individuals in populations of the groups of microorganisms that operate either as pest microorganisms or as biological control agents. For this reason, it was previously impossible to know whether a population of a target pest was genetically homogeneous, and therefore might be expected to react similarly to a biological control agent, or was genetically diverse and requiring more complex strategies of management such as in the deployment of plant disease resistance genes. One of the more important tools for implementing ecologically based pest management is that of the effective deployment of plant disease resistance genes. Because of a gene-for-gene relationship between resistance genes in a host plant and avirulence genes in a plant pathogen, it is important to understand the population genetics of the plant pathogen as well as processes that could lead to changes in the genetic structure of the population, since changes could lead the pathogen to overcome plant disease resistance. Because of the ease with which plant pathogens can now be spread around the world by human activities, we need to be knowledgeable about pathogen diversity throughout the globe to be able to deploy plant disease resistance genes in a way in which resistance will be durable. A good example of a detailed examination of a plant pest population has recently come from the laboratory of Dr. Jan Leach of Kansas State University, who studied the bacterial plant pathogen Xanthomonas oryzae, one of the most important pathogens of cultivated crops (George et al.,1997; Raymundo et al., 1999). Her analysis of the populations of this pathogen in a variety of tropical regions where rice is grown showed that the pathogen had apparently been spread widely to rice grain regions but that local variation in a pathogen population also can occur. This information is clearly applicable to regional and local plant breeding efforts to develop disease resistant rice cultivars in that it elucidates the need to include a wide variety of pathogen genotypes in selection schemes to anticipate introduction of novel pathogenic strains.
Knowledge of the population structure of soilborne pathogens is also important in implementing biocontrol procedures. For example, the soilborne plant pathogenic fungus Armillaria mellea, which can attack the subterranean parts of a variety of woody plants, has recently become a prominent disease problem in pear orchards in California. The recurrence of this disease might be associated with changes in cultural practices such as irrigation or fertilization but the introduction of novel virulent strains of the pathogen could not be ruled out without a better understanding of the population structure of the pathogen in the orchards. However, David Rizzo of the University of California at Davis was able to ascertain, using molecular techniques, that a limited number of genotypes of pathogen occur within a given orchard, and that large contiguous areas of trees are apparently infected by clonal representatives of a given strain of the plant pathogen, apparently originating from infections of native tree species hundreds of years ago (Rizzo et al.,1998). Thus, it appears that changes in cultural practices, perhaps increased amounts or altered times of irrigation in recent years, have stimulated a preexisting pathogen within the orchards. Without an understanding of the pathogen structure within these orchards, such conclusions would have been impossible and a focus of disease management based on cultural practices could not have been made.