Powdery mildew (Oidium sp.) on poinsettia (Euphorbia pulcherrima) is an economically significant problem in the United States. The effect of temperature on sporulation was quantified using leaf disks cut from mature poinsettia leaves (cv. Freedom Red). Leaf disks (1.7-cm-diameter) were placed on agar disks (2.0-cm-diameter) amended with 30 mg/L benzimidazole and were appressed to the agar by washers (2.2-cm-diameter). Leaf disks were inoculated with conidia and incubated in petri dishes for 14 days at 15 or 20 C. The number of conidiophores and length of conidial chains on the leaf disks were recorded. The number of conidiophores produced did not vary with temperature. Sporulation was initiated 9 days after inoculation. Sporulation of Oidium sp. on leaf disks was favored by the warmer temperature;maximum conidial chain lengths were 4 and 7 conidia at 15 and 20C, respectively. The percentage of conidiophores bearing no conidia were 33.0 and 15.0%, at 15 and 20C, respectively. At 20C, more conidiophores (48.8%) produced 2 or more conidia than at 15C (23.0%).