Agronomic factors affect powdery scab of potato and amounts of Spongospora subterranea DNA in soil

Abstract

Severe powdery scab (caused by Spongospora subterranea) occurred in potato tubers harvested from a field trial, which measured effects of agronomic treatments (nitrogen fertiliser rates, irrigation intensities, previous crop rotations) on potato yields. Nitrogen application (400 kg N ha⁻¹) increased weight of tubers per plant by 38 %. Incidence and/or severity of powdery scab were increased by nitrogen applications. Measurements of S. subterranea DNA in soil at harvest generally reflected the severity of powdery scab in harvested tubers. Amounts of DNA were greater after nitrogen application than without nitrogen fertiliser, less after “optimum” irrigation than “low” irrigation, and greater after a potato/wheat rotation than after potato/pea. The field trial area was used for two further growing seasons (without application of treatments) to determine if pre-planting measurements of Spongospora DNA in soil could predict powdery scab in harvested potatoes. The disease generally decreased during the next two growing seasons, and effects of the different agronomic treatments on powdery scab had disappeared by the second season. However, the greater amounts of pathogen DNA in soil in plots where nitrogen had been applied than where no fertiliser was used continued for the two following growing seasons. Relationships between amounts of pre-planting S. subterranea DNA in soil and powdery scab in subsequently harvested tubers were weak in the second growing season, and non-existent in the third. These results demonstrate that agronomic treatments (particularly nitrogen) can increase severity of powdery scab in harvested tubers. Furthermore, pre-planting measurements of pathogen DNA in soil did not give good predictions of the incidence or severity of powdery scab in harvested potatoes.