Making a Virus Visible: Francis O. Holmes and a biological assay for tobacco mosaic virus

In the early twentieth century, viruses had yet to be defined in a material way. Instead, they were known better by what they were not - not bacteria, not culturable, and not visible with a light microscope. As with the ill-defined "gene" of genetics, viruses were microbes whose nature had not been revealed. Some clarity arrived in 1929 when Francis O. Holmes, a scientist at the Boyce Thompson Institute for Plant Research (Yonkers, NY) reported that Tobacco mosaic virus (TMV) could produce local necrotic lesions on tobacco plants and that these lesions were in proportion to dilutions of the inoculum. Holmes' method, the local lesion assay, provided the first evidence that viruses were discrete infectious particles, thus setting the stage for physicochemical studies of plant viruses. In a field where there are few eponymous methods or diseases, Holmes' assay continues to be a useful tool for the study of plant viruses. TMV was a success because the local lesion assay "made the virus visible" and standardized the work of virology towards determining the nature of the virus.