Fungi that are pathogenic to seedlings in the tropical forests of Panama are multi-host and may influence forest diversity because tree species differ in their sensitivity to pathogen attack
Host-specialized pests of plants, particularly phytopathogens, have been credited with regulating the relative abundances of plant species and maintaining local plant diversity under the Janzen-Connell hypothesis. However, the relative rarity of tree species in diverse tropical forests and the passive dispersal of phytopathogens should select for phytopathogens with intermediate to broad host ranges. Despite their importance and ubiquity, knowledge of the identities and host ranges of phytopathogens in natural systems is incomplete. We hypothesized that, in the diverse tropical forests of Panama, phytopathogens tend to have broad host ranges and that tree species differ in their sensitivity to phytopathogen attack because the cost-to-benefit ratio of defenses differs among species. To test these hypotheses, we surveyed the host associations of potential phytopathogens and used shadehouse-based inoculation experiments to evaluate (i) the pathogenicity of fungi isolated from symptomatic seedlings, (ii) the host ranges of the pathogenic isolates, and (iii) variability among tree species in sensitivity to infection. Specifically, we isolated 93 fungal isolates from symptomatic seedlings of 21 tree species that were collected in Panama. Then, we inoculated seedlings of 36 tree species with 34 of the fungal isolates and documented disease symptoms and mortality in shadehouse-based experiments.
We verified the pathogenicity of 11 fungal isolates, belonging to the genera Mycoleptodiscus, Bionectria, Calonectria, and Pestalotiopsis, and found that these phytopathogens attack tree species belonging to many families. Furthermore, we documented interspecific variation among tree species in susceptibility to disease, with some tree species apparently resistant to all fungal isolates tested and others susceptible to multiple isolates. The outcome of infection by a shared pathogen also differed among tree species, ranging from tissue damage to death. Our results add to the mounting evidence that plant-associated fungi in the tropics infect a wide range of species. While the broad host ranges documented here and by other studies challenge a commonly held assumption that phytopathogens in the tropics are host specific, the differences among tree species in their sensitivity to phytopathogens suggest that multi-host phytopathogens can contribute to the maintenance of forest diversity even though they may lack the high level of host specificity traditionally proposed by the Janzen-Connell hypothesis.