The present invention relates to the use of mixtures containing secondary calcium and secondary magnesium phosphonate for combatting harmful fungi, to the use thereof as fertilizer or plant nutrient, to a mixture, containing secondary calcium and secondary magnesium phosphonate in a specific molar ratio, characterized in that the solid form of the mixture has a water solubility of at most 5 g/l, and to a method for preparing a mixture containing secondary calcium phosphonate and secondary magnesium phosphonate.