Robert E. Davis
Robert E. Davis received the B.S. degree in botany from the University of Rhode Island in 1961, and a Ph.D. in plant pathology from Cornell University, the latter under the guidance of Dr. A. F. Ross in 1967. After 18 months as a postdoctoral resident research associate with the USDA-Agricultural Research Service (ARS) in the pioneering laboratory for plant virology in Beltsville, MD, Dr. Davis became a research plant pathologist in the pioneering laboratory in 1968. In 1985, he became research leader of the molecular plant pathology laboratory of the Plant Sciences Institute, USDA-ARS, in Beltsville. He is also an adjunct professor in the Department of Plant Pathology at Cornell University.
Dr. Davis led the way toward discovery of an entirely new taxon of pathogens. He discovered in the corn stunt disease a unique helical, motile wall-less prokaryote that he named spiroplasma. This began a new field in which diverse spiroplasma pathogens have subsequently been found capable of affecting plants, insects, and mammals. While proposing a hydrodynamic theory of spiroplasma motion and the existence of a fibrillar contractile structure as the mechanism, he provided the first demonstration that a cell wall-free organism could be helical and motile. He predicted discovery of spiroplasmas in citrus and has been a central player in the characterization of spiroplasma genetic elements including plasmids and spiroplasma viruses. In the course of this latter work, he discovered the only known virus capable of infecting spiroplasmas across species.
Following his seminal discovery of spiroplasma, Dr. Davis and his research group proposed the first taxonomic classification of spiroplasmas, based initially upon serological reactions of spiroplasma membrane antigens and supported by reciprocal DNADNA hybridization analyses and electrophoresis of cellular proteins. Dr. Davis and his team initially proposed that each major spiroplasma serogroup represented at least one species of Spiroplasma. However, separate subgroups within the same serogroup cross-reacted in tests (e.g., growth inhibition) by which species of Mycoplasma could be readily distinguished. Dr. Davis and his team asserted that major criteria for species differentiation within the genus Mycoplasma could not be appropriately applied to the genus Spiroplasma without modification. Amid skepticism from established medical mycoplasmologists about taxonomic designations for spiroplasmas that cross-reacted in tests that differentiated Mycoplasma spp., the proposal by Dr. Davis and his team prevailed that major serogroups and distinct subgroups (within a major serogroup) be given the status of separate Spiroplasma spp.
Dr. Davis’s studies revealed the existence of diverse spiroplasmas and mycoplasmas on surfaces of flowers. This finding demonstrated a totally new ecological niche for cell wall-less prokaryotes and showed for the first time that spiroplasmas can exist outside of living host tissues in nature. He showed that several new mycoplasmas and spiroplasmas were epiphytes on flowers of a single plant species and that phylogenetically diverse plant species can harbor spiroplasmas. He hypothesized that flowers serve as important foci for dissemination of some spiroplasmas pathogenic in insects and possibly other fauna, including vertebrates.
Later in his career, Dr. Davis shifted the majority of his attention to the mycoplasma-like organisms (MLOs, now called phytoplasmas), a related group of cell wall-less prokaryotes that cause devastating plant diseases worldwide, impacting both nutritional and sociological needs. Because the phytoplasmas cannot be obtained in pure culture, they still present a challenge for understanding their properties. Dr. Davis, often in collaboration with his close colleague of many years, Dr. Ing-Ming Lee, took a primary role in the effort to understand the relationships among the phytoplasmas, developing a taxonomic model based on 16S rDNA analysis that is still used today. Dr. Davis’s laboratory introduced the concept of genetic cluster-specific molecular tags for phytoplasmas, and devised practical molecular methods for detecting and identifying spiroplasmas, as well as phytoplasmas. The molecular reagents and monoclonal antibodies developed by Dr. Davis and colleagues have been internationally used for pathogen detection and identification. Research by Dr. Davis’s laboratory made important contributions to understanding the phylogenetic relationships among phytoplasmas, to concepts used for phytoplasma classification based on genomic cluster and 16S rRNA, and to pioneering the taxonomy and nomenclature of phytoplasmas. In fundamental studies of the phytoplasma rRNA operon, Dr. Davis’s laboratory discovered that rRNA interoperon sequence heterogeneity was common among phytoplasmas and proposed that, where possible, descriptions of “Candidatus Phytoplasma species” include nucleotide sequences of 16S rRNA genes from both operons in the genome.
Important new insights into numerous plant diseases caused by poorly understood microbes were developed through national and international collaborative and team research by Dr. Davis and colleagues. These include insights that came from extensive data indicating that corn stunt was caused by a spiroplasma, that spiroplasma and a phytoplasma contributed to major corn disease outbreaks in the 1990s in Brazil, that Spiroplasma citri was the probable cause of devastating epidemics of horseradish brittleroot disease in Illinois, and that a coryneform bacterium, not a virus, was the probable cause of the ratoon stunt disease of sugarcane. In work on grapevine yellows diseases, which are virtually indistinguishable on the basis of symptoms in the United States, Europe, and Australia, Dr. Davis and collaborators found that phylogenetically diverse phytoplasmas were involved and that the various grapevine yellows pathogens could be distinguished on the basis of rDNA sequences These findings opened the way to accurate detection and identification of the pathogens involved in grapevine yellows disease outbreaks, and they changed approaches in research on epidemiology and efforts to control disease spread.
Based on international recognition for his scientific accomplishments and his leadership skills in organization and management, Dr. Davis has drawn together highly effective national and international research teams whose contributions continue to impact basic science and provide solutions to economic problems in agriculture in the United States and elsewhere. These have included projects with Italy, Hungary, and Australia on grapevine yellows disease, with Spain on spiroplasmal diseases, with Israel on phytoplasmal diseases of crops, with Florida on emerging diseases of strawberry, with Mexico on corn stunt disease, with Illinois on horseradish brittleroot disease, with Taiwan on sweet potato witches’-broom disease, with Brazil on biotechnology to solve economically damaging crop diseases of unknown cause, and with Lithuania on phytoplasmas and biodiversity in natural and agricultural ecosystems. His research has attracted numerous visiting scientists from the United States as well as Italy, Spain, Lithuania, Hungary, Brazil, Chile, Israel, Taiwan, India, Malaysia, and Japan to his laboratory. He has trained a number of graduate students, most of whom were degree candidates at the nearby University of Maryland. Most recently, Dr. Davis launched an ARS/University of Oklahoma/Lithuania/Brazil/Oklahoma State University project to sequence the genome of Spiroplasma kunkelii.
Dr. Davis has published over 150 refereed papers. He is a Fellow of the American Phytopathological Society and a Fellow of the Washington Academy of Sciences. In 1984, he received the Washington Academy of Sciences Award for Outstanding Research in the Biological Sciences. In 1997, Dr. Davis received the ARS Outstanding Scientist of the Year Award, followed by the USDA’s esteemed Silver Plow Award in 1998 when he was only one of three persons within the USDA to receive that award. His creativity and innovative research leadership have contributed significantly to knowledge of the causes of numerous important plant diseases. As a world authority and leader in plant diseases caused by phytoplasmas and spiroplasmas, he continues to take an active leadership role in the science of his profession, in research, teaching, and outreach. This he continues while effectively carrying out the responsibilities of chief scientist of the molecular plant pathology laboratory of the USDA-ARS at Beltsville, MD.
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