SUMMARYSeveral lines of circumstantial evidence collectively indicated that poor early growth of apple (‘replant disease’) might be associated with the effects of soil‐borne pythiaceous fungi. This hypothesis was supported by pathogenicity tests. All isolates tested ofP. sylvaticumand certain isolates of seven otherPythiumspp. significantly reduced the growth of apple seedlings. The growth reductions caused by certainPythiumisolates were of comparable magnitude to the growth increases occurring after chloropicrin‐fumigation of apple orchard soils. ThePythiumisolates most virulent to apple were of low virulence to a clonal cherry rootstock.Reappraisal of the nature of the disease as a non‐specific soil malaise is consistent with established features of the pathology ofPythiumspp. The disease, however, is an ill‐defined ‘poor growth phenomenon’ with no diagnostic symptoms and conclusive evidence thatPythiumspp. are widely causal is likel