Further studies on resistance-breaking strains of potato virus X

A stock culture of isolate CP of potato virus X (PVX) maintained by serial subculture in plants of Nicotiana glutinosa was found to contain PVX strain group four in addition to the original strain group two. The group four strain was separated from the mixture by sap-inoculation to potato cultivars Maris Piper and Pentland Dell, both of which carry PVX hypersensitivity genes Nx and Nb , by graft-inoculation to Maris Piper and by sap-inoculation to cultivar Pentland Ivory which carries Nb but not Nx. Strain group four seemed to be a minor component of the strain mixture in N. glutinosa as few potato plants became infected with it when insusceptible plants were sap-inoculated or when sap inoculum was diluted 500 times. The group four strain passed readily through tubers of infected potato plants and was stable on serial sub-culture in N. glutinosa. When stock cultures of PVX group two isolates B and EX kept in N. glutinosa were tested on cultivar Pentland Dell, they also proved to be mixtures of group two and group four, indicating that spontaneous appearance of group four in cultures of group two strains may occur readily.
When group four strains derived from isolate CP and from PVX common strain isolate DX were graft-inoculated to many plants of cultivar Cara, which carries PVX immunity gene Rx , there was no evidence of selection of a strain like HB which overcomes this gene. In mixed infection with isolate DX, HB was still present after passage through two generations of progeny tubers of cultivar Pentland Crown which lacks any resistance genes, indicating that HB is a fully competitive strain.