Pyridazinone derivatives of the formula (I) where R1 is alkyl, alkenyl, cycloalkyl or unsubstituted or substituted phenyl, R2 is chlorine or bromine, R3 is hydrogen, alkyl, cycloalkyl or unsubstituted or substituted phenyl and X is SO or SO2, with the proviso that R1 is not methyl when X is SO2, R2 is chlorine and R3 is phenyl, and fungicides containing these compounds.