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Conditions were established for inducing pear blossom blast caused by Pseudomonas syringae (Ps) on both attached and detached shoots. The incidence of disease was proportional to the log of Ps population under optimal temperature, moisture and bloom developmental stage. Highest incidence of infection followed occurrence of a major exotherm in the presence of Ps suspended in water drops on blossoms. An exotherm was detected inside ovary tissue at temperatures ranging from-1.8 to-3.5 C. Wetness duration following exotherm occurrence was less important than wetness during the freeze event. Blossoms inoculated, then either air-dried or removed from low temperature treatment prior to a freeze, had a low incidence of infection. The full bloom stage of blossom development was more susceptible to blossom blast than either the open cluster or tight cluster stages of development. Eight pear cultivars were evaluated for susceptibility to blossom blast with the red cultivars less susceptible than th