Christopher A. Clark was born in Geneva, NY, in 1949 and earned B.S. (1970), M.S. (1973), and Ph.D. (1976) degrees from Cornell University. After a post-doctoral at North Carolina State University, he joined the Department of Plant Pathology & Crop Physiology at Louisiana State University (LSU) in 1977 and remains there as a professor. His career achievements have come from his ground-breaking research on the etiology, epidemiology, and host plant resistance of sweetpotato diseases. His research has had a positive impact on the sweetpotato industry locally, nationally, and internationally while also making fundamental contributions to the discipline of plant pathology. He also has served APS with distinction.
One discovery by Clark that illustrates his exceptional research capabilities was elucidation of the highly unusual biology of the chlorotic leaf distortion disorder of sweetpotato caused by epiphytic development of Fusarium denticulatum. He demonstrated that what had been assumed to be salt deposits was actually an epiphytic phase of mycelial growth occurring on and in a mucilaginous matrix on the surface of the apical meristem and young leaves. This induces intense leaf chlorosis without invasion. Along with the epiphytic stage on the plant, the fungus becomes internally seedborne. Phylogenetic analyses of pathogen strains from a global collection of seed revealed an African origin. The biology became even more intriguing when Clark discovered that F. denticulatum offers cross-protection against Fusarium wilt, another significant sweetpotato disease. Clark addressed this complicated disease and demonstrated that F. oxysporum from at least three genetically distinct lineages can cause wilt on sweetpotato and that the host ranges of F. oxysporum f. sp. batatas and F. oxysporum f. sp. nicotianae overlap and include plants from two different families.
Streptomyces soil rot was the most destructive disease of sweetpotato in the United States and Japan. Prior to the development of resistant cultivars, it caused abandonment of fields in Louisiana. Clark demonstrated that Streptomyces ipomoeae is able to penetrate sweetpotato roots directly, a rare exception among prokaryotic plant pathogens. Strains of S. ipomoeae were separated into three groups based on interstrain inhibition patterns subsequently shown to be caused by bacteriocin-like compounds. However, rep-PCR results suggested a common origin for all the strains.
Clark’s seminal contributions also have come from developing multiple disease-resistant sweetpotato cultivars. This painstaking work has led to the identification and combination of sources of resistance to several important diseases, including Streptomyces soil rot, Rhizopus soft rot, bacterial root and stem rot, Fusarium root rot, Java black rot, and Sweet potato leaf curl virus. In addition, the nature of resistance to root-knot nematode was characterized. This enhancement of resistance screening ability contributed greatly to the development of several important cultivars, including Beauregard, which dominated U.S. sweetpotato production from 1987 to 2010 and is grown in several other countries. Because of Clark’s research, the LSU Agricultural Center program has been very successful, developing 10 cultivars from 1980 to 2008 with excellent disease resistance.
Perhaps the most significant research contribution by Clark has been the unraveling of a highly complex phenomenon known as cultivar decline. This highly challenging project has required the establishment and maintenance of single virus infection sources through grafting and indicator hosts followed up by difficult field experiments. He demonstrated that, although individual potyviruses commonly identified in sweetpotato have minimal effect on yield and quality, combinations significantly reduce yield. This suggested the tantalizing possibility of the involvement of previously unknown agents in cultivar decline. His group isolated and characterized Sweet potato leaf curl virus and Sweet potato virus 2, two viruses in this complex that were not associated with sweetpotato in the United States previously, and characterized Sweet potato virus G. They developed qPCR protocols for quantification of these viruses and have demonstrated that titer in plants in the field may limit virus spread even more than vector populations.
Based primarily on Clark’s research, the Louisiana Sweet Potato Foundation Seed Program has converted to exclusive production of ”virus-tested” seed (storage roots) for sweetpotato growers. To do this, Clark maintains a tissue culture facility that uses meristem-tip culture and virus indexing to provide virus-tested tissue cultures. Clark’s program provides cultures to the LSU Sweet Potato Research Station and a private company for micropropagation. Virus-tested seed is then made available through foundation seed programs to growers in Louisiana, Mississippi, and Alabama. His program has evaluated the role of “mericlone” variability in sweetpotato performance and eliminated viruses from heirloom cultivars and breeding lines. This extension of innovative research findings to an effective management program has had a very beneficial effect on sweetpotato production.
Clark has published numerous research findings on bacterial, fungal, nematode, and viral diseases. This includes the most authoritative and foremost book on sweetpotato diseases in the world, Compendium of Sweetpotato Diseases, Pests, and Disorders, Second Edition, 12 invited book chapters or review articles, 81 refereed journal articles, and more than 100 other extension/technical/popular articles. He has been a major advisor or coadvisor for 17 graduate students and a committee member for more than 50 students. He has developed or taught all or portions of six graduate-level courses. His grant support from federal, state, commodity groups, and the agricultural industry has been outstanding and totals more than $1,200,000.
Clark received two LSU AgCenter awards, the Tipton Team Research Award with the Sweet Potato Team, and the Doyle Chambers Research Award in 2008 (this is highest honor given by this institution based on career contributions to Louisiana agriculture), and the Outstanding Plant Pathologist Award from APS Southern Division in 2010.
Clark has served APS as an associate editor of Phytopathology and associate and senior editor of Plant Disease. He has served on the Soil Microbiology and Root Disease Committee, Nematology Committee, and the Research Committee of the Office of International Programs. He has been chair of the Archives Committee and is past president of the Southern Division of APS.
Through thoughtful, innovative, careful, and determined research, Chris Clark has provided major advances in the understanding of and ability to manage sweetpotato diseases. He has excelled at every aspect of plant pathology during a remarkable career.
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