The incidence and severity of root infection and root galling caused by Spongospora subterranea were assessed in potato plants (cv. Estima) grown under controlled environmental conditions. The effects of temperature, soil type, soil moisture regime and soil inoculum level on infection and root gall development were determined by molecular and visual methods at two plant growth stages. Root gall severity was scored at harvest, after which DNA was extracted from the roots and quantified in a real-time polymerase chain reaction (PCR) assay specific for S. subterranea. Root galling was severe at 17°C, with a disease score of 3·1 on a 0–4 scale, low (0·6) at 12°C, and did not occur at 9°C. The level of inoculum in soil, in the form of artificially added sporosori, had no effect on the incidence and severity of visual symptoms, with 21%, 41% and 33% incidence observed at 5, 15 and 50 sporosori g−1 soil, respectively. Incidence of infection, as detected by the real-time PCR assay, was greater with increasing soil inoculum concentrations, ranging from 48% at 5 sporosori g−1 to 59% (15 sporosori g−1) and 73% (50 sporosori g−1) of plants infected at maturity, but this effect was not statistically significant. No correlation was found between the occurrence of galls on roots and powdery scab on tubers of the same plants.