Field trials consisting of different tillage and soil fertility treatments were conducted to study possible causes of yield decline of monoculture winter wheat in the eastern Orange Free State. Soil fumigation with methyl bromide enhanced plant growth, reducedPythiuminfections and increased grain yields in both clean-tilled (mouldboard-ploughed) and minimum-tilled (stubble-mulch) plots.Pythium irregulärewas frequently isolated from scutellum tissue of young wheat seedlings. Roots of older plants were commonly infected withp. arrhenomanes.Higher yields were obtained under conventional tillage than under minimum tillage. Nitrogen amendments retarded growth and reduced yields in clean-tilled plots, but not in minimum-tilled plots. Phosphorus seems to act as a buffer to the damaging effects ofPythium, minimizing the combined effect of infection and phosphorus deficiency. The yield depression of wheat in minimum-tilled soil is attributed toPythiumdamage rather than to Phytotoxins previously isolated from unweathered straw.