Background/Question/MethodsWhen plant species differ in their susceptibility to particular pathogens, diseases can be creative forces that shape the diversity and spatial structure of natural plant communities. Experimental inoculations in a diverse tropical forest show that while most plant pathogens are polyphagous, each can attack only a subset of locally available hosts, and susceptible hosts tend to be closely related. The spread and impacts of a plant pathogen in a natural community will depend on the local density of all susceptible species, which is a function of the phylogenetic structure of the local community. We used empirically derived estimates of how evolutionary distance between host species affects the likelihood that plant pathogenic fungi can move from one host species to another, together with estimates of host density in forests plots, to examine how the phylogenetic structure of real forest communities would affect the likely spread of polyphagous plant pathogens.
Results/Conclusions

In diverse tropical forests, the epidemiologically significant population density of rare plant species increases many fold when likely alternative hosts (close relatives) are considered. This effect may dampen the rare-species advantage that underlies the mechanism through which plant pathogens can help maintain diversity of natural plant communities. Simultaneous, it can facilitate the spread of pathogens through natural populations, even when the density of individual species is very low.