Exclusion of viroids from potato resources and the modified use of a cDNA probe1

Threats from potato spindle tuber viroid (PSTV) to potato breeding and centralized elite seed-tuber production have been identified in world potato genetic resources. In the UK effective diagnostic testing has proved essential in preventing acquisition. Inoculation of potato nucleic acids to tomato and subsequent viroid detection by polyacrylamide gel electrophoresis (PAGE) has proved a sensitive, but cumbersome, test over 8 years. Additionally, over 2 years, ³²P-labelled PSTV cDNA was used to probe denatured sap and nucleic acid extracts: 10^-4 of peak viroid concentrations in tissue could be detected. Spurious positives were seen in particular circumstances, but could be avoided. Probing of non-denatured samples was not as sensitive. Tubers became infected and PSTV was readily detected by PAGE in leaves of potato experimentally inoculated and maintained below 20°C, but the cDNA probe could not detect infection in tuber sprouts growing at 8–10°C in darkness. Otherwise similar green-leaved sprouts were faintly positive. Detection for all sprouts was unproblematic after movement to 25°C and light for 10 days.