Fungicide resistant isolates of the cucumber powdery mildew pathogen (Podosphaera xanthii) are prevalent, and many commercially important cultivars are highly susceptible. Germicidal ultraviolet (UV-C) has shown exceptional efficacy against a broad range of powdery mildews in prior reports in greenhouse production systems. We conducted a study to optimize the UV-C application interval for control of cucumber powdery mildew in preparation for larger-scale commercial field trials. Differential effects of daytime versus nighttime UV-C doses (at 72 and 144 J/m²), and specific nighttime doses (7.2, 26, 70 J/m²) at four application intervals (from nightly to every 8^th night) were studied. Consistent with previous reports, nighttime application of UV-C allows lower doses to achieve disease suppression. At constant doses, shorter intervals were uniformly more effective than longer intervals in suppressing P. xanthii, but leaf area decreased at 70 J/m² dose when applied every night. Our results suggest that applications of 70 J/m² every 4^th to 8^th night will effectively suppress cucumber powdery mildew without leaf injury. These results provide guidance for end-users in defining dose and application intervals for future field efficacy trials.