Emerging pathogens in Microstegium-invaded plant communities: Biogeographic distribution, species richness, and disease severity
The success of invasive plants is often attributed to the lack of co-evolved natural enemies, such as pathogens and herbivores, in the new range. However, recent work suggests that resident enemies may accumulate over time on non-native plants, potentially mitigating the effects of enemy release and leading to the decline of invasive plant populations. Alternatively, emerging pathogens may have no effect on invaded communities or may have negative impacts if the pathogens are capable of spilling back to native plants. We investigated patterns of pathogen emergence using the particularly noxious invasive annual grass Microstegium vimineum (stiltgrass) in the eastern United States as a model system. Assuming pathogens accumulate over time, we hypothesized that older populations would host more fungal pathogen species and sustain more lesion damage than newer populations. To test this hypothesis, we conducted a broad-scale biogeographic survey of M. vimineum populations across a chronosequence of sites to identify dematiaceous hyphomycete fungi infecting M. vimineum and co-occurring native grasses, quantify disease incidence and severity, and identify environmental variables that correlate with disease prevalence.
Our results show that disease incidence and severity were not explained by population age, but lesion damage was significantly correlated with factors affecting light availability and plant species richness. In addition, phylogenetic analysis indicated that our survey of 81 Microstegium vimineum populations across 18 states yielded eight distinct species of pathogenic fungi in the genus Bipolaris infecting M. vimineum, of which four also occurred on associated native grass species. One of these pathogens is a previously undescribed species and was surprisingly the most common and widespread; it was also found infecting several native grasses. Our results indicate that pathogens do not accumulate as a function of age in this system. However, we found an unexpectedly large number of pathogens infecting M. vimineum and co-occurring native species, suggesting that pathogens are emerging on this invasive plant through other mechanisms such as the presence of resident plant hosts and site characteristics. It remains unclear if these pathogens were co-introduced with M. vimineum from their native range or occur as a result of an evolutionary or ecological shift from native species. Further research is necessary to identify the origins of these emergent pathogens and to determine their long-term effects on M. vimineum and native plants.