The invention relates to salts of the pyrophosphorous acid of formula I ##STR1## wherein X.sym. is a cation equivalent selected from the group consisting of sodium, potassium, lithium, beryllium, magnesium, aluminum, zinc, manganese, copper and NH.sub.4, to the preparation thereof and to novel compounds of formula I, to the use of compounds of formula I as microbicides and as seed dressing agents.
