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 <i>Rhizoctonia solani</i> AG3 and <i>Streptomyces scabies</i> DNA and disease symptoms associated with these pathogens on seed-tubers could predict subsequent disease in progeny tubers and for <i>R. solani</i>, stolons. In contrast abundance of <i>Spongospora subterranea</i> DNA and disease symptoms on seed-tubers were not consistently associated with powdery scab in progeny tubers. The relationship between <i>S. subterranea</i> DNA and seed-tuber symptoms on root galling was stronger. Symptomless seed-tubers that carried high levels of <i>S. subterranea</i> 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.