The present invention relates to isolated strains of a Streptomyces spp. which are endophytes of dicotyledonous plants and to methods for selecting such strains. The present invention also relates to a biologically active compound called coronamycin obtained from endophytic Streptomycetes isolated from higher plants. The present invention further relates to compositions of such compounds and to methods of protecting plants against attack by a plant pathogen and methods of inhibiting bacterial growth, fungal growth, viral infection, growth of parasitic organisms, and cancer cell growth with such compositions.