Problems and progress in the biological control of wheat take-all^

It is truly a priviledge and an honour to be invited to present this inaugural Garrett Lecture, It was also a priviledge and an honour the first time I met Professor S, D, (Denis) Garrett, which was in October 1963, when I was a graduate student at the University of California, Berkeley, under the late W, C, Snyder, Professor Garrett was the invited keynote speaker (Garrett, 1965) at the First International Symposium on Ecology of Soil-bome Plant Pathogens (Baker & Snyder, 1965), Little did I know then that I would spend my career working on root and crown diseases of cereals, including take-all of wheat that had already, by then, been the subject of his own research for about 30 years. The concepts developed by Professor Garrett and expressed in his extensive writings created the area of science that we know today as ecology of soil-borne plant pathogens. Contemporary microbial ecologists planning to investigate and develop models to predict the fate of new genotypes of microorganisms released into the environment would do well to examine the works and consider the concepts set forth by Professor Garrett during the 60 years of his career. All of us working on ecology and management of soil-borne plant pathogens would do well to read his writings again. In preparation for this lecture, I took particular note ofthe paper that Professor Garrett presented at the Berkeley symposium, as this was his first to focus so completely on biological control, Reading this paper again was a humbling experience because I realized that most of the concepts established by the work I present below were stated already in Professor Garrett's chapter written over 30 years ago. My research as an employee of the US Department of Agriculture, Agricultural Research Service, has focused on methods to manage the root diseases of wheat and barley in modem farming systems, I use the term 'modern' to cover several factors, including the use of short, or no, crop rotation, as opposed to more traditional long rotations, little or no tillage (i,e, direct drilling) to conserve water, soil, and fuel, in contrast to conventional clean tillage, and centre pivot irrigation in the arid and semi-arid (once-desert) parts of the Pacific Northwest, An assignment to develop methods to manage root diseases of wheat and barley without using crop rotation, tillage, host-plant resistance (no useful genetic resistance to take-all has been found in wheat; Scott & Hollins, 1985), or soil fumigation may give the impression that biological control through the use of microorganisms was the last resort. On the contrary, the influence of Garrett's work, as well as the strong influences of K, F, Baker and W, C, Snyder, prepared me both philosophically and scientifically for development of a programme based first on ecological principles, of which biological control is a major application. In this lecture, I focus on the problems and progress in the development of biological control for wheat take-all, caused by Gaeumannomyces graminis var, tritici. Take-all can serve as both a model system for biological control research and a practical disease problem of major significance waiting to be solved by use of biological control. It has become a model system in large part because of the excellent foundation set in place on the ecology of this fungus by the work of Garrett, It has become a disease problem of major practical significance because, among the arable crops, wheat occupies the most agricultural land world-wide, and take-all is the most important root disease of wheat. Indeed, there may well be more agricultural land infested with G. g, var, tritici world-wide than with any other specialized root pathogen,