Potato X virus. Coloured transmission electron micrograph (TEM) of virus particles (virions) of the potato x virus (PVX). This plant virus is a potexvirus, and has an RNA (ribonucleic acid) genome. It was first identified in 1931 in the potato plant (Solanum tuberosum), one of the world's major food crops. This virus also infects turnips. Symptoms are streaks of dead tissue. The virus is transmitted by contact between plants. Magnification: x125,000 when printed 10cm high.