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Exotic species invasion may occur because of the lack of natural enemies, such as pathogens, in introduced ranges. 

Infection by the fungal pathogen Ustilago bullata, which causes head smut disease, has been shown to affect cheatgrass, although stand failure occurs infrequently and is difficult to predict. USGS researchers explored patterns of post-fire infection of Ustilago bullata on cheatgrass. They investigated how infection level affects cheatgrass density, and how weather and wildfire patterns affected infection. Fungal infection levels increased with greater prior year cheatgrass cover, suggesting that host density influences disease severity, whereas moderate levels of infection had minimal stand-level impacts on cheatgrass persistence. Unburned fire refugia may be important for pathogen persistence right after disturbance, while warm and humid winters facilitated infection in subsequent years. These results indicate the limited conditions under which natural enemies may realistically control invasive populations in heterogeneous landscapes. 

Applestein, C.V., Simler-Williamson, A., Germino, M.J., 2021, Weather and distance to fire refugia limit landscape-level occurrence of fungal disease in an exotic annual grass: Journal of Ecology, 

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