Infection of wild rice (Zizania palustris) flag leaves by Bipolaris oryzae was studied at temperatures of 5 to 35°C and wet periods of 2 to 36 h after inoculation. Lesion densities (lesions/cm2) increased with increasing wet periods depending on optimum temperature. High rates of infection occurred at 25 and 30°C and generally increased with continuous wet periods of 16, 18, 24, and 28 h. There were no lesions at 5°C and few at 10 and 35°C. Lesion densities declined when wet periods of 2, 4, or 6 h were interrupted by dry periods of 4, 6, 8, 10, or 12 h followed by a final 14 h of wetness. Lesion densities decreased at all temperatures with increased dry periods regardless of the initial wet period. The interaction of dry period length × wet period length × temperature was significant at the 0.5% level. With continuous wet periods, lesion numbers were highest at 25 to 30°C.