RESPONSE OF SELECTED SOLANUM SPECIES TO VIRUS ERADICATION THERAPY

Thirty-four Solanum genotypes infected with one or more potato viruses were evaluated in an in vitro virus eradication system combining heat therapy and the antiviral chemical ribavirin. This system was compared with in vitro heat therapy or ribavirin treatment alone. The effect of explant size on virus eradication following therapy was also evaluated. Infected plantlets were established in ribavirin lacking and amended medium; half of the plantlets in each medium were subjected to a 30-day heat treatment (35 C/31 C, 4-hr alternating periods), while the remaining plantlets were kept at room temperature (23 C). Following therapy, plantlets were tested quantitatively (OD at A405) for the appropriate virus by enzyme linked immunosorbent assay (ELISA) and divided into 3 categories: low (less-than-or-equal-to 0.05 OD), medium (> 0.05 to < 1.0 OD) and high (greater-than-or-equal-to 1.0 OD). Three plantlets were selected per group and further propagated as nodal cuttings or meristemptips. Regenerants which tested negative (less-than-or-equal-to 0.05 OD) were transplanted to plug trays in a greenhouse and were retested after a 30-day grow-out period. Seventy-four and 91% of the genotypes yielded virus-free (VF) plants following propagation as nodal cuttings and meristem-tips, respectively. The combined therapies generated twice as many VF plants as thermotherapy alone and 5% and 15% more VF plants for nodal cutting and meristemtip derived plantlets, respectively, than for chemotherapy alone. Eleven of the 34 genotypes exhibited susceptibility to heat, but VF plants were obtained from survivors or by shortening the thermotherapy period. The efficiency of the system was enhanced by selecting plantlets for propagation which tested less-than-or-equal-to 0.05 OD after treatment with 92%, 93%, 71%, and 100% of them remaining VF from potato viruses S, X, Y and leafroll, respectively. Twice as many VF plantlets were produced when treated plantlets were then propagated from meristem-tips than from nodal cuttings, but the rapid regeneration rate of plantlets from nodal cuttings makes it an attractive propagation method. Plantlet evaluation by ELISA after therapy to select plantlets with a high probability of producing VF plantlets and after further propagation to assure virus freedom are strongly recommended.