Wind tunnel experiments carried out on single pustules of Erysiphe graminis DC. f. sp. hordei confirmed the passive pattern of conidial liberation. Dryness of the pustule was required. The wind-speed threshold of release depended on the length of the conidial chain and on the total number of spores likely to be released, so long quiet periods with effective sporulation resulted in a considerable decrease in the wind-speed threshold. The basal immature spores of the chain, released at high wind-speeds, germinate poorly. The curve relating the cumulative number of liberated spores to wind speed was typically S-shaped, depending on the initial density of conidia in the pustule. The relative humidity of the air surrounding the pustule did not matter, in contrast to results from field trials showing a negative correlation between relative humidity (meteorological data) and the number of trapped spores. This result can be explained by the presence of water (dew or rain) in the canopy at high relative humidity.