Abstract

Raspberry bushy dwarf virus (RBDV) is transmitted to raspberry plants pollinated with infected pollen. Spread of the virus is controlled by the planting of immune cultivars; this is effective against the common strain but is threatened by the recent discovery of a resistance-breaking strain. The two strains cannot be distinguished from each other serologically or by symptoms in test plants. RBDV particles are about 33 nm in diameter and sediment as a single rather broad zone (so20,w = 115S) in sucrose density gradients. They contain a major protein species of mol. wt 29 000 (30 K) and three major species of ssRNA of mol. wt. (x 10-6) about 2.0 (RNA-1), 0.8 (RNA-2) and 0.3 (RNA-3).
In studies with one isolate of the common strain and one of the resistance-breaking strain, translation of unfractionated RBDV RNA in wheat germ extracts gave many polypeptides of which the largest had mol. wt. about 190 000 (190 K) and the most abundant was about 30 K. The 30 K product co-migrated with the protein from RBDV particles and reacted specifically with RBDV antiserum. When separated RBDV RNA species were translated in reticulocyte lysates, RNA-1 gave many polypeptides, the largest of about 190 K; RNA-2 and RNA-3 gave almost exclusively single polypeptides of about 46 K and 30 K respectively. The 190 K and 30 K products account for most of the coding capacity of RNA-1 and RNA-3 respectively.
Preparations of double-stranded RNA (dsRNA) from RBDV-infected Chenopodium quinoa leaves contained two major electrophoretic species of mol. wt. (x 10-6) 3.6 and 1.7; these are presumably the replicative forms of RNA-1 and RNA-2 respectively. Many minor dsRNA bands were observed, at least one of which (0.3 106 mol. wt.) occurred in dsRNA extracts from healthy C. quinoa leaves. There was no major dsRNA band corresponding in position to that expected for a replicative form of RNA-3. Slight differences were observed between the isolates of the common and resistance-breaking strains in the pattern of minor dsRNA bands but it is not clear whether this is a useful diagnostic character.
These preliminary results suggest that RBDV has a genome strategy unlike that described for any other plant virus.