Low temperature during infection limits Ustilago bullata (Ustilaginaceae, Ustilaginales) disease incidence on Bromus tectorum (Poaceae, Cyperales)

Ustilago bullata is frequently encountered on the exotic winter annual grass Bromus tectorum in western North America. To evaluate the biocontrol potential of this seedling-infecting pathogen, we examined the effect of temperature on the infection process. Teliospore germination rate increased linearly with temperature from 2.5 to 25°C, with significant among-population differences. It generally matched or exceeded host seed germination rate over the range 10-25°C, but lagged behind at lower temperatures. Inoculation trials demonstrated that the pathogen can achieve high disease incidence when temperatures during infection range 20-30°C. Disease incidence was drastically reduced at 2.5°C. Pathogen populations differed in their ability to infect at different temperatures, but none could infect in the cold. This may limit the use of this organism for biocontrol of B. tectorum to habitats with reliable autumn seedling emergence, because cold temperatures are likely to limit infection of later-emerging seedling cohorts.