Spongospora subterranea root infection assessed in two potato cultivars differing in susceptibility to tuber powdery scab

Infection by Spongospora subterranea of roots of two potato (Solanum tuberosum) cultivars, either very resistant or very susceptible to powdery scab on their tubers, was studied in a glasshouse experiment. Plants grown in sand/nutrient solution culture were inoculated with S.subterranea sporosori 2weeks after planting. Plant parameters, the intensity of zoosporangium infection in roots, numbers of Spongospora root galls and amounts of Spongospora DNA in roots, measured using quantitative PCR (qPCR), were assessed at sequential harvests. Inoculation with S.subterranea reduced water use (56days after planting) by 26% in the tuber resistant cultivar compared with uninoculated plants, and by 60% in the susceptible cultivar. Inoculation did not affect growth of the resistant cultivar, nor shoot mass of the susceptible cultivar, but caused a 38% reduction in root mass of the susceptible cultivar. The intensities of zoosporangium development in both cultivars were similar. The susceptible cultivar had approximately four times more Spongospora root galls g(-1) root mass than the resistant cultivar. Quantitative PCR detected S.subterranea DNA in roots 1week after inoculation, and indicated a twofold greater amount of pathogen DNA in roots of the susceptible than the resistant cultivar. This study suggests that the S.subterranea zoosporangium stage in host roots is affected differently by host resistance factors than the sporosorus (root gall and tuber scab) stages. The study has also demonstrated the usefulness of qPCR for sensitive and consistent detection of S.subterranea across the duration of potato root infection.