Charles R. Howell
Charles R. Howell was born in Fort Worth, Texas, on December 26, 1935. He received his B.S. degree in biological sciences from California State Polytechnic College, Pomona, in 1962. He received his Ph.D. degree in plant pathology from Washington State University in 1967, having conducted his dissertation research on the biochemistry of resistance to onion smut with Ruben Duran and Shirl Graham. Howell was then employed by the USDA to work on Verticillium wilt of cotton at the Texas A&M Experiment Station in Lubbock, Texas. After 4 years, he moved to the Cotton Pathology Research Unit in College Station, Texas, where he remained until his retirement in January 2006. Howell continued is work on Verticillium wilt until 1977, during which time he discovered that mutants of V. dahliae deficient for pectinase activity were still pathogenic to the cotton host, and that production of catechin and tannins in cotton leaves was related to their resistance to the pathogen. He then initiated a research program on biological control of cotton seedling diseases, with an emphasis on the mechanisms employed by biocontrol agents to suppress soilborne pathogens.
Howell has made numerous landmark discoveries in the field of biological control of plant disease, with an initial focus on rhizosphere bacteria for suppression of seedling diseases of cotton. In the late 1970s, he isolated Pseudomonas fluorescens Pf-5, an effective biological control agent of damping-off diseases caused by Pythium ultimum and Rhizoctonia solani. With his colleague Robert Stipanovic, he published two classic papers in Phytopathology demonstrating that Pf-5 produces the antibiotics pyrrolnitrin and pyoluteorin, which suppress disease when applied to seed surfaces. With agrocin 84, whose structure was published by another group during the same year, pyrrolnitrin and pyoluteorin were the first structurally characterized antibiotics with a demonstrated role in biological control of plant disease. Before that time, antibiotics were considered to be too unstable to play an important role in soil ecology, so these manuscripts ushered in a new era focusing on antibiosis as a central mechanism in biological control. Strain Pf-5 has since been used as a benchmark for other strains by biological control researchers worldwide, and it has served as a model organism for molecular and ecological studies of biological control. It was the first biological control agent for plant disease whose complete genome was sequenced. Howell’s subsequent work with pseudomonads, done in collaboration with CIBA-GEIGY Inc. (now Syngenta), led to the patenting of several strains for seedling disease control, and to the production of a pyrrolnitrin analog (Maxim) as a seed and foliar fungicide. Maxim provides the first example in which a natural product with a demonstrated role in biological control has been developed as a highly successful fungicide for management of plant disease. In addition to his work with Pseudomonas spp., Howell also discovered the novel mechanism of ammonia production for suppression of Pythium damping-off of cotton by the biological control bacterium Enterobacter cloacae.
Howell also discovered several secondary metabolites produced by fungal biological control agents and determined their roles in the suppression of seedling diseases. This research began with the isolation and characterization of a mycoparasite of R. solani that proved to be Trichoderma (Gliocladium) virens. From cultures of T. virens, he and a colleague isolated the novel antifungal compound gliovirin, determined its structure, and demonstrated that it is toxic to phycomycetes. He demonstrated that strains of T. virens could be separated into two distinct groups on the basis of their antibiotic production: “Q” strains produce the antibiotic gliotoxin, whereas “P’ strains produce gliovirin. Howell and a colleague also isolated and characterized the phytotoxin viridiol that is produced by T. virens and demonstrated that viridiol could be used effectively to control weeds. He also found that viridiol production was influenced by the substrate on which the fungus was grown and that its production could be suppressed by the addition of sterol inhibitors to the growth medium. Howell was one of the first scientists to derive mutants of fungal biocontrol agents and employ them in experiments to determine the role of compounds such as gliovirin in biological control. In later studies, Howell demonstrated that the principal mechanism in the control of R. solani-incited seedling disease of cotton was the induction of terpenoid phytoalexin synthesis in the developing root system by T. virens. With a colleague, he isolated and characterized the protein produced by T. virens that induces phytoalexin production. In studies on the causes and biological control of preemergence seedling disease of cotton, Howell discovered that seed of susceptible cotton cultivars released a compound to the spermosphere during germination that stimulated pathogen propagules to germinate and infect. He also found that Trichoderma spp. controlled this disease by metabolizing the stimulatory compound before it reached the pathogens. Howell also discovered that one of the primary inciters of preemergence damping-off of cotton seedlings is Rhizopus oryzae, a fungus not previously known as a soilborne pathogen of cotton, and one that is not sensitive to the fungicides normally used to control this form of the disease. Howell’s reputation as an expert in the field of biological control is evidenced by the fact that he has conducted much collaborative research work with scientists in industry and academia and by the review articles, book chapters, and popular articles that he has been asked to write on the subject. He is also the author or coauthor of four patents relating to biological control.
Howell is a member of the graduate faculty of the Department of Plant Pathology and Microbiology at Texas A&M University, where he serves on the thesis committees of M.S. and Ph.D. students. He has been an active member of APS since 1963 and of the Cotton Disease Council since 1966, where he has served as secretary, vice chair, and chair of the CDC. Howell is currently an associate editor of Phytopathology.