Modeling pathogen DNA content and visual disease assessment in seed tubers to inform disease in potato progeny root, stolon and tubers

Measurement of pathogens on seed-tubers is essential for informing likelihood of subsequent potato disease. Here we utilised quantitative polymerase chain reaction assessment of pathogen DNA and visual assessment of disease to measure seed-tuber inoculum and used this to model development of disease in potato grown in pathogen-free soil. Analysis by recursive partitioning and modelling using receiver operating curves indicated both abundance of Rhizoctonia solani AG3 and Streptomyces scabies DNA and disease symptoms associated with these pathogens on seed-tubers could predict subsequent disease in progeny tubers and for R. solani, stolons. In contrast abundance of Spongospora subterranea DNA and disease symptoms on seed-tubers were not consistently associated with powdery scab in progeny tubers. The relationship between S. subterranea DNA and seed-tuber symptoms on root galling was stronger. Symptomless seed-tubers that carried high levels of S. subterranea DNA were also associated with greater root galling than those with low pathogen DNA levels. There was a modest association between root galling and powdery scab in progeny tubers. These results highlight the importance of using certified seed-tubers, and demonstrate a statistical tool for measuring the impact of seed tuber-borne inoculum.