The infectivity of soils containing Phytophthora infestans

Infectivity of soil was measured by incubating samples on potato tuber slices during and after outbreaks of potato blight in three years. Surface soil was always more infective than deeper soil, the most infective surface soil containing the equivalent of 2100 sporangia of P. infestans per ml. Infectivity usually diminished rapidly with increasing depth, and only occasionally did infectivity below 2 in. exceed the equivalent of 100 sporangia per ml. Exceptionally large infectivity was once found at all depths sampled after heavy rain, but rain did not consistently affect infectivity. The surface soil remained infective for at least 32 days after the haulm was killed with acid, but the concentration of viable spores declined to a small value during the first week after treatment. In the laboratory infectivity persisted for 11 weeks after sporangia were mixed with unsterilized soil originally with a moisture content of 20%. P. infestans mycelium grew only in sterilized soils, perhaps because some soil micro‐organisms inhibit its growth, and others (e.g. Trichoderma viride) parasitize it or lyse it (e.g. by Rhizoctonia solani). The three soils tested all contained substances that inhibit the germination of sporangia. RESP-529