Eight years of annual monitoring of DNA of soil-borne potato pathogens in farm soils in south eastern Australia

Concentrations of DNA of the potato pathogens Rhizoctonia solani, Spongospora subterranea and Streptomyces scabies in surface soil (0–150 mm depth) from potato farm paddocks were monitored annually for 8 years in South Australia and Tasmania. A non-linear model was fitted to these data to test if changes in DNA concentrations could be explained by changes in land use in the paddocks, and so identify land uses with potential to either increase or decrease the risk of disease caused by these pathogens. Spongospora subterranea and R. solani anastomosis group (AG) 2.1 were the most prevalent pathogens in both states, followed by St. scabies and R. solani AG3, and R. solani AG4 in Tasmania. The fitted model showed prior crops of poppy and potato to be associated with increased R. solani AG2.1 DNA concentrations; and prior crops of carrot, fallow, poppy and ryegrass with increased R. solani AG3 DNA concentrations, while a prior potato crop was associated with increased concentrations of the DNA of Sp. subterranea and St. scabies. Prior potato crops resulted in average increases of Sp. subterranea DNA of 3300 (South Australia) to 7300 (Tasmania) pg DNA/g soil. Based on the average concentrations of Sp. subterranea DNA in the monitored soils, the model predicted that soils in which potato is grown at least once in every 5 years are likely to remain at high risk of powdery scab. Potato growers in these areas will need to take steps to minimize the risk of this disease, e.g. choose less risky paddocks, grow less susceptible cultivars, and where possible manage soil moisture to avoid wet soil at tuber initiation.