SUMMARYIncubation time ofErysiphe graminisf.sp.hordeivaried seasonally, but between early May and late September it was always between 5 and 8 days. This is approximately the time of year when the 30‐year weekly mean air temperature exceeds 10oC. Amount of infection during summer was decreased by hot, dry weather. In winter, incubation times were more variable and long incubation times were usually associated with few infections. Variation in incubation time during the summer is unlikely to be of practical importance in determining the rate of epidemic developmen