Studies on Potato Virus X Ii. Criteria of Relationships Between Strains

The range and type of symptoms produced by strains of virus X are briefly described. Of ten strains examined, six were identical in serological cross‐absorption tests, and two (T.B.R. X and B ) differed considerably from the others. The possibility of preparing strain‐specific antisera is indicated. The protection afforded by mild strains against severe strains in tobacco and Datura tatula was complete for all strains tested except T.B.R. X and B , which gave only partial protection. Thus there is a correlation between wide serological differences and a breakdown in the protection afforded in the plant. There was no correlation between type of symptoms caused and serological relationships. There was some evidence to show that strains from the same source were more closely related than strains from different sources.