Pycnidia were produced by six of seven isolates ofB. ribis at one or more intensity levels of continuous illumination at 21 °C. Under conditions of alternating light (12 h−27 °C) and darkness (12 h−21 °C) pycnidia formed in cultures of six isolates at three or more intensity levels, while one isolate failed to form pycnidia at any intensity level. Pycnidia did not develop when cultures were incubated in complete darkness. Exposure periods as brief as 2 days under continuous illumination at 21 °C induced pycnidial formation. In alternating light (12 h−27 °C) and darkness (12 h−21 °C), the shortest period of exposure which induced pycnidial formation was 4 days. Continuous illumination at 21 °C favored development of uniloculate pycnidia, while alternating light (12 h−27 °C) and darkness (12 h−21 °C) favored formation of multiloculate pycnidia.