What determines which plant species are susceptible to a given plant pathogen is poorly understood. Experimental inoculations with fungal pathogens of plant leaves in a tropical rain forest show that most fungal pathogens are polyphagous but that most plant species in a local community are resistant to any given pathogen. The likelihood that a pathogen can infect two plant species decreases continuously with phylogenetic distance between the plants, even to ancient evolutionary distances. This phylogenetic signal in host range allows us to predict the likely host range of plant pathogens in a local community, providing an important tool for plant ecology, design of agronomic systems, quarantine regulations in international trade, and risk analysis of biological control agents. In particular, the results suggest that the rate of spread and ecological impacts of a disease through a natural plant community will depend strongly on the phylogenetic structure of the community itself and that current regulatory approaches strongly underestimate the local risks of global movement of plant pathogens or their hosts.fungal pathogen ͉ plant disease ecology ͉ tropical forest ͉ plant quarantine ͉ host specificity M ost species of plant pathogen can attack a broad diversity of plant species (1), but the number of plant species with which a pathogen interacts in a local community is generally much lower (2). However, we have limited abilities to predict which species within a plant assemblage are most likely to be susceptible to a particular pathogen. Existing databases of pathogen-host range (e.g., ref. 1) have limited value for quantitative assessments because they are based primarily on haphazard records of pathogens on economically important plants. More importantly, only plant species that are susceptible to particular pathogens were recorded but not which plants are resistant. Nevertheless, several economically and scientifically important issues require predicting the likely host range of plant pathogens. The idea of selectivity among local plant species underlies the theory for the role of natural enemies in the maintenance of plant diversity (3-5) and biological invasions (6, 7). Host selectivity is used in studying plant disease epidemics (8, 9), estimating fungal biodiversity (10), managing agriculture and forestry systems (11,12), and in risk analysis for global movement of plants and pathogens (13,14).Conventional wisdom is that two closely related plant species should be more likely to be susceptible to the same plant pathogens than would plants that are evolutionarily distant, because the morphological and chemical traits of plants that regulate interactions with pathogens are often phylogenetically conserved (15). Indeed, recent work has shown a strong phylogenetic signal in host range of herbivorous insects in tropical rain forest (16,17), and there is qualitative support for such a signal for fungal pathogens (7). The presumption of phylogenetic signal in host range often underlies important agronomic and econ...