Invention relates to a strain of Brevibacillus parabrevis B50, NCAIM (P) B 001413, which present superior characteristics of: in vitro antagonism against phytopathogen agents； biosynthesis of compounds which stimulate the formation of mycorrhiza； endophyte development； innocuity on Galleria mellonella test； protection of maize plants, used as host plants for the multiplication of mycorrhizal fungi, and ornamental plants of geranium, pansies, petunias and cockscomb, against soil phytopathogen agents； stimulation of mycorrhiza on the host plants used for multiplication of mycorrhizal fungi, with the formation of suppressive mycorrhizal composts. The controlled-release composition based on strain B50 is made up of 76 parts oatmeal flour, 3 parts ethyl esters of fatty acids, 4 parts polyvinyl alcohol, 3.5 parts lecithin, 0.75 parts potassium soap, 0,45 parts glycerol, 0,4 parts fats from rapeseed oil, the balance to 100 parts being water, and min. 108 cfu/g B. parabrevis B50
