Date of this Version
The first infectivity assays of plant viruses relied on the percentage of inoculated plants becoming systemically infected. The use of systemic infections for assays became unfashionable, however, after the discovery of local lesions and the demonstration that their numbers depended on virus concentration. Continued emphasis on local lesions over the years led to the widely held belief that systemic infections could not be used for a reliable and accurate assay of plant viruses. There was never any good statistical evidence for this viewpoint. In fact, no thorough investigation has been published of the accuracy of "systemic assays" of plant viruses, as I shall term those assays based on the development of systemic symptoms in inoculated plants.