Raspberry bushy dwarf virus (RBDV) is transmitted through Rubus pollen, from which infection spreads not only to the seed but also to the pollinated plant. No other means of natural spread is known, and the only method of control is by the use of resistant cultivars.
Existing staple cultivars in the UK (Malling Jewel, Glen Clova) have remained free from infection in the field, and were not infected in graft transmission tests with the type isolate from Scotland (RBDV-S). However, in recent graft-inoculation experiments reported from East Malling Research Station a culture of RBDV found in raspberry plants raised from seed imported from the USSR infected many cultivars previously regarded as immune, including Malling Jewel and Glen Clova. This resistance-breaking strain (RBDV-RB) is serologically indistinguishable from RBDV-S and induces similar symptoms in herbaceous hosts; it differs from RBDV-S in failing to infect cucumber systemically, but it is not known whether this property is completely correlated with the resistance-breaking character. If RBDV-RB is able to spread by pollination to cultivars previously regarded as immune, it could pose a new threat to Rubus crops in any country where resistance-breaking strains do not already exist. Resistance in Rubus to RBDV-S is thought to be conferred by a single dominant gene designated Bu, which segregates independently of five other genes studied. Further work is required to show whether resistance exists to RBDV-RB.
Quarantine regulations should be introduced to prevent the movement of RBDV and other seed-borne viruses in Rubus seed and breeders should take steps to prevent the introduction and establishment of such viruses in imported germplasm.