Potato mop top virus. Coloured transmission electron micrograph (TEM) of virus particles (virions) of the potato mop top virus (PMTV). This plant virus is a furovirus, and has an RNA (ribonucleic acid) genome. It was first identified in 1966 in the potato plant (Solanum tuberosum), one of the world's major food crops. Symptoms are dwarfing (mopping), and blotching and tissue death. The virus is transmitted by the potato powdery scab fungus, Spongospora subterranea. Magnification: x125,000 when printed 10cm high.