Definitions

spotted-wilt

Tomato spotted wilt virus

The Tomato Spotted Wilt Virus (TSWV) is a type species of the Tospovirus genus which is a member of the Bunyaviridae family. TSWV is a plant virus that has a broad host range and can infect over 800 different plant species from 82 different families. Some of the most common hosts are tomato, pepper, many other vegetables and ornamental plants. The TSWV is transmitted from plant to plant by insect vectors that include several species of thrips.

Genome

This virus has a single stranded RNA genome with negative polarity, therefore it is classified as a Class V virus ((-)ssRNA viruses). The structure of the genome resembles that of the genus Phlebovirus. The genome is linear and is 17.2 kb in size. It is segmented into three segments termed S (2.9kb), M (5.4kb) and L (8.9kb). The M and S RNA segments encode for proteins in an ambisense orientation.

Disease

Early symptoms of infection are difficult to diagnose. In young infected plants the characteristic symptoms consist of inward cupping of leaves and leaves that develop a bronze cast followed by dark spots. As the infection progresses additional symptoms develop which include dark streaks on the main stem and wilting of the top portion of the plant. Fruit may be deformed, show uneaven ripening and often have raised bumps on the surface. Once a plant becomes infected the disease cannot be controlled.

Epidemology

TSWV is prevalent in warm climates in regions with a high population of thrips. This virus is an agricultural pest in Asia, America, Europe and Africa. Over the past 15 years outbreaks of Tomato spotted wilt disease have become more prevalent in these regions. Therefore TSWV is described as an emerging viral disease of plants. The increased prevalence is largely because of the successful survival of the thrips vector Frankliniella occidentalis. However, the successfulness of this virus is also attributed to the acquisition of a gene in the M segment of the genome which encodes a movement protein. This protein allows the virus to infect a range of hosts. The gene encoding this protein was likely acquired by recombination from either a plant host or from another plant virus.

Prevention of Spotted Wilt disease

Control of this disease is difficult. One of the reasons for this is that the wide host range allows the virus to successfully overseason from one crop to the next. To prevent spread of the virus infected plants should be immediately removed away from neighbouring plants. Control of insects, especially thrips, is important to reduce spread of the virus by vectors.

References

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