The most dangerous diseases of celery are leaf spot disease (Septoriakrankheit), and the celery scab.
The first is incorrectly called celery rust, because numerous rust-brown spots appear on the leaves. The consequences of this disease are mainly small tubers. The celery scab has a particularly strong effect on the tubers. The scab-like brown appearances on the tubers turn to rot in the impaction, so that the celery becomes useless. On the plants, celery scab causes the outer leaves to turn yellow. Both diseases are particularly prevalent in wet summers. They are also favored by one-sided nitrogen fertilization and watering with liquid manure. As a preventive measure, the seeds are dressed. Spraying with copper preparations does not always have the hoped-for success.