Host range and properties of potato black ringspot virus

Potato black ringspot virus (PBRV), obtained from cultivated potato in Peru, was found to have a very wide host range resembling that of tobacco ringspot virus (TRSV-B), to which PBRV is distantly related serologically. However, PBRV caused the more severe symptoms in many species and, unlike TRSV B, infected Lycopersicon esculentum and Cyamopsis tetragonoloba. In Solanum tuberosum, PBRV caused necrotic spots and ringspots in systemically infected leaves in the year of infection and was readily transmitted through tubers to progeny plants, most of which developed no obvious symptoms although systemically infected. TRSV-B infected non-inoculated S. tuberosum leaves only sporadically, did not induce symptoms in them and was not transmitted through tubers to progeny plants. PBRV was cultured in Nicotiana clevelandii and infectivity was assayed in Cheno-podium amaranticolor or C. quinoa. Virus particles were purified from leaf extracts, after clarification using chloroform, by precipitation with 6% polyethylene glycol and differential centrifugation. Purified preparations contained 25 nm diameter isometric particles with somewhat angular outlines, sedimenting as three components (T, M and B) at 49, 84 and 117 S, and containing a single protein species of mol. wt 59 000. Preparations of PBRV nucleic acid contained two species, estimated by polyacrylamide gel electrophoresis in non-denaturing conditions to have mol. wt of about 25 10⁶ (RNA-1) and 15 10⁶ (RNA-2). Infectivity was associated with B particles, preparations of which contained RNA-1 and RNA-2, presumably in different particles. M particles contained RNA-2, were not infective and enhanced infectivity only slightly when added to B particles. Similar relative amounts of RNA-1 and RNA-2 were extracted from unfractionated virus using phenol or Pronase, but preparations obtained using phenol were much the more infective. PBRV has properties typical of nepoviruses; its present cryptogram is (R/1):2–5/41 + 15/28 or 2 1 5/46:S/S:S/*, nepovirus group.