Production and development of the chasmothecia of Erysiphe necator on Vitis vinifera leaves were studied using potted plants in controlled and outdoor environments and grapevines in a vineyard. The optimum temperature for ascocarp production was 20°C; fewer chasmothecia were produced at 15°C and even fewer at 25°C; at 10 and 30°C, no or very few chasmothecia were observed, and none reached maturity. Nonlinear equations describing ascocarp development as a function of time and temperature were developed, parameterized with data from experiments at constant temperatures, and evaluated under fluctuating temperatures. Goodness-of-fit showed high agreement between observed and predicted data: the model efficacy ranged from 0·74 to 0·97 (1·0 indicates a perfect fit), and the root mean square error ranged from 0·001 to 0·01 (zero indicates a perfect fit). The high proportion of the observed variability accounted for by these equations (R2 = 0·83–0·98) supported the hypothesis that temperature has a predominant role in ascocarp development under natural conditions, when all environmental factors interact. The equations tended to overestimate the production of mature chasmothecia (the coefficient of residual mass was −0·23), but this inconsistency mainly occurred when rainfall apparently washed the mature chasmothecia from leaves during the logarithmic phase of the ascocarp developmental curve. Results from this work will be useful for predicting the development of chasmothecia in a vineyard and for timing the use of natural products, fungicides or biocontrol agents for reducing the population of chasmothecia, which are all more effective when they are applied to immature chasmothecia.