Cirsium arvense is a perennial weedy plant species found in most parts of the temperate world, including mountain ecosystems. In both the native and non-native ranges C. arvense impacts forage production and reduces crop yield. Puccinia punctiformis is a fungal pathogen specific to C. arvense that has been shown to impact its host and is naturalized in the non-native range of C. arvense. However, in the non-native range, P. punctiformis is less common than C. arvense and there is interest in increasing its presence as a biocontrol agent. There is a need to better understand the abiotic factors shaping its distribution in its non-native range. Using data from 197 sites, some intentionally inoculated, across six states in the western USA, this study aimed to 1) identify climate variables important for P. punctiformis presence on C. arvense and model its distribution/climate envelope in the western USA. (2) Assess P. punctiformis along an elevation gradient, and (3) quantify the effect of inoculation with P. punctiformis on C. arvense individuals and patches through time. Mean annual temperature of the driest quarter, and mean annual temperatures were important variables predicting the occurrence of P. punctiformis. Habitat suitability was greatest in mid montane elevations in mountainous regions. Consistent with these findings, we also found that P. punctiformis probability of occurrence increased up to upper range limit of C. arvense. Infection increased with time since inoculation, and as the number of infected stems increased the average C. arvense stem size decreased. We also observed that as P. punctiformis infected stems increased in a patch through time there was an associated decrease in mean C. arvense cover and stem density within the patches, and in those patches where infection was present the negative relationship between native cover and C. arvense stem density was greater. These findings highlight the importance of a niche based perspective when considering use of biocontrol. Niche overlap between the host and pathogen can vary along an elevation gradient, affect population dynamics, and inform intervention opportunities.