The Ascomycete Blumeria graminis f. sp. hordei, a serious pathogen of barley, reproduces asexually by conidia. Infection - conidial germination and the formation of primary germ tube, secondary germ tube, appressorium and haustorium - and host response are documented in detail by time-lapse cinemicrography. The processes are shown in compatible and incompatible combinations of host and parasite. In incompatible combinations the defence response, effected by the ml-o resistance gene, is visible as an increased formation of papillae, and the hypersensitive reaction takes place as an effect of the ml-a resistance gene. The video is suitable for use in higher education.