SUMMARYSporulation ofSeptoria nodorumwas assessed on inoculated spring wheat cvs Kolibri and Maris Butler. The time from inoculation to first production of spores (latent period) was similar over a range of constant temperatures (12–18°C) but was shorter for Kolibri at 6°C and at 24°C. Spore production was greatest during the period of stem extension and, for equal amounts of disease, was twice as great on Kolibri as on Maris Butler. Spore production on leaves was much more intense withSeptoria triticithanS. nodorumfor equal amounts of disease, and was less intense on heads than on leaves for both pathogens. Inoculation of plants resulted in significant losses in yield. Mans Butler consistently out‐performed Kolibri on each component of yield. Kolibri was particular affected: 1000‐grain weights were reduced at each growth stage tested especially byS. nodorum, grain numbers and yield/head were reduced particularly following inoculation at heading. Spore production ofS. nodorumduring the period of stem extension increased to a peak in early July and then declined but no reliable relationship with monitored weather accounted for this season