Viruses

Abstract

Man has for centuries recognised the symptoms caused by viruses without realising their cause. The garish colour breaks in tulip flowers so highly prized in seventeenth-century Europe resulted from virus infection spread during vegetative propagation. It was not until the late nineteenth century, however, that evidence was produced that such symptoms were caused by pathogens. It was then shown that mosaic disease to tobacco could be transmitted to healthy plants by inoculation with sap from an infected plant. Proof that such symptoms were the result of particles smaller than those of bacteria was obtained when sap from mosaic-infected plants was passed through a bacteria-proof filter and it was shown that the resultant eluant was still infective. From this Beijerinck proposed the theory of contagium vivum fluidum, from which the present term virus is derived. The requirement for vectors to transmit viruses began to be realised in this period. Workers in Japan showed that dwarf disease of rice was spread by a leaf hopper (Nephotettix nigropictus), while in the USA it was demonstrated that a single leaf hopper (Circulifer tenellus) taken from a sugar beet plant infected with curly top and placed for five minutes on a healthy beet plant could transmit the organism causing disease. Following this there has been intensive work to isolate and identify viruses.