Karen-Beth G. Scholthof

Karen-Beth G. Scholthof


Professor, Department of Plant Pathology and Microbiology


Texas A&M University

Current Project

As a practitioner-historian I am studying the development of the science of plant virology in the United States in the early 20th century. In particular, I am investigating how tools were designed and used by plant virologists, chiefly to develop quantitative assays to measure Tobacco mosaic virus (TMV) in tobacco. Most of my research is focused on the work of Francis O. Holmes, a scientist at the Boyce Thompson Institute for Plant Research (Yonkers, NY) and the Rockefeller Institute for Medical Research (Princeton, NJ). In 1929 Holmes determined that the necrotic primary lesions observed on tobacco after inoculation with TMV could be used to accurately assay virus concentrations. This became an essential tool for scientists working on the physicochemistry of plant viruses in the early 1930s.

The method became "Holmes' local lesion assay"—possibly the only eponymous assay for plant pathology. From the local lesion work, Holmes reasoned that this necrotic (N) response by Nicotiana glutinosa tobacco plants to infection with TMV was a resistance response. He performed a series of genetic crosses to introgress the N-gene into previously susceptible tobacco, which then responded with necrotic primary lesions. This is a first example of genetic breeding for virus disease resistance using a single dominant gene. Both the assay and the N-gene work, and several other events that originated with Holmes, were paradigm breaking. Of equal importance, this work did not require anything but the simplest of tools—a plant and a virus; a crucial aspect since it allowed Holmes' work to be quickly and easily tested (and accepted) by others, and to be successfully applied to plant breeding programs.


Travel, especially with an historical bent; hiking; photography; reading

Favorite Historical Figure

Tobacco mosaic virus

Favorite Agricultural Book

Ship Fever by Andrea Barrett

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