Raspberry bushy dwarf virus
Transmission
Raspberry bushy dwarf virus (RBDV) is a pollen-transmitted virus that has been reported from eastern and western Europe, Scandinavia, Russia, North and South America, Australasia and South Africa, and probably occurs wherever susceptible varieties are grown. In the field the virus infects both red raspberry and black raspberry, although, symptoms vary depending on the cultivar and some cultivars are immune to the virus. RBDV is associated with a range of symptoms including yellowing of the leaf, reductions in cane height and diameter, and decreases in fruit size and yield. In addition, RBDV infection can interfere with the correct development of drupelets in the berry resulting in the phenomenon of crumbly fruit, seriously reducing fruit quality. Severe stunting of the plant, referred to as bushy dwarf disease, only occurs when the plant is infected with both RBDV and a second virus, most likely Black raspberry necrosis virus (BRNV). Indeed, all the symptoms associated with RBDV are greatly enhanced when additional viruses are also present.
The transfer of RBDV in infected pollen leads to infection not only of the seed but also the mother plant. The mechanism for this transmission is not known but may be aided by pollinating insects, although, experimentally infection can occur (at low frequency) by mechanical transfer of infected pollen. No other natural route for transmission (e.g. aphid feeding) is known, whereas in the glasshouse RBDV can be transmitted between raspberry plants by grafting.
Resistance
Experimentally RBDV also infects quite a wide range of herbaceous plants and can be transferred by manual inoculation using sap extracted from infected raspberry plants. This is one of the routine tests used at SCRI to identify RBDV in raspberry plants from the field and in the Certification Scheme for the production of high-health plants that are to be used for commercial propagation. The ability of RDBV to infect herbaceous plants also means that it is possible to isolate pure preparations of the virus which can be used to produce highly specific anti-virus antibodies. Using these reagents, RBDV can be readily detected in infected plants by an enzyme linked immunosorbent assay (ELISA). The susceptibility of different cultivars of raspberry to RBDV has been assessed by two approaches, firstly by recording the situation in the field over time where infection results from natural transmission in pollen, and secondly by grafting with infected scions in the glasshouse. The second approach identifies where varieties are immune to the virus, whereas the first approach also identifies cultivars that may have lower levels of resistance but which are still of practical use. A single, dominant resistance gene, denoted as Bu, was identified in the cultivar Glen Clova in a crossing experiment with the fully susceptible variety Lloyd George. Other varieties showing good resistance to RBDV include Malling Admiral, Malling Jewel, Malling Orion, Malling Promise, Haida and Willamette, although whether these reactions are all due to the Bu gene or to other genes is not clear. Particular strains of RBDV, reported from Russia, Germany, the former Yugoslavia and also present in the south of England, are known to be able to overcome Bu resistance. At present such resistance-breaking (RB) strains have not been reported to occur in the field in Scotland.
Description of genome
RBDV produces quasi-isometric (i.e. nearly spherical) virus particles of about 33nm diameter which contain three different molecules of single-stranded positive-sense RNA. RNA1 (5449 nucleotides) encodes a large protein of c.190 kDa molecular mass that is likely to be the viral RNA replicase. A 12 kDa protein is potentially encoded by a small open reading frame (ORF) that overlaps the C-terminus of the 190K ORF in a different reading frame. Expectations that the 12K protein may be involved in suppression of RNA silencing have not yet been confirmed. The 5 half of RBDV RNA2 (2231 nt) encodes a 39 kDa protein (appears as 44 kDa in PAGE gels of in vitro translation products) which has some weak similarities to virus cell-to-cell movement proteins from other viruses, while the 3 half of RNA2 encodes the 30 kDa coat protein (CP). RNA3 (946 nt) is derived from the 3 part of RNA2 and is probably a subgenomic RNA for CP synthesis. There is little or no nucleotide sequence homology at the 5 termini of the viral RNAs, whereas the 3 terminal 18 nt of RNA1 and RNA2 are identical. 5 capping of the RNAs is suspected but not proved, and the 3 termini are not polyadenylated but may fold into a highly-base-paired structure.
