Purification and properties of blackberry chlorotic ringspot, a new virus species in subgroup 1 of the genus Ilarvirus found naturally infecting blackberry in the UK

A mechanically transmissible virus was isolated from Bedford Giant blackberry plants showing chlorotic mottling and ringspot symptoms growing in Scotland. It infected several herbaceous test plants, many of them symptomlessly. This virus was also transmitted to several Rubus species and cultivars by graft inoculation with scions from the field‐infected Bedford Giant plant. Most grafted plants were infected symptomlessly, but Himalaya Giant blackberry and the hybrid berry Tayberry developed symptoms similar to those in the infected Bedford Giant plant. In the sap of infected Chenopodium quinoa, the virus lost infectivity when diluted 10^?4 but not 10^?3, after 6 h and 48 h when kept at 20°C and 4°C, respectively, but was infective for more than 8 days when kept at ?15°C. Preparations of purified virus from infected C. quinoa or spinach sedimented as three major nucleoprotein components and consisted of quasi‐isometric particles that varied in size from 24 to 32 nm in diameter and that were not penetrated by negative stain. Such virus particle preparations contained a major polypeptide of ca 28 kDa and three single‐stranded RNA species of estimated size 3.2, 2.8 and 2.1 kb. The complete sequence of the largest RNA (RNA 1, 3478 nt) and the partial sequence of the other RNAs (1863 and 2102 nt long, respectively) were determined and compared with sequences in databases. These findings, together with the biological and biochemical properties of this virus, indicate that it should be regarded as a distinct species in subgroup 1 of the genus Ilarvirus even though it was serologically unrelated to existing members of this subgroup. The virus showed a very distant serological relationship with prune dwarf virus (PDV) but differed significantly from it in the amino acid sequence of its coat protein, experimental host range and symptomatology and was unrelated to PDV at the molecular level. The virus, tentatively named blackberry chlorotic ringspot virus, is therefore a newly described virus and the first ilarvirus found naturally infecting Rubus in the UK.