SUMMARYIn spore‐trapping studies in 1965, conidia ofAlternaria porri(Ellis) Neerg. were detected in the atmosphere immediately above onions affected by purple blotch disease. On calm summer days, peak conidium concentration occurred between 08.00 and 14.00 hr., few conidia being trapped between 20.00 and 06.00 hr. On windy days, there were marked increases in concentration. Concentration tended to increase with increasing wind velocity. Increased conidium concentrations were also recorded after rainfall and irrigation, and during spraying operations.Laboratory observations indicate that inA. porri, the attachment of conidium to conidiophore is weakened, if not broken, by movements in the conidial apparatus which occur in response to a decrease in vapour pressure.Possible causes of fluctuations in atmospheric content ofA. porriconidia are discusse