SUMMARYThe relative importance of infected celery seed, infected leaf debris in the soil, and infected wild celery, in the incidence ofSeptorialeaf spot in cultivated celery has been investigated. Infection can be caused when the sole source of inoculum is viable spores on the seed surface; such spores are considered to be the main cause of disease outbreaks.Of all the seed samples examined, 93% were infected bySeptoriaspp. In untreated seed samples, 40% carried viable spores which survived for up to 15 months on seed stored in the laboratory, and for longer periods on seed stored at ‐20d̀ C. However, ageing of seed is not recommended as a commercial control measure.The fungus was not found in seed embryos or endosperms but mycelium was present in pericarps and testas. Unconfirmed evidence suggests that in favourable circumstances new spores might be produced in old seed‐borne pycn