Postbloom Fruit Drop (PFD), caused by <i>Colletotrichum acutatum </i>and <i>C. gloeosporioides,</i> is a serious problem in most humid citrus production areas of the Americas and can be very severe when the rainfall occurs during the bloom. The objective of the present study was to develop an agro-climatic approach to determine the risk of citrus PFD epidemics. For such purpose, twenty years of daily weather data, from ten representative citrus areas in São Paulo state were analyzed. Based on previous studies, the flowering period was estimated based on degree-days and the accumulated rainfall data from the flowering peak until the final 50% of flowers was determinate for each year for all locations. Non-linear Gompertz regression was used to fit the observed PDF incidence (%) to the accumulated rainfall, generating an estimating model. Finally, the percentages of years with PDF incidence in different intervals were obtained in order to define a PDF climatic risk index. The accumulated rainfall of 70.8 mm between the flowering peak and the last flowering 50% provided a PDF incidence of 50%. The locations in the south and southwest regions of the state were those with the greatest risk for PDF. According to our findings, traditional citrus areas presented a medium PDF risk, showing the importance of monitoring this disease for management strategies. The present approach can be very useful to predict disease incidence and to develop a new warning system for this disease.