The effect of temperature and of two levels of relative humidity (RH) on maturity of pseudothecia of Pleospora allii (teleomorph of Stemphylium vesicarium) was studied under controlled environmental conditions at temperatures of 5, 10, 15, 20, 25, and 30°C and high (≥98%) and low (≈60%) RH. Pseudothecia developed only at high RH, and the optimum temperature was between 10 and 15°C. A regression model of the form ln(1/1 − y) = 0.12550 + 0.005048x, which related the proportion of mature pseudothecia (y) to cumulative degree-days (x), was developed with data from two controlled environment experiments. Maturation of pseudothecia also was studied in leaf debris in pear orchards affected by brown spot of pear in seven field trials during 4 years. Asci with mature ascospores were observed in leaf debris from mid-January to May. A significant linear relationship was observed between the predicted values according to the monomolecular model and observed values of the percentage of mature pseudothecia in the field trials (r 2 = 0.91, intercept=8.718, and slope=0.903). This model can be used to predict the onset of ascospore discharge and for determining the initiation of fungicide applications to control the primary inoculum and prevent primary infections.