists from year to year in flower bulbs or roots. Lilies and dahlias are a common source of the virus. If dahlias have oak leaf or ring spot patterns on the foliage, there is an excellent chance that these plants are infected with TSWV. When infected dahlias are dug and stored for the winter, the virus is reintroduced to the area the following year when the rhizomes are planted in the spring. Suspect plants should be destroyed.

Weeds:

Ornamentals are not the only source of the disease. Over wintering thrips harbor the TSWV virus (Chamberlain et al., 1993); perennial weeds such as bindweed, nettle, mallow, and chickweed also harbor this viral plant pathogen. Weed species help over wintering thrips survive (Beckham, et al. 1971) and reproduce (Duffus, 1971).

The Vector:

Tospoviruses are transmitted from plant to plant by nine species of thrips (Thysanoptera) (Tsuda, 1999), to include the onion and common flower thrips (Mound, 1996). When the juvenile stage of the thrips ingests the virus from an infected plant, it enters the midgut epithelial and muscle cells and replicates. Once infected the juvenile thrips continues its normal development creating an infective adult within 5 to 9 days. The virus moves to the salivary glands during the insect's pupation stage. The virus is then transmitted through the feeding activity of the adult thrips (Ohnishi, et al., 2001; Tsuda, et al., 1996).

Infective adult thrips move readily from plant to plant infecting plants with the virus via the saliva produced in the thrips salivary glands (German, et al., 1992). Plant symptoms begin to show 7 to 20 days after infection.

TSWV is known to be acquired by first instar thrips feeding on infected host plant material (Lewis, 1997). Ohnishi, et al. report TSWV enters and replicates within the midgut of adult Thrips setosus (the light brown soybean thrips). The virus, however, remains confined to the midgut epithelia cells of these newly virus-infected adults thereby preventing the infected adult thrips from spreading the virus.

Control:

Several varieties of tomato, developed in Hawaii, resistant to TSWV include Pearl Harbor, Oaku, Lanai, Hawaii, Maui, Molakai, Kauak, and Niilhau. These are late maturing and therefore may have little value in colder areas with short growing seasons.

Sprays to control thrips have not been successful, probably because viruliferous thrips are constantly being blown into fields from external virus reservoirs. Systemic insecticides are not recommended due to the health problems associated with the use of these materials on edible crops.

Controlling weeds, avoiding contaminated host plants near the vegetable crop, and eliminating thrips in greenhouses and solariums is the best way to manage this problem.

Edibility:

According to Mike Davis (Personal phone call - August 1990), Plant Pathologist, University of California at Davis, there have been no problems with eating tomatoes or any other vegetables infected with Tomato Spotted Wilt and there are no changes in the acidity level of tomato due to this viral infection.

References Used:

Beckman, C.M., Beshear, R.J., and Tippins, H.H. 1971. Some winter host plants of thrips. University of Georgia Agric. Exp. Stn. Bull. 86.
Chamberlain, J.R., Culbreath, A.K., Todd, J.W., and Demski, J.W. 1993. Detection of tomato spotted wilt virus in tobacco thrips (Thysanoptera: Thripidae) Triplicate in harvested peanut fields. J. Econ. Entomol. 86:40-45.
Duffus, J.E. 1971. Role of weeds in the incidence of virus diseases. Annu. Rev. Phytopathol. 9:319-340.
German, T.L., Ullman, D.E., and Moyer, J.W. 1992. Tospoviruses: Diagnosis, molecular biology, phylogeny, and vector relations. Annu. Rev. Phytopathol. 30:315-348.
Lewis, T. 1997. Flight and dispersal. Pages 175-196 in:Thrips as Crop Pests. T. Lewis (ed). CAB international, Oxon, U.K.
Mound, L.A. 1996. The Thysanoptera vector species of Tospoviruses. Acta Hortic. 431:298-309.
Murphy, F.A., Fauquet, C.M., Bishop, D.H.L., Ghabrial, S.A., Jarvis, A.W., Martelli, G.P., Mayo, M.A., and Summer, M.D. 1995 Tospovirus. Pages 313-314 in: Virus Taxonomy, Archives of Virology. Suppl. 10. Springer-Verlag, New York.
Ohnishi, J. Knight, L.M. Hosokawas, D., Fujisawa, I, and Tsuda, S. 2001. Replication of Tomato spotted wilt virus after infection by adult Thrips setosus is restricted to midgut epithelial cells. Phytopathology 91:1149-1155.
Peters, D. 1998. An updated list of plant species susceptible to tospoviruses. Pages 107-110 in: Fourth International Symposium on Tospovirus and Thrips in Floral and Vegetable Crops. D. Peters and R. Goldbach, eds. Wageningen, the Netherlands.
Tsuda, S. 1999. Tomato spotted wilt tospovirus: Plant-infecting Bunyaviridae. Virus 49:119-130.

This page was initially input by Jean Morris Diller, Advanced Master Gardener Tri River Area.