Technical Abstract: Growth and development of Erysiphe necator (syn. Uncinula necator) has been extensively studied under controlled conditions, primarily with a focus on development within the optimal temperature range and the lethal effects of high temperatures. Little is known of the effect of cold temperatures on pathogen development or host resistance. Pretreatment of V. vinifera leaf tissue that was ordinarily highly-susceptible to powdery mildew by exposure to cold temperatures (2 deg C to 4 deg C for 2 to 8 hr) reduced infection efficiency and colony expansion when tissues were subsequently inoculated. Furthermore, nascent colonies exposed to similar cold events exhibited hyphal mortality, reduced expansion, and increased latent periods. Historical weather data and an analysis of the radiational cooling properties of leaf tissue indicated that early-season cold events capable of inducing the foregoing responses occur commonly and frequently across many, if not most viticultural regions worldwide, and may partially explain: (i) unexpectedly slow development of powdery mildew during the first month after budbreak, and (ii) the sudden increase in epidemic development once seasonal temperatures increase above the threshold for acute cold events.