The invention relates to a bactericide composition based on bacteriophages for the control of black plague in plants or parts thereof, preferably walnuts, a preparation method and application. The invention provides methods for the isolation, propagation and application of bacteriophages against phytopathogens affecting trees/plants that are of commercial interest for their fruit, flowers etc., for the prevention, treatment or reduction of signs, in particular, for Xanthomonas A. pv juglandis in walnuts.