Indirect effects of non-native Spartina alterniflora and its fungal pathogen (Fusarium palustre) on native saltmarsh plants in China

1. Pathogens can affect their hosts and change community composition and structure. Pathogens may be key determinants of biological invasions. However, few empirical studies exist examining how non-native plants drive their invasions through indirect effects involved with pathogens. Here, indirect effects refer to how one species alters the effect that another species has on a third. 2. Fusarium palustre was associated with the dieback of Spartina alterniflora in its native North American saltmarshes. Native plant Phragmites australis was also found to die back in the Dongtan wetland of the Chinese Yangtze River estuary invaded by non-native Spartina alterniflora. This phenomenon suggests that Spartina might not escape from its pathogen when being introduced from its native North America and has indirectly caused the dieback of Phragmites in China. 3. To investigate the indirect effect of Spartina involving Fusarium, we sampled plants and soils in dieback patches to isolate the pathogen. Next, we used an artificial inoculation study to determine the virulence of Fusarium to both Phragmites and Spartina. Finally, the spatial distribution of Fusarium was studied through examining its incidence in saltmarshes along the east coast of China. 4. The endophytic fungus F. palustre was found to be closely associated with Phragmites dieback in the Dongtan wetland and it is likely that it was transported by non-native Spartina from its native North American saltmarshes to the Chinese saltmarshes. The spillover of F. palustre from non-native Spartina to native Phragmites might subsequently facilitate Spartina invasion. 5. Synthesis. Invasive plants do not only directly compete with native plants, but also indirectly cause pathogen infection on the latter, by acting as vectors and reservoirs for pathogens shared with native plants. Our findings highlight the significance of indirect effects involving pathogens in biological invasions. It is necessary to consider these pathogen-mediated indirect effects of non-native plant species in multi-host-pathogen systems for management and restoration purposes.