Cyrstal Conrad and Dr. Mikel R. Stevens, Agronomy and Horticulture

The Tomato Spotted Wilt Virus (TSVRV) is a Tospovirus. It, along with its relatives, is identified as infecting more than 550 monocot and dicot species throughout the world.1 In some areas of the United States as much as 38% of crops were lost.2 In Hawaii and South Africa 80-90% losses in crops were reported.3.4 TSWV mottles fruits, causes stunting in plant growth, and reduces yields in commercially grown tomatoes. Research has shown the most effective solution is found in genetic resistance.

Resistance was discovered in the wild species of Lycopersicon peruvianum (L.) Mill. Two different tomato lines, one from Hawaii and the other from South Africa, carry this resistance. A single dominant gene named Sw-5 was identified in the South African line.5 The gene in the Hawaiian line is considered to be the same as Sw-5 or allelic.6 In an unpublished test, done some time ago, it was suggested that they may actually be different genes. In order to back up these results more testing was needed.

Along with the clarification of these two genes, new resistance was discovered in L. chilense Dun.7 It is not as effective as Sw-5 but allows higher levels of virus infection in the plant without affecting plant performance. As the plant matures its resistance to TSWV improves with time. In order to further study the strength of this new form of resistance, five different TSWV isolates were used to infect L. chilense plants.

In both the Hawaiian/African study and the L. chilense study, plants were grown from seed and inoculated with TSWV isolates. Six flats containing F2’s of both the Hawaiian line and the African line were grown. Five flats of L. chilense plants were inoculated with five different TSWV isolates. Checks, which included plants with and without Sw-5, were included in each of the flats. For the Hawaiian/African study three flats were inoculated with the TSWV isolate HR-1. The other three flats were inoculated with the TSWV isolate from Arkansas. The Arkansas isolate was used in the previous unpublished experiment which showed differentiation between the two genes. HR- 1 and the Arkansas isolate were also used for inoculating L. chilense along with JF-1, Hawaii, and H40. The JF-1, and Hawaii isolates were shown previously to overcome the Sw-5 resistance gene. Plants were rated visually and with enzyme-linked immunosorbent assay (ELISA) after two inoculations and a few weeks of growth. Plants with an optical density of 0.186 or greater were considered infected.

The TSWV Arkansas isolate did infect the Hawaiian and African lines, but the infection was so low that no distinguishing patterns could be established. Visually, the tobacco check was severely infected, yet the ELISA readings showed no infection. These two factors suggest a mutation has occurred in the Arkansas isolate rendering it less virulent. These mutations are not unusual as new strains of TSWV are constantly being found. As a result no determination could be made in this study.

A comparison between the Hawaiian and African lines using the TSWV isolate HR- 1 yielded more promising results. The African line followed the segregation expectations for a single dominant gene. With a single dominant gene 3/4 or 75% of all of the plants should be resistant in an F2 population. Thirty eight plants (66%) came up resistant using ELISA and twenty (34%) came up susceptible. A Chi-Square test yielded a test statistic of 4.32. These numbers are not unusual and fall within the limits of acceptability for a single dominant gene. In the Hawaiian line 47% of the plants were resistant and 53% of the plants were susceptible according to ELISA. The test statistic for the Hawaiian line came up 62.72. With 95% confidence we can be sure these results do not fit the 3:1 ratio for a single dominant gene in an F2 population.

These results help support the hypothesis of two different, though allelic, genes in these two tomato lines. In spite of this new evidence much more testing needs to be done to support these findings before a definite decision can be made whether the two genes are different or not.

In the L. chilense study three of the TSWV isolates showed low virulence; H40, Arkansas, and JF- 1. When the HR- 1 isolate was compared with the Hawaii isolate, however, L. chilense showed greater resistance to the Hawaii isolate. Ten plants (38%) of the L. chilense infected with the Hawaii isolate came up resistant while only one plant (1%) of the L. chilense infected with the HR- 1 isolate came up resistant. Their resistance is expected to increase over time. This shows L. chilense’s promise as a source of TSWV resistance to other isolates. It is not as effective as Sw-5, but may be used in conjunction with Sw-5 in the future to provide tomatoes with a greater resistance to TSWV.

References

1. I. Wij Karnp and D. Peters, (1993) Determination of the median latent period of two tospoviruses in Frankliniella occidentalis, using a novel leaf disk assay. Pytopathology 83:986-991.
2. R.G., Paterson (1987) Epidemiology and genetic resistance in tomato to the tomato spotted wilt virus in Arkansas. M.S. Thesis. University of Arkansas, Fayetteville, Arkansas, USA.
3. J.J., Cho et.al., (1989) A multi disciplinary approach to management of tomato spotted wilt virus in Hawaii. Plant Dis. 73:375-383.
4. J.J.B., Van Zijl et.al., (1986) Breeding tomatoes for processing in South Africa Acta. Hort. 194:69-75.
5. M.R., Stevens et.al., (1992) Inheritance of a gene for resistance to tomato spotted wilt virus (TSWV) from Lycopersicon peruvianum Mill. Euphytica 59:9-17.
6. 6. S.H., Brommonschenkel et.al., (1994) Molecular tagging of tomato spotted wilt tospovirus (TSWV) resistance derived from Lycopersi conperuvianum. (Abstract). J. Cell. Biochem. Suppl. 18A: 116.
7. M., Canady, (1997) TSWV Tolerance as derived from Lycopersicon Chilense LA 1938. M.S. Thesis. Brigham Young University, Provo, Utah, USA.