The survival of Didymella bryoniae and the incidence of ascospores in glasshouses, outdoors and under controlled conditions were studied. The fungus was able to overwinter in the open as dormant mycelium. Dry and undecomposed crop residues remained a source of infection for more than one year. Moisture and a minimum temperature between 5 and 10 ~ were needed for fructification. For ascospore release a high relative humidity was not sufficient, the substrate had to be moist during a short period. Ascospores could be trapped throughout day and night both outdoors and in glasshouses, but there was a marked peak during a period of 3 h in the evening. Both on days with and without rain about the same numbers of ascospores : were trapped from crop residues in the open. Ascospore release was favoured by watering the plants in the glasshouse. Under controlled conditions the release of ascospores was determined by humidity and not by light or darkness.In a cucumber crop in the glasshouse the first ascospores were trapped at about the same time the first symptoms on the plants appeared. In the glasshouse with introduced diseased plant debris, particularly when the debris became wet when the plants were watered, the disease was more severe and yield was less than in a glasshouse without introduced plant debris. Airborne ascospores may cause the primary infection of a cucumber crop. Therefore, hygienic measures must be taken to eliminate plant debris as source of infection, both in glasshouses and outdoors.