David M. Weller
David Weller was born 19 February 1951 in Covington, KY, and received his primary and secondary education in Cincinnati, OH. In 1973, he graduated from Miami University in Oxford, OH, with a B.A. degree in botany. He went on to graduate school at Michigan State University in the Department of Botany and Plant Pathology, where he received his M.S. and Ph.D. degrees in plant pathology in 1975 and 1978. During his graduate career, he conducted research on the epidemiology and control of bean common blight caused by Xanthomonas campestris pv. phaseoli. In 1979, Dr. Weller began work with the USDA, ARS Regional Cereal Disease Research Laboratory at Washington State University as a postdoctoral microbiologist with Dr. R. J. Cook. In 1981, he was appointed research plant pathologist with responsibilities for biological control of soilborne pathogens of small grain crops. Dr. Weller has conducted international cooperative research projects with scientists in Australia, the Netherlands, Switzerland, and England. In 1998, he was appointed research leader of the ARS Root Disease and Biological Control Research Unit.
Dr. Weller is recognized as an international authority on biological control of soilborne plant pathogens, suppressive soils, and molecular mechanisms of pathogen suppression by rhizobacteria. His scientific contributions have been a major force in moving research on biological control of soilborne pathogens in new directions. His work focuses on take-all, Rhizoctonia root rot, and Pythium root rot, three diseases that are major constraints to the production of wheat in the Pacific Northwest and worldwide. Rhizobacteria were isolated so that when applied individually, or in combination, they protected wheat and barley against root diseases in the field. He has developed concepts relating biological control to the process of root colonization and the mechanisms by which introduced bacteria suppress root diseases. Working with strain 2-79 and other isolates, he described the movement of fluorescent Pseudomonas spp. from the seed to the roots of wheat and documented the pattern of multiplication on roots in the field. With his graduate student, he demonstrated, for the first time, a direct correlation between take-all suppression and the population size of an introduced antagonist on the roots. With other colleagues, he showed that biotic and abiotic factors including soil matric potential, rhizosphere pH, wheat cultivar, and competition from indigenous microflora profoundly influence the rhizosphere population size and the biocontrol activity of rhizobacteria introduced on wheat seed. These studies, and related investigations with Trichoderma koningii, provide novel insight into the effects of soil physical and chemical factors on the activity of biocontrol agents. This helped define a strategy for targeting biocontrol agents to sites where the soil environment is supportive of their activity.
Seminal research conducted by Dr. Weller, Dr. L. S. Thomashow, Dr. R. J. Cook, and others, showed that the production of phenazine antibiotics by strains 2-79 and 30-84 is a major determinant in their biocontrol activity and their ability to grow and survive in the rhizosphere. This and related work with strains that produce a different antifungal metabolite, 2,4-diacetylphloroglucinol (DAPG), established unequivocally that rhizobacteria not only produce antibiotics in the rhizosphere environment, but also that these metabolites have a significant role in the ecology of rhizobacteria in natural habitats. The genetic and biochemical protocols outlined in these landmark studies are now considered essential to any definitive investigation of mechanisms of bacterial suppression of soilborne pathogens.
Equally important are Dr. Weller’s contributions toward understanding the molecular basis of pathogen suppressive soils. A majority of the thousands of rhizosphere strains he has isolated throughout his career originated from take-all decline soils—soils that, after sustained wheat monoculture, have developed a natural and microbiologically based suppressiveness of take-all disease. Dr. Weller demonstrated that take-all decline, long-recognized and poorly understood, results in Pacific Northwest soils from the buildup of fluorescent Pseudomonas spp. that produce DAPG. Using genetic probes and primers specific for the DAPG biosynthetic locus, he and his postdoctoral associates showed that DAPG producers are present in take-all decline soils but not in conducive soils at population densities above a threshold level required for suppressiveness. Suppressiveness was lost when population densities dropped below the threshold level, and could be transferred with small quantities of take-all decline soil containing DAPG producers. These results have reinvigorated research worldwide on the molecular basis of soil suppressiveness. More significantly, they led to the identification of a specific “premier” class of DAPG producers with a unique genetic fingerprint and exceptional biological control and root colonizing abilities. These rhizobacteria represent a major advance in biocontrol technology because they are effective in the rhizosphere when applied to seed or soil at doses much lower than those typically needed to achieve control of take-all, and apparently regardless of soil composition. These strains have been licensed for commercial development.
Dr. Weller has been active in the American Phytopathological Society as a member, vice chair, and chair of the Biological Control Committee, member of the Root Disease and Soil Microbiology Committee, and organizer and moderator of discussion sessions and symposia at numerous annual meetings. He served as an associate editor of Plant Disease, an associate and senior editor of Phytopathology, and is now on the editorial advisory board of Biocontrol Science and Technology. He has been ARS representative to regional project W-147 since 1991, and has been a member of several CSRS review teams and USDA competitive research grants panels. Since 1995, he has been a member of the ARS Pacific West Area Patent Committee. He was a joint recipient of the APS Ruth Allen Award in 1997, has received many meritorious awards from ARS, and was elected a Fellow of the American Association for the Advancement of Science in 1999. He is a member of the graduate faculty at Washington State University, where he is active in the affairs of the Department of Plant Pathology and serves on the University’s Marketing and Communications Leadership Council.