The relationship of potato black ringspot virus to tobacco ringspot and allied viruses

The relationship between potato black ringspot virus (PBRV), isolates of tobacco ringspot virus from blueberry (TRSV-B), cherry (TRSV-C) and calico-diseased potato (TRSV-P), and eucharis mottle virus (EuMV) was examined in tests of three types. In gel-diffusion precipitin tests, the reaction end-points of antisera, and spur formation, indicated that PBRV and TRSV-P are very closely related but not identical antigenically, as are TRSV-B and TRSV-C, and that these two pairs are more distantly related to each other and to EuMV. In plant-protection tests in Nicotiana angustifolia, PBRV, TRSV-B and EuMV conferred protection against the homologous virus but not against one another. PBRV, but not TRSV-B, conferred protection against TRSV-P. In tests with the two RNA species of PBRV, infectivity increased greatly when preparations of RNA-1 and RNA-2 were mixed, and both species are probably needed for infection. Infectivity did not increase when RNA-1 or RNA-2 of PBRV was mixed with RNA-2 or RNA-1, respectively, of TRSV-B; the two viruses seem too distantly related to form pseudo-recombinants. It is concluded that PBRV and tobacco ringspot virus should be considered separate viruses, and that TRSV-P should be considered a strain of PBRV. EuMV should perhaps be recognised as a virus distinct from, but related to, PBRV and tobacco ringspot virus.