Synthetic chemicals are mainly used for the control of fungal diseases in tomato, causing the phytopathogens to generate resistance to the chemical active ingredient, with a consequent risk to human health and the environment. The use of plant extracts is an option for the control of these diseases, which is why the main objective of this research was to study an alternative biocontrol strategy for the management of plant diseases caused by fungi through obtaining polyphenol extracts from mistletoe plants growing on three different tree species—mesquite (Prosopis glandulosa), cedar (Cedrus), and oak (Quercus), which contain flavones, anthocyanins, and luteolin. The overall chemical structure of the obtained plant extracts was investigated by RP-HPLC-ESI-MS liquid chromatography. The antifungal effect of these extracts was examined. The target phytopathogenic fungi were isolated from tomato plantations located in Altamira, Tamaulipas, Mexico. The microorganisms were characterized by classical and molecular methods and identified as Alternaria alternata, Fusarium oxysporum, Fusarium sp., and Rhizoctonia solani.