Ecological consequences of pathogen accumulation on an invasive grass

Abstract

Release from natural enemies can contribute to the success of biological invasions. However, the advantage gained through enemy release may diminish over time as pathogens, parasites, or herbivores accumulate on invasive species. Pathogen accumulation may result from the development of novel host-pathogen interactions in the invasive range or the introduction of pathogens from the invader's home range or other areas. Previous studies suggest that pathogen accumulation may reduce the performance of invasive species but experimental demonstrations are lacking. Microstegium vimineum is a highly invasive annual grass with an expanding distribution in eastern North America. Bipolaris species and other fungal pathogens have recently been shown to cause foliar blight on invasive populations of Microstegium. Here we used experimental inoculations in controlled laboratory and greenhouse conditions and fungicide applications in the field to evaluate host relations of pathogens infecting invasive Microstegium and the ecological consequences of pathogen infection in nature. Four of seven native plant species tested, including three native grasses and one forb, became infected by at least one of the five pathogens isolated from Microstegium, suggesting that the native species might be sources or alternative hosts for Microstegium pathogens. Microstegium populations may also provide a reservoir of pathogens that could spillover to co-occurring native species and exacerbate the effects of invasions. All five pathogens caused foliar lesions on Microstegium, but there was over a four-fold difference in lesion area caused by different pathogens. Fungicide applications to infected field populations significantly reduced disease symptoms, increased invader biomass by up to 50% and increased seed head production by up to 200% compared to controls. Our results show that pathogen infection of invasive Microstegium caused significant declines in invasive host performance in natural populations, suggesting that the release of invasive species from natural enemies may be temporary.