Induction of Disease by Viruses, with Special Reference to Turnip Yellow Mosaic Virus

Historically. we can distinguish three phases in the study of disease symptoms induced by viruses. first, from about 1900 to 1935, might be called The Symptom or The Era of Chaos. Hundreds of diseases were described in detail and given names. There was widespread confusion about the dis­ eases and their unknown causative agents. Goldstein (30) was the first to attempt to relate disease patterns to the developmental processes of the plant. She found that the distinctive mosaic patterns in successive leaves of tobacco plants infected with tobacco mosaic virus (TMV) were related to the age of the leaf at the time it became infected. She also noted that dark green tissue appeared cytologically normal. discovery, about 1936, that several plant viruses were ribonucleopro­ teins focussed attention on the virus particles themselves. With the develop­ ment of molecular biology after World War II, the study of virus symptoms fell into general disrepute, as being rather second-grade purely descriptive science. This might be called The Era of Prejudice. Since about 1965 a new trend has begun to emerge. knowledge about the structure and replication of viruses provided by molecular biology allows us to think much more constructively about possible ways in which viruses might induce symptoms; very sensitive and specific analytical procedures have been developed; and the widespread availability of high resolution elec­ tron microscopes has helped to bridge the gap between biochemical knowl­ edge and descriptions of a disease based on macroscopic observation and the light microscope. To understand how a virus causes disease we need to be able to explain, in terms of molecular biology and biochemistry, how a single virus particle con­ taining a piece of genetic material with about 2000 base triplets can infect and cause disease in a growing plant, organized into a variety of tissues and organs with specific structures and functions and containing when mature