Winter wheat (Triticum aestivum L.) is one of the most important arable crops. Copper-containing fertilizers are used in crop production to treat or reduce the negative effects of copper deficiency. Various copper compounds are also widely used as effective fungicide agents. The influence of a copper-containing foliar fertilizer prepared from a copper solution and sucrose had been tested on the yield and raw protein content of winter wheat in previous field experiments. In this work, we studied the antifungal effect of this particular foliar fertilizer in an in vitro trial, wherein the fungicide effect on growth and reproduction of specific plant pathogens, i.e., Fusarium graminearum and Drechslera sorokiniana, was evaluated with the agar well diffusion method. The results showed that doses of the foliar fertilizer corresponding to field applications of at least 1 kg ha−1 had antifungal effects, which were significant (P < 0.01) above the concentration level of 3337 mg L−1, corresponding to 2 kg ha−1 application, against both fungal pathogens. A linear correlation (r2 > 0.95) was found between copper concentration and fungicide efficiency in the concentration range studied. The fungicide efficiency of the foliar fertilizer was lower than that of the commercially available copper-oxychloride fungicide. The sole use of the foliar nutrient tested probably cannot replace fungicide treatments. However, in addition to its plant nutrition effect, its antifungal properties should also be taken into consideration.