The mainDidymellaascospore season in Derby is June-September. 10 day running means for the years 1986–1988 have been compared with 10 day running means of rainfall and maximum temperatures. The high temperatures reached in 1987 together with substantial rainfall were very favourable forDidymellaascospore release. Diurnal periodicity showed that the expected night time pattern was only occasionally disturbed by day time rainfall.Didymellaascospore liberation is greatly increased when both temperature and rainfall are high. Temperatures of about 20° in the period before rainfall appear to greatly increase the numbers ofDidymellaascospores liberated.