Spatio-temporal analyses of plant community dynamics suggest pathogens play a central role in the maintenance of local forest diversity by conspecific negative density- and/or distance-dependent mortality (CNDD). However, such analyses cannot determine whether observed patterns are caused by pathogens with relatively narrow or broad host ranges. To assess the host ranges of fungal pathogens attacking seedlings, I: (i) isolated fungi from diseased seedlings of 26 tree species; (ii) identified and assigned the axenic isolates to operational taxonomic units (OTUs) based on 99% ITS sequence similarity; (iii) documented the host(s) from which a given OTU was isolated (host associations); and (iv) experimentally assessed the pathogenicity and specificity of isolates with greenhouse-based inoculations. However, the ITS region provides limited taxonomic resolution and there is no single sequence similarity threshold that appropriately captures lineage-specific variability across fungal genera. Thus, for a subset of genera, I am delineating species based on multi-gene phylogenetic analyses and re-evaluating initial ITS-based conclusions about host associations. The ITS-based description of fungal host associations and experimental assessments of specificity suggest that pathogens with host ranges spanning plant families and orders are common in Panama, with one pathogenic Mycoleptodiscus isolate causing significant disease when inoculated onto tree species in three orders. Preliminary multi-locus analyses leave some ITS-based OTUs intact and split or collapse other ITS-based OTUs, depending on the fungal genus. For example, a single Calonectria “species” based on the ITS region is split into four different species complexes when additional loci are used. Two of the four putative Calonectria species were isolated from heterofamilial tree species, while two putative species appear host-specialized. While these preliminary multi-gene analyses lend some support to past conclusions of host generalism being common among fungal pathogens of seedlings, they also demonstrate that specialized pathogens may go unobserved when fungal species discrimination is based on the ITS region alone. ITS-based species delineations are likely to over- and underestimate the host ranges of plant-associated fungi, impacting conclusions about plant-pathogen interactions and the mechanisms by which pathogens contribute to the maintenance of forest diversity. Undoubtedly, generalist and specialist pathogens both play an important role in diversity maintenance. Generalist pathogens can unevenly affect seedling recruitment across host species through host-specific coinfections, host affinities, and host- and age-specific impacts. Experimental tests of these potential mechanisms are pivotal future directions for understanding the processes maintaining tropical forest diversity and such tests depend on an investment in culture-based research.
seedling pathogens, host specificity, forest diversity, multi-host pathogens, multi-locus, ITS