Light‐colouredSphagnum fuscumpeat is the substrate most widely used in glasshouses in Finland. A number ofStreptomycesstrains which are strongly antagonistic against a number of soil and seed‐borne fungal pathogens, bothin vitroandin vivo, have been isolated from light‐coloured peat. Seed dusting withStreptomycesprevented or reduced damping‐off and foot‐rot disease caused byAlternaria brassicicolaandRhizoctonia solanion crucifers. Seed treatment slightly increased yields of cereals in fields experiments. Spraying the substrate with a suspension ofStreptomycesreduced root diseases on cucumbers caused byPythium, and in many cases successfully prevented fusarium wilt of carnations. A dry powder preparation has been made from theStreptomycesisolates, and applied at a dose of 5‐10 g kg‐1in seed dusting, and 0.1‐10 g 100 m‐2when spraying the substrate. The microbe was most effective in peat soil, although good results have also been obtained in fine sa