SUMMARYLong wave ultraviolet light, peaking at 366 nm, inhibits pycnidium development. The extent of this inhibition is related to: (1) the duration of the ultraviolet light exposure, (2) the intensity of ultraviolet light, (3) the carbohydrate in the substrate, and (4) the concentration of the carbohydrate. At low concentration of the carbohydrate (0.6 g C/liter) inhibition occurred on all the substrates within 15 min. Longer light exposures, up to 40 min, were required to inhibit pycnidia at the high carbohydrate level (6.0 g C/liter). When the ultraviolet light intensity was increased from 100μW/cm2to 320μW/cm2, shorter exposures inhibited pycnidium formation. Identical dosages of ultraviolet light inhibited pycnidium formation on some substrates but not on others. When cultures were irradiated, inhibition occurred readily on maltose media at both concentrations. Inhibition on other substrates and substrate concentrations generally required exposures of a longer duration.