The production of haustoria by colonies of Erysiphe graminis DC. ex Mérat f.sp. hordei Marchai was studied in detached leaf segments of susceptible spring barley ( Hordeum vulgare L.) incubated for 108 h under various combinations of daylength and irradiance arranged to give comparable levels of energy input. Under all irradiances haustorial production was faster under short (6 h) than long days (18 h). Irradiance had an independent effect on haustorial production, but although 3.77 W/m 2 was optimal under both daylengths, the influence of irradiance level was relatively small compared with that of daylength. Faster haustorial production under short days was associated with greater synchrony in the production of haustorial generations formed subsequent to the primary, but the clearest distinction between secondary and tertiary generation haustoria was obtained with a short day/high irradiance treatment. Although this treatment was not optimal for haustorial production, such incubation conditions aid confident identification of tertiary generation haustoria which allows sensitive measurement of colony size useful for quantifying host resistance of a partial nature.