Key Words plant community dynamic, plant diversity, density dependence, Janzen-Connell hypothesis, local adaptation s Abstract Plant pathogens cause mortality and reduce fecundity of individual plants, drive host population dynamics, and affect the structure and composition of natural plant communities. Pathogens are responsible for both numerical changes in host populations and evolutionary changes through selection for resistant genotypes. Linking such ecological and evolutionary dynamics has

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... been the focus of a growing body of literature on the effects of plant diseases in natural ecosystems. A guiding principle is the importance of understanding the spatial and temporal scales at which plants and pathogens interact. This review summarizes the effects of diseases on populations of wild plants, focusing in particular on the mediation of plant competition and succession, the maintenance of plant species diversity, as well as the process of rapid evolutionary changes in host-pathogen symbioses. 16 GILBERT forest-gap-specialist pioneer species and fire-released species from prairie and chaparral habitats. However, a major lacuna in research remains the effects of pathogens on seeds in the soil. Seedling Diseases Damping-off diseases of seedlings are probably the most widely studied lethal diseases of plants in natural communities. Augspurger and coworkers (25, 27, 28, 142) first brought damping-off to the attention of plant ecologists with a series of studies of seedling mortality in the tropical forest of Barro Colorado Island (BCI) in Panama. Damping-off affected 80% of tested species (25) and was the primary cause of death for seedlings of six of nine tree species, killing up to 74% of a parent tree's annual reproductive output (28). Most studies of damping-off in natural ecosystems have been done in tropical forests, but Packer & Clay (170) found that black cherry seedlings (Prunus serotina, Rosaceae) in Indiana suffered up to 65% mortality from Pythium. Environmental heterogeneity can have strong effects on the development of damping-off caused by Oomycetes like Pythium and Phytophthora. For seedlings of several tropical trees, damping-off was more severe in soils with blocked drainage (92) or in areas of deep shade (25, 28, 222) than in drier or sunnier areas. Such effects of environmental heterogeneity are common for dampingoff caused by Oomycetes, which rely on water-filled soil pores for zoospore motility. 20 GILBERT Recent years have witnessed significant growth in the number of studies of plant diseases in natural ecosystems. Nevertheless, the number of empty or nearly empty cells in Table 1 indicates the need for studies on a diversity of pathogen types and their effects on various stages in the host life cycle.