The Enemy Release Hypothesis proposes that invasion by exotic plant species is driven by their release from natural enemies (i.e. herbivores and pathogens) in their introduced ranges. However, in many cases, natural enemies, which may be introduced or managed to regulate invasive species, may fail to impact target host populations. Landscape heterogeneity, which can affect both the population dynamics of the pathogen and the susceptibility and the density of hosts, may contribute to why pathogens fail to control hosts, despite established negative disease impacts.

We explored patterns of post‐fire infection of the fungal head‐smut pathogen (Ustilago bullata) on the invasive annual cheatgrass (Bromus tectorum), which has caused the notorious grass‐fire cycle and ecosystem degradation across Western North America. We asked whether infection level was a driver of host density or vice‐versa, how weather affected infection, and how spatial patterns of infection varied with time since fire, using a combination of structural equation modeling (SEM), proportional odds modeling, and entropy‐based local indicator of spatial association (ELSA) on data from >700 plots spanning >100,000 ha remeasured annually for 4 years.

Observed infection levels increased with greater prior‐year cheatgrass cover, and disease severity did not suppress cheatgrass populations. Warm, humid fall/winters and proximity to fire refugia (unburned patches) were associated with more infections. Infection clustering was most evident 2‐3 years following fire with warm‐wet fall‐winter conditions and decreased after two drier, colder winters.

Synthesis: Disease severity in an invasive host did not result in a measurable reduction in its populations, which suggests that natural enemies may not strongly regulate this invasive species in its introduced range. Landscape heterogeneity associated with disturbance and weather limited population‐level infection of hosts by the fungal pathogen. Disturbance (specifically wildfire) and variable weather are key components of this and similar invasion systems, and likely need to be considered when evaluating disease dynamics and potential for natural enemies to influence invasion potential.