Potato root exudates stimulate zoospore release of Spongospora subterranea

Spongospora subterranea is a soil-borne pathogen which causes powdery scab and root infection in potato. Root infection is less well studied than powdery scab, but is now gaining more attention. The role of root exudates in zoospore release of S. subterranea hasn’t been fully investigated and the metabolic profile of potato root exudates, which may influence zoospore releases, is also unreported. We studied the effect of root exudates, from three-week old potato cv. Gladiator (resistant) and cv. Agria (susceptible), on S. subterranea zoospore release in two laboratory assays. Sporosori (2.3 x 104) were added to root exudates and distilled water (control), incubated at fluctuating temperature of 15-18 °C for 10 days, and zoospores were assessed microscopically. We also report the qualitative metabolic profile of root exudates characterised by ultra performance liquid chromatography – mass spectrometry (UPLC-MS). Release of zoospores was observed as early as four days after incubation (DAI) in all test exudates and none in the control solutions. Significant differences were observed between root exudates and distilled water at 4, 7 and 10 DAI in both experiments (P<0.01). Qualitative UPLC-MS chromatogram characterisation of root exudates detected 43 low molecular weight organic compounds, out of 135 analytes screened. This study provided strong evidence root exudates released by potato as early as three weeks from planting stimulates S. subterranea zoospore release. Moreover, potential root metabolites responsible for the observed effect were successfully determined by single UPLC-MS method.