For many years ecologists have identified pathogens as theoretically important forces influencing basic processes in natural plant populations. These processes include, first, the evolution and/or maintenance by natural selection of sexual reproduction (which provides novel combinations of genes that may enable offspring to resist local pathogens genetically) and, second, the evolution of dispersal by offspring (which may help them physically escape local pathogens). In the first process the pathogen population responds to host plant genetics, while in the second it responds to host plant density. In addition, the pathogen can be expected to respond strongly to its physical and biological environment. This research will determine for one plant-pathogen system which of these three factors or their combinations the pathogen population responds to most strongly. This then sheds light on the actual role the pathogen population is having in the plant population's biology. The plant being studied is the rush, Juncus dichotomous and the pathogen is a parasitic smut fungus, Cintractia junci. These organisms occur together in the Long Island pine barrens. The results of this research directly apply only to nonagricultural plants and pathogens, but its methodology can be a model for applied problems. With increasing public concern about the health and environmental costs of using agricultural pesticides and given the fact that they are not fully effective, detailed population studies on the interaction of pest organisms with crop plants are needed to suggest safe and effective ways of reducing crop losses. Population studies involving pathogens and crops and incorporating genetics, density, and variables together could be very useful. **
