Tomato Spotted Wilt Virus: TSWV in Vegetable Crops: Tospoviruses

Tospoviruses In Solanaceae and Other Crops in The Coastal Plain of Georgia

Introduction

John Sherwood

The Tomato spotted wilt virus (genus Tospovirus; family Bunyaviridae) (TSWV) is transmitted by thrips (Thysanoptera: Thripidae) and replicates in both the thrips vectors and the plant hosts. Although worldwide in distribution, TSWV was of limited and sporadic significance until the mid-1980s. In peanut, for example, TSWV had not caused significant losses in the United States until 1982, but now the disease has become epidemic in peanut, tobacco, vegetables and many ornamentals. The emergence of Frankliniella occidentalis as a worldwide resident, and the interstate and intercontinental shipping of plant materials likely contributed to TSWV becoming a global problem. The epidemiology of the diseases caused by TSWV remains poorly understood. It was only in 1993 that it was confirmed that TSWV multiplies in its vector, and it is not yet understood why some species of thrips transmit TSWV and others do not. The wide host range of TSWV and the occurrence of at least 13 species of thrips that transmit TSWV make eliminating the sources of primary inoculum of the virus impractical.

Understanding the relationship of TSWV with thrips requires a fundamental understanding of the structure of TSWV. Virus particles (virions) of TSWV are complex compared to many plant viruses. There are three RNAs in the virus genome that are individually encapsidated in many copies of the nucleocapsid protein, and the three RNAs are collectively bound by a membrane envelope that is of host origin, much like three strings of spaghetti inside a tennis ball. Also inside the envelope are several copies of a virus-encoded “replicase” protein that is required to initiate virus replication in a new host. This complex virion structure is a characteristic that distinguishes TSWV from most other plant viruses. TSWV virions are roughly spherical and are 80-110 nm in diameter. Two virus proteins processed during replication to contain sugars, i.e. glycoproteins (GPs), are dispersed throughout the surface of the viral envelope (much like the fuzz on a new tennis ball). These proteins are called glycoprotein 1 (GP1) and glycoprotein 2 (GP2) and differ slightly in size. The GPs in the envelope function in the maturation and assembly of virions, and in acquisition of virus in thrips leading to transmission.

Thrips deposit their eggs into plant tissue and the eggs hatch after 2-3 days, depending on the temperature and the plant host species. There are two feeding larval stages that are followed by two non-feeding pupal stages. The life cycle takes about 20-30 days from egg to adult, again depending on the temperature. Thrips are dispersed over long distances by wind. TSWV must be acquired by thrips during the larval stage of their development to be transmitted. Thus, only immature thrips that acquire TSWV, or adults derived from such immatures, transmit the virus. The capability of thrips to acquire TSWV decreases as the thrips age. Once acquired by the larvae, the virus is passed transtadially, i.e., TSWV persists through insect molts from larval to adult stages. The virus replicates in thrips, and the thrips can transmit the virus within about 24 hours after acquisition as larva and during their entire adult life. There are at least 13 species of thrips that transmit tospoviruses. Frankliniella occidentalis, the western flower thrips (WFT), is considered to be the most important vector species because it is globally distributed and can transmit most tospoviruses. Frankliniella fusca is considered the most important vector of TSWV in Georgia.

The first site of TSWV infection in larval thrips is the cells of the midgut. The virus then moves to other cells and organs, and becomes well established in the muscle cells. Eventually, the virus enters the salivary glands. Virions are excreted with the saliva into host plants when the thrips feed. If the plant is a suitable host for virus, the virus begins to replicate. If the plant is a suitable host for thrips, the thrips reproduce. As those thrips emerge they feed on the virus infected plant and the cycle begins again.

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