The seedling-infecting pathogen Ustilago bullata Berk. is a naturally occurring biological control agent for cheatgrass (Bromus tectorum L.). The effects of temperature and nutrients on pathogen teliospore germination behavior and the effects of temperature on host seed germination were examined. The effects of temperature on sporidial proliferation, host infection in a temperature-controlled environment and in a field setting for eight populations were investigated. The infection success of Ustilago bullata on Bromus tectorum in cultivated fields as a function of seeding date, inoculation method, inoculum density, supplemental watering, and litter was also investigated. Teliospores germinated faster on potato dextrose agar than on water agar. Teliospores germinated slowly at temperatures far from the optimum of 15 and 20 C. There were among population variations in teliospore germination and sporidial proliferation, but differences among populations were much more pronounced at temperatures below 15 C. Infection also decreased and varied far from the optimum with almost no infection at 2.5 C in a controlled-environment and in the field for the December-planted seeds. Warmer early fall rather than the colder late fall was suitable for successful infection. This agreed with both laboratory and controlled-environment experiments. Intratetrad mating was observed with teliospores at 2.5 C. Teliospore germination tracked seed germination closely with teliospore germination rate exceeding the host seed germination rate over the range of 10 to 25 C where both were measured. Below 10 C, teliospore germination rate fell below host seed germination. This phenomenon was associated with lower infection percentages, suggesting that teliospore germination needed to be ahead of the seed for maximum infection. Inoculum density was positively correlated with infection rate. Litter significantly increased infection, while supplemental watering significantly increased plant establishment. Since teliospores from different populations showed similar germination patterns at temperatures typical of autumn seedbeds in the Intermountain West, it may not be necessary to use locally-adapted pathogen populations in biological control program. A biocontrol program is most likely to be effective under a scenario where autumn precipitation permits emergence of most of the host seed bank as a fall cohort.