The spots often are circular in shape but may be angular when they are delineated by leaf veins. As the disease progresses, leaf lesions enlarge and usually grow together.
Mature lesions turn tan and dry out, becoming papery.
Do not confuse this disease with late blight of tomato and potato caused by Phytophthora infestans, which does not infect celery. The primary inoculum source for late blight is contaminated celery seed. The pathogen can survive in undecomposed plant residue. Septoria apiicola may also be found on celeriac. Because this pathogen is seedborne, use Septoria -indexed seed.