SummarySeasonal changes in numbers of conidia ofRhynchosporium secalison debris from previous barley crops infected with leaf blotch (primary inoculum) were monitored in 1985–86 and 1986–87. In 1986–87, changes in numbers of conidia on leaves of plants in the new winter barley crop (secondary inoculum) were also recorded. The greatest increases in production of primary inoculum were in early spring after rain, when temperatures were increasing after periods of sub‐zero temperatures when there was little conidial production. Subsequently, more conidia were recovered from this debris after cycles of drying and rewetting than when it remained wet. After January 1987, amounts of secondary inoculum produced on the crop were much greater than amounts of primary inoculum on debris. Most spores were produced on the basal leaves and more spores were present on the September‐sown than on the November‐sown crop. Thus, while primary inoculum was a source of disease when plants were emerging, secondary inoculum on basal leaves was the main source of disease at stem extension, especially on early