Effects of Pythium species on the growth of apple and their possible causal role in apple replant disease

Abstract

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 of P. sylvaticum and certain isolates of seven other Pythium spp. significantly reduced the growth of apple seedlings. The growth reductions caused by certain Pythium isolates were of comparable magnitude to the growth increases occurring after chloropicrin‐fumigation of apple orchard soils. The Pythium isolates 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 of Pythium spp. The disease, however, is an ill‐defined ‘poor growth phenomenon’ with no diagnostic symptoms and conclusive evidence that Pythium spp. are widely causal is likely to be elusive.