Potato mop-top virus (PMTV) is a plant pathogenic virus transmitted through the vector Spongospora subterranea that affects potatoes. PMTV belongs to family of Virgaviridae, and the genus Pomovirus (Potato mop-top virus). The virus was first identified in 1966 by Calvert and Harrison in Britain, and is now reported in many other potato cultivating regions of the world including U.S.A., Canada, China, Pakistan, Japan, South American countries and many parts of Europe. Many disease management systems have been found to be ineffective against the virus, although a combination of sanitation and vector controls seems to work well. Hosts and symptoms As the name implies, the main host is potato; however, this virus also affects some common vegetable plants and weeds. Some of them include tomato, black nightshade, lambsquarters and ground cherry. The potato mop top virus' primary hosts are plants in the Solanaceae and Chenopodiaceae. Potatoes that are infected by PMTV generally show hollow necrotic spots on the inside. This virus shows different symptoms in Europe; for example, the infected plants usually show dark brown necrotic arcs that discolor the tuber's flesh (these symptoms are very similar to those caused by alfalfa mosaic virus). Plants showing no symptoms of PMTV tend to produce larger quantities of infected tubers if they are derived from plants that have shown foliar symptoms in the previous year. Disease cycle The spores that PMTV is vectored in can live in the soil for up to 18 years giving the virus a long period of survival. The critical period for infection of S. subterranea and consequently PMTV is earlier in the potato growth cycle, during stolon formation and tuber set, which lasts 3–4 weeks. The disease cycle of PMTV begins with the virus entering the host plant's cell and disassembling its capsid to release the viral RNA into the cell. As a pomovirus, PMTV uses the host plant's machinery for replication and translation which both follow positive-stranded RNA models. After replicates of the viral RNA and of the capsid proteins are made in the cytosol, the virus reassembles itself and exits the cell to infect other cells. Because it is vectored by a protist, it generally appears in cooler and more moist times of the year. The naturally occurring virus has been found to have systemic effects on Nicotiana benthamiana. However, when the gene for TRIPLE GENE BLOCK1 (TGB1), which is a movement factor protein, was silenced,