Nighttime application of ultraviolet radiation (UV), particularly wavelengths below ~280nm, has been shown to reduce populations of species causing powdery mildew at relatively low doses, while avoiding harm to the plants being exposed. The Bunsen-Roscoe principle, or dose reciprocity, describes a photochemical reaction where the outcome is determined solely by the dose level used, not how quickly or intensely that dose is supplied. Several microorganisms, however, have shown sensitivity to the intensity of UV used at a given dose. No study has investigated whether the method in which UV-C (100-280nm) is applied impacts the level of control achieved, or the consequences for the implementation of field application. Erysiphe necator, the causal agent of grape powdery mildew, was examined to determine if dose reciprocity held within the dose range (4 - 210 J/m 2 ) for germination, colony expansion rates, and latency. Initial experiments comparing all doses at UV exposures over 4 and 400 seconds found dose reciprocity held, with increasing dose leading to decreased odds of germination success (P<0.05). Experiments to confirm this finding using larger sample sizes and irradiance of 0.3 and 30 W/m 2 found conidia exposed to higher irradiance trended toward increased detrimental effect on E. necator at a dose of 120 J/m 2 (P=0.2); at 210 J/m 2 mean germination was significantly different (P<0.05) between the higher irradiance (4%) and the lower irradiance (9%). Colony expansion and latency for colonies exposed to 210 J/m 2 were affected to the same degree irrespective of irradiance of duration of exposure. This study is the first to examine dose reciprocity at irradiance levels closer to those shown to be effective in field applications. These results suggest that UV dose may be lowered, if irradiance is increased, to achieve powdery mildew disease suppression.