Abstract
Protoplasts derived from shoot cultures of potato cv. Cara, which carries immunity gene Rx, supported only limited virus multiplication after inoculation with particles or RNA of isolate DX, a group 3 strain of potato virus X, as compared to similarly inoculated protoplasts of cultivars King Edward and Pentland Ivory which lack Rx. The Cara protoplasts were, however, able to support extensive replication of the resistance-breaking strain HB. This strain-specific resistance did not appear to be mediated by a failure or inhibition of the uncoating mechanism or of virus assembly.