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When tomatoes were planted in steamed soil inoculated with spores of Didymella lycopersici Kleb., a large proportion became infected but very few became infected in unsteamed soil similarly inoculated. In soil treated with formalin and subsequently inoculated the number of diseased plants was about half that in steamed soil. Reinfecting steamed soil with soil micro-organisms before inoculation with D. lycopersici reduced the amount of disease. Covering steamed soil with a layer of maiden loam filtered out a large proportion of D. lycopersici spores applied in water suspension and reduced the amount of disease. Addition of stable manure either before or after steaming did not affect the number of diseased plants Steamed soil remained susceptible to infection by D. lycopersici for 4 months