Weather records were compared with severity of Septoria [tritici] leaf blotch of wheat [Triticum aestivum] over a 20 yr period at Lafayette, Indiana, [USA]. In 3 yr of very severe epidemics, in which all leaves of susceptible cultivars were killed prematurely, there were at least 40 days of rain, 1 April-17 June, 8 days above average. A severe epidemic occurred when there were 34 days of rain if there was no excess of 2 day periods with minimum temperatures of 7.degree. C or lower (34 such periods compared to an average of 26). It may be possible to forecast a very severe epidemic at the time flag leaves emerge by examining weather data from the previous 40 days (1 April-10 May) and predicting weather for the next 35 days. A severe epidemic is likely to develop if past weather has been favorable, if the pathogen is well-established and if the 30 day outlook is for much above-normal precipitation (17 or more days of rain, 11 May-14 June.).