The present invention relates to the use of 2,3,5,6-tetracyano-[1,4]dithiine and the N-oxides and the salts thereof for combating phytopathogenic fungi on cultivated plants, and to seeds coated with 2,3,5,6-tetracyano-[1,4]dithiine. The invention also relates to methods for combating harmful fungi, comprising treating the plants or seeds to be protected against fungal attack.