Non-native Microstegium vimineum populations collapse with fungal leaf spot disease outbreak.
Non-native plants may meet little resistance in the novel range if they leave their biological enemies at home. As a result, species invasion can be rapid and appear unlimited. However, with time, organisms may acquire novel enemies in the novel range, or home-range enemies also may colonize the novel range. For plants, several authors have suggested that enemy release may give way to enemy acquisition in which pathogens accumulate and suppresses non-native plants. The 'naturalization' that occurs with acquired enemies may take decades to develop, yet most species invasion research lasts less than 4 years, and data tracking plant invasion before and after the appearance of pathogens are rare. Microstegium vimineum is an Asian grass that has invaded deciduous forest habitats in the southern Midwestern and Southeastern USA and is currently expanding in the Northeastern USA. We recorded widespread expansions in M. populations in North Carolina and Georgia (USA) between 2009 and 2011 but noticed that a fungal pathogen (indicated by leaf lesions; Bipolaris sp.) appeared on several of the populations in 2011. In 2019, we re-sampled these populations to determine whether the appearance of the fungal pathogen corresponded with a suppression of M. vimineum expansion. We found the once-expanding M. vimineum populations in retreat in 2019, and the plant population contractions were greater (and seed production lesser) where the fungal leaf spot disease was most extensive. These results suggest that enemy acquisition suppressed an active non-native plant invasion. We also found that where M. vimineum populations declined (or disappeared) native plants appeared to fill in the gap. Hence, whereby exotic species may gain advantage in novel habitat with the loss of their native-range pathogens, with longer time spans, enemy release may give way to enemy acquisition and native populations may recover if they are immune to the pathogens.