Occurrence and establishment of Pythium in soils

Abstract

Species of Pythium were isolated from cultivated soils by using susceptible host seedlings as baits, but could not be found in soils taken from woodland or other non-cultivated areas. No isolates were obtained from very acid soils in which Trichoderma viride was particularly abundant. The behaviour of inocula of Pythium mamillatum in a cultivated (native) soil, and in a woodland (alien) soil was assessed by means of the host test described. Maize meal added to the cultivated soil became colonized by P. mamillatum after only 2 days, indicating, that under these conditions, the parasite behaves as a saprophytic ‘sugar fungus’. In the alien woodland soil P. mamillatum did not colonize maize meal. After neutralization of this acid soil with lime, however, the fungus rapidly utilized the material. The parasite could not be grown in culture at the low pH value of the alien soil. The behaviour of vegetative hyphae of P. mamillatum in these contrasting soils was studied by the glass-fibre tape technique. In native, cultivated soil, oospores and resting sporangia were formed abundantly, whereas in alien woodland soil these bodies were not produced. Thus the inability of Pythium hyphae to produce a resting phase may prevent long-term survival of the parasite in alien soils. This effect was shown to be due to low pH value of the soil, which may directly limit the normal life cycle of Pythium in acid woodland soils.