Patterns of conidial release by Helminthosporium turcicum Pass. (Trichometasphaeria turcica Luttrell) on sweet corn (Zea mays L.) were studied over 24-h periods under controlled environmental conditions. During the night (dark for 10 h, 100% relative humidity (RH)), relatively few conidia were trapped below diseased seedlings and there were no obvious peaks of spore release. During the day (light for 14 h, 70% RH), however, many conidia were caught (usually four to six times as many as during the night), and there were either one (with 10-week-old seedlings) or two (with 6-week-old seedlings) peaks of spore release. Conidia trapped during the day period were considered to have been released as a result of drying of the leaves. Those trapped during the night were considered to have been washed passively from leaf surfaces. No evidence was obtained to link conidial release with increasing RH.