Plant pathogens may play an important role in maintaining forest community composition by reducing host abundance and by altering the outcomes of competition among host and non-host plant species.  Tests of the Janzen-Connell hypothesis have exclusively used seed and seedling mortality to measure the effects of plant pathogens, perhaps because these life stages are most accessible.  However, pathogens can affect the demography of a host plant population through a variety of ways, not only through reducing seed and seedling survival but also though reducing both sexual and vegetative reproduction.  To test the total effect of a specialist plant pathogen on host abundance across multiple host-life history stages, I have conducted experiments and field surveys on the interaction between the host-specific rust fungus, Puccinia podophyii, and its host, Podophyllum peltatum, an abundant, colonial, herbaceous plant. 

Results/Conclusions

Seedling survival experiments demonstrate that the pathogen significantly and dramatically reduces seedling survival from 35% in the control treatments to less than two percent in the treatments exposed to the wind-borne form of the pathogen.  The survival of seedlings grown in soil that contained the pathogen spores and were not exposed to infected adults is significantly greater than the treatments that were exposed. This implies that infected adult plants, the primary source of wind-borne inoculum, are largely responsible for seedling mortality and thus seedling mortality is independent of seedling density. A multi-year pathogen exclusion experiment shows that the pathogen significantly reduces the host plant’s sexual and vegetative reproduction.  Pathogen exposure reduced flowering effort, the proportion of flowering stems, by nearly 15% and seed set by 40%.  These reductions in sexual reproduction likely are the result of resource loss due to leaf damage caused by the pathogen and to the reduction in the host’s ability to acquire resources.   Indeed, ramets exposed to the pathogen were significantly smaller, both in leaf area and in stature.  The pathogen also significantly slows vegetative growth though indirect effects on rhizome branching rates and direct and indirect effects on average rhizome length.  Only in considering these total effects of specialist plant enemies can we understand their role in maintaining plant community composition.