This study used existing databases to test the hypothesis that fern species harbor fewer pathogen species than angiosperm species. Analysis was limited to fungal pathogens because of their visibility and to herbaceous perennial dicots (forbs) because they have a similar growth form to ferns. From complete listings in the United States Department of Agriculture plant-fungal database, the number of pathogen species recorded on 200 randomly chosen ferns and herbaceous perennial dicot species were assessed. To control for differences in study effort, the number of citations to these species in the Web of Science was determined. The results showed that the major predictor of the number of fungal pathogen species known to occur on a plant species was study effort, but after controlling for this, the likelihood of a fern species being recorded as having a fungal pathogen species was much less than that for a forb. When pathogens were present, there were approximately 50% fewer pathogen species recorded on fern species than on forb species. This pattern is present even though fern species were cited on average more often than forb species, and it is consistent with impressions in the literature from studies in other parts of the world. Testable hypotheses to explain this difference are evaluated in the context of evolutionary processes leading to variation for pathogen incidence in different phylogenetic lineages, but the physiological or molecular processes that determine the higher resistance of ferns to fungal pathogens remain unknown.
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Vol. 110 • No. 3