The development of plant virology as a science is, almost totally, a 20th century phenomenon. Early papers on plant viruses emphasize pathogenicity and ultra-filterability and report transmission by leafhopper and aphid vectors, and from soil or through seed. Successive advances were largely driven by access to new techniques. For example, biochemical approaches, and local- lesion and serological assays, led to purification and crystallization of virus particles, and their identification as nucleoproteins; electron microscopy and X-ray crystallography revealed their detailed structure. Biological tests revealed mite, nematode, chytrid and plasmodiophorid vectors. Viroids were discovered. Proof of the infectivity of viral RNA and DNA, followed by sequencing of complete viral genomes, allowed their genes and control elements to be defined. Although some viral genomes were monopartite and others multipartite, each typically encoded proteins involved in replication, movement within the plant, particle structure and, commonly, transmission by vectors. The processes involved in vector transmission, viral replication, disease induction and resistance, and the role of gene silencing, were then studied intensively. Genetic variability was analyzed and short-term evolution explored. A soundlybased virus taxonomy was devised and now includes over 70 plant virus genera containing some 800 species. Virus ecological and epidemiological research established patterns of virus spread and aided the development of control measures. These include schemes for producing virus-tested stocks of vegetatively propagated plants, quarantine regulations, application of vectorcontrolling chemicals to crops and use of virus-resistant cultivars that contain conventional or man-made resistance genes. These and other changes in agricultural practices have resulted in substantial increases in crop yields and food security. Since about 1950, results of research on viruses infecting the whole spectrum of host Corresponding author: B.D. Harrison Fax: +44.1382562426 E-mail: [email protected]
taxa have been integrated to constitute virology, with plant virology contributing many seminal discoveries. The biological phase of plant virology has progressed to molecular biological and molecular genetic phases, and plant viruses have become tools in molecular biology, cell biology and biotechnology.