QPCR testing seed potato tuber for pathogens – identifying hidden threats

Utilising certified seed is an important principle in providing a potato crop with the greatest opportunity to grow to its full potential free of disease. Within Australia key soil-borne diseases affecting potato tuber production and quality include Black scurf, Common scab and Powdery scab, caused by infections with the pathogens, Rhizoctonia solani AG3, Streptomyces scabies and Spongospora subterranea f.sp. subterranea, respectively. The significance of these pathogens as seed-borne threats is reflected by the inclusion of these diseases (pathogens) in all key certification schemes worldwide. Typically a crop is certified by visual subjective assessments but recently developed qPCR tools provide an additional quantitative measure to elucidate seed tuber health. This project examined and compared both the visual and qPCR status of potato crops passing through Tasmanian certification schemes over a three year period. Further, selected tubers carrying varying tuber-borne inoculum loads of the three pathogens were grown on in pathogen-free potting soil to establish whether concentration of pathogen DNA (from the tuber peel) effected progeny crop disease outcomes. Key outcomes suggest that for certification; visual assessment, in most circumstances provides an accurate measure of tuber seed health, however qPCR was able to more accurately differentiate difficult to distinguish diseases (powdery vs common scab) and also identify significant levels of pathogen DNA from symptomless tubers. Additionally, it was shown that planting tubers with high inoculum loads (measured visually or by qPCR) led to enhanced progeny disease compared to planting ‘certified’ tubers. Further advantages of planting certified seed are also discussed.