Comparison of tomato bioassay and slab gel electrophoresis for detection of potato spindle tuber viroid in potato.

SCHUMANN, G. L., H. D. THURSTON, R. K. HORST, S. O. KAWAMOTO, and G. I. NEMOTO. 1978. Comparison of tomato bioassay and slab gel electrophoresis for detection of potato spindle tuber viroid in potato. Phytopathology 68: 1256-1259. Potato plants from a randomly selected set of 146 Cornell When 20 were tested a second time, results supported the University breeding lines were tested for the presence of original electrophoretic data in all but one case. Potato plants potato spindle tuber viroid (PSTV) by both polyacrylamide of four commercial cultivars and 45 breeding lines gel electrophoresis and tomato bioassay. The electrophoretic representing diverse germplasm were inoculated with mild or assay was an adaptation of Morris and Smith's procedure to severe strains of PSTV. The electrophoretic assay accurately a slab gel apparatus that permitted testing of 25 samples at detected PSTV in the inoculated plants. Assay for PSTV by once. Results of both tests were in agreement in 123 cases. In gel electrophoresis is more rapid and reliable than the tomato 23 cases, results of the original tests were not in agreement. bioassay. Because potato spindle tuber viroid (PSTV) is both A selected set of tubers of 45 breeding lines representing seedand pollen-borne in potatoes (Solanum tuberosum diverse germplasm was planted in pots in a glass greenL.) (2) it is desirable to use only parent plants that are not house maintained at about 25 C with supplemental light infected with PSTV for potato breeding. A bioassay to provide a 16-hr photoperiod. Each breeding line was described by Fernow et al. (1) using tomato as a test plant represented by a set of three plants all grown from the has been used for over a decade to detect PSTV in potato same tuber. One plant of each set was not inoculated, one breeding lines at Cornell University. Although the test has was inoculated with a mild strain of PSTV, and one was been useful, it has two main disadvantages: (i) 6-8 wk are inoculated with a severe strain of PSTV. All inoculations required to complete the test, and (ii) it lacks accuracy were made as soon as the leaves began to expand by under unfavorable environmental conditions. An assay rubbing crushed PSTV-infected tomato leaves on the described by Morris and Smith (4) which uses polyupper leaf surfaces of Carborundum-dusted potato acrylamide gel electrophoresis is an appealing procedure plants. Samples from all three plants of each breeding line because it requires days rather than weeks to perform. were assayed 4-5 wk postinoculation by the electroThe accuracy of the electrophoretic assay was not known phoretic assay described below. Assay results were for testing a wide variety of potato lines infected with a compared to a coded list to determine the accuracy of the range of isolates of PSTV of unknown titers. We made a electrophoretic assay. comparison of the bioassay and a modified electroA third set of potato plants of four commercial phoretic assay to determine their relative reliability and cultivars, Sebago, Katahdin, Kennebec, and Russet usefulness. Rural, was grown. Eighteen plants of each cultivar were inoculated with a severe strain of PSTV, and eighteen MATERIALS AND METHODS were inoculated with a mild strain of PSTV. Inoculations were made as described previously. Samples of young Plant culture.-Tubers of Cornell University potato leaves were assayed 4-6 wk postinoculation by the electrobreeding lines to be assayed for PSTV were planted in phoretic assay described below. pots in a polyethylene greenhouse maintained at about 25 Tomato bioassay.-Bioassays were initiated with C. Samples of young leaves were taken from 4to 5-wkRutgers tomato plants at the cotyledon stage (1). old plants of a randomly selected set of 146 breeding lines Cotyledons of two tomato plants were dusted with 0.15for bioassay and electrophoretic assay. All tomato bioy.m (400-mesh) Carborundum, rubbed directly with assay plants were grown in a glass greenhouse maintained crushed leaves from the potato plant to be tested, but not at about 25 C with supplemental light during a 16-hr rinsed. If the potato plant was infected with a severe strain photoperiod. of PSTV, the tomato plant should develop severe symptoms. If no symptoms appeared, then the potato 00032-949X/78/000 226$03.00/0 plant was either healthy or infected with a mild strain of Copyright © 1978 The American Phytopathological Society, 3340 PSTV. Pilot Knob Road, St. Paul, MN 55121. All rights reserved. After 14-16 days one of the tomato plants was challenge