"An Introduction to Plant Viruses
and TSWV"

A.K. Culbreath, J.W. Todd, S.L. Brown,
and H.R. Pappu

Viruses are extremely small organisms that consist simply of nucleic acid and protein. These organisms are not capable of living independent of another organism, therefore, they are considered obligate parasites. They are completely dependant upon their host's living cells to produce new virus nucleic acid and structural proteins. Individual viruses cannot be seen with a standard light microscope, yet they cause tremendous problems in animals and plants.

Most of the more familiar viruses are those that cause disease in humans. From cold sores, warts and the common cold and to influenza, polio, rabies and yellow fever, viruses can cause diseases in humans that range from minor irritation to death. Likewise with plants, viruses cause diseases that have a tremendous range in their effects.

Plant viruses enter their host plants only through wounds, via pollen transmission, or by vectors. Vectors are animals, plants and fungi that are able to transmit the virus from an infected plant to a healthy one, such that the second plant also becomes infected. Insects are the most common and most economically important vectors of plant viruses. Aphids, leafhoppers, plant hoppers, treehoppers, whiteflies, mealybugs, plant bugs, beetles, grasshoppers and thrips (with an "s") can transmit one or more plant viruses. Aphids are the most common insect vectors, but all of the insects mentioned can be extremely important for transmission of specific viruses or virus groups.

Members of the group of plant viruses called "Tospoviruses" cause severe problems in many of the world's agricultural production systems. The tospoviruses in many ways are more similar to some of the viruses that are pathogens of humans and other animals than they are to other plant viruses. In fact, the tospoviruses are closely related to the hantavirus that has caused disease and death in humans in the Four Corners area of the southwestern U.S.

Tospoviruses are typically vectored or spread by thrips, tiny insects that live, feed and reproduce on plant leaves and flowers. Several species of thrips have been proven to transmit one or more tospovirus(es). Tomato spotted wilt virus (TSWV) is the virus for which the tospovirus was named. It is spread by thrips. To be able to transmit the virus, a thrips (There is no such word as "thrip;" it is spelled with an "s" even if there's just one.) must acquire the virus from an infected plant as an immature larva. Once the larva has acquired the virus, it can transmit the virus to healthy plants as long as it lives. Tomato spotted wilt virus can infect several hundred different species of plants. Many of these are important food and oil crops. As the name indicates, this virus does infect tomato, and causes serious damage to tomato crops. It is not, however, just a tomato virus.

Peanut is an extremely important crop to areas of southern Georgia and Alabama, and northern Florida. Spotted wilt of peanut, caused by TSWV, has become one of the most serious and complex disease problems in production of peanut (Arachis hypogaea L.) and other crops since 1985 in the southeastern U.S. Spotted wilt is now common across the peanut growing areas of Georgia, Florida, and Alabama. In 1997 and 1998, losses to spotted wilt in peanut were estimated at $40 million/year for Georgia alone.

Spotted wilt and the thrips that vector TSWV present a perplexing challenge in epidemiology and disease control. Peanut appears to be an extremely good host for TSWV in the field. Tobacco thrips, Frankliniella fusca and western flower thrips, Frankliniella occidentalis, are both good vectors of TSWV and both infest peanut plants in Georgia and the southeastern U.S. Since the virus is spread by thrips, it follows logically that growers should be able to control the disease with insecticides that kill the thrips. However, this has not been the case. Since the emergence of spotted wilt as an important problem in peanut production in the U.S., numerous studies have generally failed to show that even long term intensive insecticide applications for control of thrips gives a reduction in incidence of spotted wilt in peanut. Growers have been educated to the fact that intensive applications of insecticides for thrips control are ineffective for preventing spotted wilt, wasteful of time and money, and serve as an unnecessary risk to the environment. One exception to these findings was the discovery that in-furrow application of phorate insecticide suppressed epidemics of the disease. However, that does not appear to be due to actual killing of the thrips.

Unlike the situation with many of the fungal disease and insect problems that are controlled by fungicides or insecticides, there are no viricides for control of this disease in peanut. Although several factors have been shown to provide some suppression of spotted wilt in peanut, no single measure alone has been effective in field situations of heavy disease pressure.

A team approach in research and extension has been employed to improved management of spotted wilt in Georgia and Florida. This team has succeeded in identifying and combining some critical management inputs that collectively minimize the risk of losses to spotted wilt. The most important inputs employ genetic resistance and other cultural practices. Initial experiments to elucidate the effects of these inputs were done with only one or combinations of two factors. Results from intensive cooperative research have shown that the cumulative effects of multiple management practices can have a huge impact on epidemics of spotted wilt, whereas one input alone may result in only marginal suppression of the disease.