The effect of soil amended with stalks of sunflower (Helianthus annuus L.), alfalfa (Medicago sativa L.) and Hungarian vetch (Vicia pannonica Crantz) on onion bulb rot caused byFusarium oxysporum Schlecht. f.sp.cepae (Hanz.) Snyder Hansen andAspergillus niger v. Tieghem was tested in a soil naturally infested with the pathogens under field conditions. All treatments significantly reduced bulb rot incidence. The fungal and antibacterial populations varied among all of the amended tested soils. Heterotrophic fungal populations were more effective than heterotrophic bacterial populations in the suppression of disease. The highest population of fungal microflora was found in plots amended with sunflower stalks. The carbon-to-nitrogen ratio and available moisture content of amended soils have an indirect suppressive role on disease, by increasing the heterotrophic fungal population.