In this study, the phytochemicals effects of smoothseed alfalfa dodder (Cuscuta approximata Bab.) which causes significant yield loss in agricultural production were determined to develop plant-derived formulations for controlling plant diseases. For this purpose, the methanol extract of Cuscuta approximata was used in different concentrations (10, 20, 30 %) against the important fungal plant pathogens as follows: Alternaria alternate, Fusarium oxysporum and Rhizoctonia solani. The best result was obtained at 30% concentration for all fungus species. Findings show that, increasing concentration of extract has negative impact on the development of micelle in three different fungal species while the best micelle development was observed in control petries. In conclusion the phytochemicals effects of smoothseed alfalfa dodder could be used against some of the plant pathogenic fungus.