Gray mold (Botrytis cinerea) is an important postharvest disease of blueberries (Vaccinium corymbosum) in Chile, favored by the long (>15 days) transportation to reach the international markets. The aims of this research were to study the critical blueberry growth stages for postharvest gray mold control and to determine the infection risks on the basis of weather conditions. The critical stages for gray mold control were studied on blueberry 'Brigitta' and 'Duke' in two planting localities. Differential fungicide applications (0.5 g/L fenhexamid), performed between the early pink bud stage and mature fruit stage, showed that the best control of postharvest gray mold was obtained when fungicides were applied between the first blue fruit and mature fruit stages. The infection risks for B. cinerea infection were defined as >6 h of wetness and temperatures between 14 and 25 degrees C. This algorithm to estimate the infection risks was studied in blueberry 'Brigitta', 'Duke' and 'Liberty' in four planting localities. A significant correlation between the infection risk and gray mold prevalence in stored fruit was obtained (r = 0.96, P < 0.0001), suggesting that this algorithm could be used to optimize fungicide applications, but field validation remains to be determined. In conclusion, the mature fruit stage appears as the most critical stage for postharvest gray mold control if weather conditions, defined by this algorithm, occur.