Testing of Imported Potato Genotypes for Potato Spindle Tuber Viroid with a Tomato‐intermediate /Electrophoresis Combined Method

Abstract

The polyacrylamide gel electrophoresis (PAGE) test of Morris & Smith (1977) was evaluated for detection of potato spindle tuber viroid (PSTV) in breeding material. Number, density and mobility of nucleic acid bands in the electropherograms were influenced by genotype and growing temperature. So direct testing of genotypes was not reliable. After an intermediate viroid multiplication in tomato host plants at about 30oC and high irradiance, PSTV was reliably detectable with PAGE in inocula of potato samples of diverse origin. A 4‐week incubation period proved to be suitable for inocula with low and high concentrations of a mild strain of PSTV (m‐PSTV) as well as a severe strain of PSTV (s‐PSTV). If incidence of PSTV is expected to be low, testing can be speeded up by bulking samples. With the combined tomato‐intermediate/ PAGE assay, one m‐PSTV or one s‐PSTV infected leaf disk in 200 healthy ones was consistently detectable. Occasionally gels with a nucleic acid band of about the same relative mobility as the viroid band were found. Evidence that these bands were not caused by viroid is presented. A procedure to resolve such questionable test results is described. Infectivity of s‐PSTV was higher than that of m‐PSTV. Concentration of viroids in the inoculum influenced appearance of mild or severe symptoms and the rate of symptom production.